ISO-IEC 14882-2017 Information technology — Programming languages — C++ [5 ed.]

Citation preview

I N TERNATIONAL S TANDARD

ISO/IEC 14882 Fifth edition 2 0 1 7 -1 2

Programming languages — C++ Langages de programmation — C++

Reference number I SO /I EC 1 48 8 2 : 2 0 1 7 (E)

©

I SO /I E C 2 0 1 7

ISO/IEC 14882:2017(E)

COPYRIGHT PROTECTED DOCUMENT © I SO /I E C 2 0 1 7, Published in Switzerland

All rights reserved. Unless otherwise specified, no part o f this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country o f the requester.

ISO copyright o ffice C h. de B landonnet 8 • C P 40 1 C H -1 2 1 4 Vernier, Geneva, Switzerland Tel. + 41 2 2 749 0 1 1 1 Fax + 41 2 2 7 49 0 9 47

[email protected] www. iso. org

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© I SO /I E C 2 0 1 7 – All rights reserved

ISO/IEC 14882: 2017(E)

Contents Foreword

xi

1

Scope

1

2

Normative references

2

3

Terms and definitions

3

4

General principles

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 5

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Lexical conventions

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 6

7

Implementation compliance . . . . Structure of this document . . . . . Syntax notation . . . . . . . . . . . The C++ memory model . . . . . . The C++ obj ect model . . . . . . . Program execution . . . . . . . . . Multi-threaded executions and data Acknowledgments . . . . . . . . . .

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Separate translation . . . . Phases of translation . . . . Character sets . . . . . . . . Preprocessing tokens . . . . Alternative tokens . . . . . Tokens . . . . . . . . . . . . Comments . . . . . . . . . . Header names . . . . . . . . Preprocessing numbers . . . Identifiers . . . . . . . . . . Keywords . . . . . . . . . . Operators and punctuators Literals . . . . . . . . . . .

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Basic concepts

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11

Contents

7 8 8 8 9 11 15 20 22 22 23 24 25 25 26 26 26 27 28 29 29 39

Declarations and definitions One-definition rule . . . . . Scope . . . . . . . . . . . . Name lookup . . . . . . . . Program and linkage . . . . Start and termination . . . Storage duration . . . . . . Obj ect lifetime . . . . . . . Types . . . . . . . . . . . . Lvalues and rvalues . . . . . Alignment . . . . . . . . . .

© ISO/IEC 2017 – All rights reserved

39 41 44 50 63 66 70 74 77 83 84

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ISO/IEC 14882: 2017(E)

7

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 8

Lvalue-to-rvalue conversion . . . . . . Array-to-pointer conversion . . . . . . Function-to-pointer conversion . . . . . Temporary materialization conversion Qualification conversions . . . . . . . . Integral promotions . . . . . . . . . . . Floating-point promotion . . . . . . . Integral conversions . . . . . . . . . . . Floating-point conversions . . . . . . . Floating-integral conversions . . . . . . Pointer conversions . . . . . . . . . . . Pointer to member conversions . . . . Function pointer conversions . . . . . . Boolean conversions . . . . . . . . . . Integer conversion rank . . . . . . . . .

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Primary expressions . . . . . . . . . . . . . . . . Postfix expressions . . . . . . . . . . . . . . . . . Unary expressions . . . . . . . . . . . . . . . . . . Explicit type conversion (cast notation) . . . . . Pointer-to-member operators . . . . . . . . . . . Multiplicative operators . . . . . . . . . . . . . . Additive operators . . . . . . . . . . . . . . . . . Shift operators . . . . . . . . . . . . . . . . . . . Relational operators . . . . . . . . . . . . . . . . Equality operators . . . . . . . . . . . . . . . . . Bitwise AND operator . . . . . . . . . . . . . . . Bitwise exclusive OR operator . . . . . . . . . . . Bitwise inclusive OR operator . . . . . . . . . . . Logical AND operator . . . . . . . . . . . . . . . Logical OR operator . . . . . . . . . . . . . . . . Conditional operator . . . . . . . . . . . . . . . . Throwing an exception . . . . . . . . . . . . . . . Assignment and compound assignment operators Comma operator . . . . . . . . . . . . . . . . . . Constant expressions . . . . . . . . . . . . . . . .

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Expressions

8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 8.18 8.19 8.20 9

86

Standard conversions

93

Statements

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

Contents

Labeled statement . . . Expression statement . . Compound statement or Selection statements . . Iteration statements . . Jump statements . . . . Declaration statement . Ambiguity resolution . .

87 87 88 88 88 89 89 89 90 90 90 90 91 91 91 96 109 120 129 130 131 131 132 133 133 135 135 135 135 135 136 137 137 138 139 144

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© ISO/IEC 2017 – All rights reserved

145 145 145 145 148 150 152 153

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ISO/IEC 14882: 2017(E)

155

1 0 Declarations

10.1 10.2 10.3 10.4 10.5 10.6

Specifiers . . . . . . . . . Enumeration declarations Namespaces . . . . . . . . The asm declaration . . . Linkage specifications . . Attributes . . . . . . . . .

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Type names . . . . . . . . . . . Ambiguity resolution . . . . . . Meaning of declarators . . . . . Function definitions . . . . . . . Structured binding declarations Initializers . . . . . . . . . . . .

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1 1 Declarators

11.1 11.2 11.3 11.4 11.5 11.6

201

1 2 Classes

12.1 12.2 12.3 12.4

Multiple base classes . Member name lookup Virtual functions . . . Abstract classes . . . .

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. . . base . . . . . . . . . . . . . . .

. . . class . . . . . . . . . . . . . . .

Contents

268 269 272 275 276 276 276 278

Constructors . . . . . . . . . . . Temporary obj ects . . . . . . . . Conversions . . . . . . . . . . . . Destructors . . . . . . . . . . . . Free store . . . . . . . . . . . . . Initialization . . . . . . . . . . . . Construction and destruction . . Copying and moving class obj ects

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1 6 Overloading

16.1 16.2 16.3

256 258 261 265 267

Access specifiers . . . . . . . . . Accessibility of base classes and Friends . . . . . . . . . . . . . . Protected member access . . . . Access to virtual functions . . . Multiple access . . . . . . . . . Nested classes . . . . . . . . . .

1 5 Special member functions

15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8

239 241 251 254 255

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1 4 Member access control

14.1 14.2 14.3 14.4 14.5 14.6 14.7

202 203 204 216 219 220 237

Class names . . . . . . . Class members . . . . . Unions . . . . . . . . . . Local class declarations

1 3 Derived classes

13.1 13.2 13.3 13.4

157 174 178 191 191 194

278 281 283 286 289 291 298 301 309

Overloadable declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Declaration matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overload resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

© ISO/IEC 2017 – All rights reserved

309 311 312

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16.4 16.5 16.6

Address of overloaded function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overloaded operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Built-in operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 7 Templates

17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8 17.9

342

Template parameters . . . . . . . . . . . . Names of template specializations . . . . . Template arguments . . . . . . . . . . . . Type equivalence . . . . . . . . . . . . . . Template declarations . . . . . . . . . . . Name resolution . . . . . . . . . . . . . . . Template instantiation and specialization Function template specializations . . . . . Deduction guides . . . . . . . . . . . . . .

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1 8 Exception handling

18.1 18.2 18.3 18.4 18.5

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General . . . . . . . . . . . . . . . . The C standard library . . . . . . . . Definitions . . . . . . . . . . . . . . . Method of description (Informative) Library-wide requirements . . . . . .

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Conditional inclusion . . Source file inclusion . . . Macro replacement . . . Line control . . . . . . . Error directive . . . . . Pragma directive . . . . Null directive . . . . . . Predefined macro names Pragma operator . . . .

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Contents

433 435 436 441 442 442 442 442 444 445

21 Language support library

21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 21.9 21.10

423 425 425 427 430 432

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20 Library introduction

20.1 20.2 20.3 20.4 20.5

343 347 348 354 355 373 388 400 421 422

Throwing an exception . . . . Constructors and destructors Handling an exception . . . . Exception specifications . . . Special functions . . . . . . .

1 9 Preprocessing directives

19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9

333 334 339

445 446 446 449 454 476

General . . . . . . . . . . . . . Common definitions . . . . . . Implementation properties . . . Integer types . . . . . . . . . . Start and termination . . . . . Dynamic memory management Type identification . . . . . . . Exception handling . . . . . . . Initializer lists . . . . . . . . . . Other runtime support . . . . .

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© ISO/IEC 2017 – All rights reserved

476 476 481 490 491 492 500 502 507 508 vi

ISO/IEC 14882: 2017(E)

511

22 Diagnostics library

22.1 22.2 22.3 22.4 22.5

General . . . . . . . Exception classes . . Assertions . . . . . . Error numbers . . . System error support

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General . . . . . . . . . . . . . . . . . . . . Utility components . . . . . . . . . . . . . . Compile-time integer sequences . . . . . . . Pairs . . . . . . . . . . . . . . . . . . . . . . Tuples . . . . . . . . . . . . . . . . . . . . . Optional obj ects . . . . . . . . . . . . . . . Variants . . . . . . . . . . . . . . . . . . . . Storage for any type . . . . . . . . . . . . . Bitsets . . . . . . . . . . . . . . . . . . . . . Memory . . . . . . . . . . . . . . . . . . . . Smart pointers . . . . . . . . . . . . . . . . Memory resources . . . . . . . . . . . . . . . Class template scoped_allocator_adaptor Function obj ects . . . . . . . . . . . . . . . Metaprogramming and type traits . . . . . Compile-time rational arithmetic . . . . . . Time utilities . . . . . . . . . . . . . . . . . Class type_index . . . . . . . . . . . . . . . Execution policies . . . . . . . . . . . . . . .

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23 General utilities library

23.1 23.2 23.3 23.4 23.5 23.6 23.7 23.8 23.9 23.10 23.11 23.12 23.13 23.14 23.15 23.16 23.17 23.18 23.19

528

24 Strings library

24.1 24.2 24.3 24.4 24.5

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General . . . . . . . . . . . . . . Container requirements . . . . . . Sequence containers . . . . . . . Associative containers . . . . . . Unordered associative containers Container adaptors . . . . . . . .

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Contents

723 723 729 762 772 778

General . . . . . . . . . . . Header synopsis . Locales . . . . . . . . . . . . Standard locale categories C library locales . . . . . .

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26 Containers library

26.1 26.2 26.3 26.4 26.5 26.6

528 528 536 537 541 553 567 580 586 592 607 634 645 651 675 699 702 719 720 723

General . . . . . . . . . . . . . . Character traits . . . . . . . . . . String classes . . . . . . . . . . . String view classes . . . . . . . . Null-terminated sequence utilities

25 Localization library

25.1 25.2 25.3 25.4 25.5

511 511 515 515 517

778 778 780 787 825 826

© ISO/IEC 2017 – All rights reserved

826 826 864 896 918 942

vii

ISO/IEC 14882: 2017(E)

952

27 Iterators library

27.1 27.2 27.3 27.4 27.5 27.6 27.7 27.8

General . . . . . . . . . . . Iterator requirements . . . . Header synopsis Iterator primitives . . . . . Iterator adaptors . . . . . . Stream iterators . . . . . . . Range access . . . . . . . . Container access . . . . . .

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General . . . . . . . . . . . . . . . Header synopsis . . . Algorithms requirements . . . . . . Parallel algorithms . . . . . . . . . Non-modifying sequence operations Mutating sequence operations . . . Sorting and related operations . . . C library algorithms . . . . . . . .

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General . . . . . . . . . . . . . . . . . . . . . . Definitions . . . . . . . . . . . . . . . . . . . . . Numeric type requirements . . . . . . . . . . . The floating-point environment . . . . . . . . . Complex numbers . . . . . . . . . . . . . . . . . Random number generation . . . . . . . . . . . Numeric arrays . . . . . . . . . . . . . . . . . . Generalized numeric operations . . . . . . . . . Mathematical functions for floating-point types

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28 Algorithms library

28.1 28.2 28.3 28.4 28.5 28.6 28.7 28.8

986

29 Numerics library

29.1 29.2 29.3 29.4 29.5 29.6 29.7 29.8 29.9

Contents

1047 1047 1047 1048 1049 1059 1102 1122 1136 1 153

General . . . . . . . . . . . . Iostreams requirements . . . . Forward declarations . . . . . Standard iostream obj ects . . Iostreams base classes . . . . Stream buffers . . . . . . . . . Formatting and manipulators String-based streams . . . . . File-based streams . . . . . . File systems . . . . . . . . . . C library files . . . . . . . . .

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General . . . . . . . . . . . . . . . Definitions . . . . . . . . . . . . . . Requirements . . . . . . . . . . . . Header synopsis . . . . . . Namespace std: : regex_constants Class regex_error . . . . . . . . .

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31 Regular expressions library

31.1 31.2 31.3 31.4 31.5 31.6

986 986 1005 1006 1009 1017 1027 1046 1047

30 Input/output library

30.1 30.2 30.3 30.4 30.5 30.6 30.7 30.8 30.9 30.10 30.11

952 952 958 961 964 977 984 985

1153 1154 1154 1156 1158 1175 1184 1211 1221 1235 1288 1292

© ISO/IEC 2017 – All rights reserved

1292 1292 1293 1295 1301 1304 viii

ISO/IEC 14882: 2017(E)

31.7 31.8 31.9 31.10 31.11 31.12 31.13

Class template regex_traits . . . . . . . . . . . . Class template basic_regex . . . . . . . . . . . . . Class template sub_match . . . . . . . . . . . . . . Class template match_results . . . . . . . . . . . Regular expression algorithms . . . . . . . . . . . . Regular expression iterators . . . . . . . . . . . . . Modified ECMAScript regular expression grammar

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32 Atomic operations library

32.1 32.2 32.3 32.4 32.5 32.6 32.7 32.8 32.9

1 338

General . . . . . . . . . . Header synopsis Type aliases . . . . . . . . Order and consistency . . Lock-free property . . . . Class template atomic . . Non-member functions . . Flag type and operations . Fences . . . . . . . . . . .

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Keywords . . . . . . . . . Lexical conventions . . . . Basic concepts . . . . . . . Expressions . . . . . . . . Statements . . . . . . . . Declarations . . . . . . . . Declarators . . . . . . . . Classes . . . . . . . . . . . Derived classes . . . . . . Special member functions Overloading . . . . . . . . Templates . . . . . . . . . Exception handling . . . . Preprocessing directives .

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33 Thread support library

33.1 33.2 33.3 33.4 33.5 33.6

General . . . . . . Requirements . . . Threads . . . . . . Mutual exclusion . Condition variables Futures . . . . . .

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Contents

1408 1408 1413 1413 1417 1418 1422 1424 1425 1426 1426 1426 1427 1428 1 430

C Compatibility

C++ and ISO C . . . . . C++ and ISO C++ 2003 C++ and ISO C++ 2011 C++ and ISO C++ 2014 C standard library . . .

1355 1355 1358 1363 1384 1391 1 408

B Implementation quantities

C.1 C.2 C.3 C.4 C.5

1338 1338 1342 1342 1344 1345 1352 1352 1353 1 355

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A Grammar summary

A.1 A.2 A.3 A.4 A.5 A.6 A.7 A.8 A.9 A.10 A.11 A.12 A.13 A.14

1305 1307 1313 1318 1324 1329 1335

1 432

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© ISO/IEC 2017 – All rights reserved

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. 1432 . 1441 . 1447 . 1449 . 1453 ix

ISO/IEC 1 4882:201 7(E)

1456

D Compatibility features

D.1 D.2 D.3 D.4 D.5 D.6 D.7 D.8 D.9 D.10 D.11 D.12 D.13 D.14 D.15 D.16

Redeclaration of static constexpr data members Implicit declaration of copy functions . . . . . . . . Deprecated exception specifications . . . . . . . . . C++ standard library headers . . . . . . . . . . . . C standard library headers . . . . . . . . . . . . . . char* streams . . . . . . . . . . . . . . . . . . . . . uncaught_exception . . . . . . . . . . . . . . . . . Old adaptable function bindings . . . . . . . . . . . The default allocator . . . . . . . . . . . . . . . . . Raw storage iterator . . . . . . . . . . . . . . . . . Temporary buffers . . . . . . . . . . . . . . . . . . Deprecated type traits . . . . . . . . . . . . . . . . Deprecated iterator primitives . . . . . . . . . . . . Deprecated shared_ptr observers . . . . . . . . . . Deprecated standard code conversion facets . . . . Deprecated convenience conversion interfaces . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

1456 1456 1456 1456 1457 1457 1466 1466 1471 1472 1473 1474 1475 1475 1475 1477

Bibliography

1 482

Cross references

1 483

Cross references from ISO C

++

201 4

1504

Index

1506

Index of grammar productions

1539

Index of library names

1543

Index of implementation-defined behavior

1 601

Contents

© ISO/IEC 2017 – All rights reserved

x

ISO/IEC 1 4882:201 7(E)

Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies) . The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subj ect for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www. iso. org/directives ) . Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified during the development of the document will be in the Introduction and/or on the ISO list of patent declarations received (see www. iso. org/patents ) . Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement. For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISO’s adherence to the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following URL: www. iso. org/iso/foreword. html . This document was prepared by Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 22, Programming languages, their environments and system software interfaces. This fifth edition cancels and replaces the fourth edition (ISO/IEC 14882:2014) , which has been technically revised. The main changes compared to the previous edition are as follows: — expression evaluation order is specified in more cases — removal of trigraphs — adj ustments to value categories resulting in copy elision being mandatory — additional character and floating point literal syntaxes — lambda expressions extended to permit capture of *this and use in constant expressions — initializer statements for if and switch statements — addition of constexpr if statements — range-based for statement generalized to support heterogeneous begin and end types — addition of structured bindings — addition of inline variables — list initialization extended to support enumerations and aggregates with base classes — message in static_assert is now optional — addition of nested namespace definition syntax Foreword

© ISO/IEC 2017 – All rights reserved

xi

ISO/IEC 14882: 2017(E)

— extended support for attributes — exception specifications are now part of function types — template argument deduction is now supported for class templates — addition of fold expressions — pack expansion can be performed on using declarations — permitted forms of template parameters and template arguments have been generalized — dynamic allocation is supported for over-aligned types — preprocessor can detect presence of header files with

__has_include

— new utility functions, types, and templates in the standard library, including — an

any type

— an

optional class template

— a

variant class template

— a

clamp function

— a

std: : byte type

— a

not_fn function

— a

void_t alias template

—

conj unction, disj unction, and negation templates

— an

invoke function, and is_invocable and invoke_result type traits

— an

is_swappable type trait

— extended constant expression evaluation support in the standard library — elementary conversion functions between strings and numeric types added — constructors for —

pair and tuple are conditionally-explicit

shared_ptr s of array types now supported

— additional algorithms for managing uninitialized memory — addition of polymorphic memory resources — addition of substring search facilities providing the Boyer-Moore and Boyer-Moore-Horspool search algorithms — addition of variable templates for type traits — addition of a non-owning string view template — ability to splice elements between containers for maps and sets — better support for element insertion in unique-key maps — support for incomplete types in containers — addition of parallel algorithms — addition of

sample algorithm

— addition of mathematical special functions, and

gcd, lcm, and three-argument hypot functions

— addition of support for operations on file systems — addition of shared mutexes and variadic lock guards — removal of deprecated features Foreword

© ISO/IEC 2017 – All rights reserved

xii

ISO/IEC 1 4882:201 7(E)

1 1

Scope

This do cument requirement

[intro.scope] ++

sp ecifies requirement s for implement at ions of t he C

is t hat

t hey implement

and relaxat ions of t he firs t

2

C

++

is

a

IS O / IEC

general

p urp ose

9899: 201 1

t he language,

requirement

app ear at

programming

t he facilit ies provided by C , op erat or overloading,

C

++

based

on

t he

C

programming

( hereinaft er referred t o as t he

provides addit ional dat a typ es ,

funct ion name overloading,

programming language. also defines C

.

The firs t

such

O t her requirement s

various places wit hin t his do cument .

language

Programming languages — C ++

so t his do cument

references,

classes,

language

C standard

t emplat es,

as

).

except ions,

free st ore management op erat ors,

describ ed

in

In addit ion t o namespaces,

and addit ional

lib rary facilit ies.

S cop e

© IS O / IEC

2 0 1 7 – All right s reserved

1

ISO/IEC 14882: 2017(E)

2 1

Normative references

[intro.refs]

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

(1. 1 )

— Ecma International, ECMAScript Language Specification , Standard Ecma-262, third edition, 1999.

(1. 2)

— ISO/IEC 2382 (all parts) , Information technology — Vocabulary

(1. 3)

— ISO/IEC 9899:2011, Programming languages — C

(1. 4)

— ISO/IEC 9945:2003, Information Technology — Portable Operating System Interface (POSIX)

(1. 5)

— ISO/IEC 10646-1:1993, Information technology — Universal Multiple-Octet Coded Character Set (UCS)

— Part 1: Architecture and Basic Multilingual Plane

(1. 6)

— ISO/IEC/IEEE 60559:2011, Information technology — Microprocessor Systems — Floating-Point

arithmetic

(1. 7)

— ISO 80000-2:2009, Quantities and units — Part 2: Mathematical signs and symbols to be used in the

natural sciences and technology

2

The library described in Clause 7 of ISO/IEC 9899:2011 is hereinafter called the C standard library . 1

3

The operating system interface described in ISO/IEC 9945:2003 is hereinafter called POSIX.

4

The ECMAScript Language Specification described in Standard Ecma-262 is hereinafter called ECMA-262 .

With the qualifications noted in Clauses 21 through 33 and in C.5, the C standard library is a subset of the C++ standard library. 1)

Normative references

© ISO/IEC 2017 – All rights reserved

2

ISO/IEC 14882: 2017(E)

3 1

Terms and definitions

For

t he

purp oses

definit ions,

2

IS O

of t his

do cument ,

and IEC

—

IS O

O nline browsing plat form:

(2. 2)

—

IEC

Elect rop edia:

4

available at

2 0 . 3 defines addit ional t erms t hat

Terms t hat

t erms

and

80000- 2: 2009,

definit ions

given

in

IS O / IEC

available at

2382- 1 : 1 993,

t he

t erms,

and t he following apply.

maint ain t erminological dat abases for us e in st andardiz at ion at

(2. 1 )

3

t he

and symb ols given in IS O

[intro.defs] t he following addresses:

http: //www. iso. org/obp

http: //www. electropedia. org/

are used only in C lauses 2 0 t hrough 3 3 and Annex D .

are used only in a small p ort ion of t his do cument

are defined where t hey are used and it aliciz ed

where t hey are defined.

3. 1

[defns. access]

access 〈 execut ion- t ime act ion〉

read or mo dify t he value of an ob j ect

3. 2

[defns. argument]

argument 〈 funct ion call expression〉

expression in t he comma- separat ed list

b ounded by t he parent heses (8 . 2 . 2 )

3. 3

[defns. argument. macro]

argument 〈 funct ion- like macro〉

sequence of prepro cessing t okens in t he comma- sep arat ed list

b ounded by t he parent he-

s es ( 1 9 . 3 )

3. 4

[defns. argument. throw]

argument 〈 t hrow expression〉

op erand of

throw

( 8 . 1 7)

3. 5

[defns. argument. templ]

argument 〈 t emplat e inst ant iat ion〉

constant-expression type-id ,

,

or

id-expression

in t he comma- separat ed list

b ounded

by t he angle bracket s ( 1 7. 3 )

3. 6

[defns. block]

block wait

for some condit ion ( ot her t han for t he implement at ion t o execut e t he execut ion st eps of t he t hread of

execut ion)

t o b e sat isfied b efore cont inuing execut ion p ast

t he blo cking op erat ion

3. 7

[defns. cond. supp]

conditionally-supported program const ruct

[

t hat

Note 1 to entry: — end note

s upp ort .

an implement at ion is not

required t o supp ort

Each implement at ion do cument s all condit ionally- supp ort ed const ruct s t hat

it

do es not

]

3. 8

[defns. diagnostic]

diagnostic message mes sage b elonging t o an implement at ion- defined subset

§ 3. 8

© IS O / IEC

of t he implement at ion’ s out put

2 0 1 7 – All right s reserved

messages

3

ISO/IEC 14882: 2017(E)

3. 9

[defns. dynamic. type]

dynamic type

〈glvalue〉 type of the most derived obj ect (4.5) to which the glvalue refers [ Example: If a pointer (11.3.1) p whose static type is “pointer to class B ” is pointing to an obj ect of class D , derived from B (Clause 13) , the dynamic type of the expression *p is “ D ”. References (11.3.2) are treated similarly. — end example ] 3. 1 0

[defns. dynamic. type. prvalue]

dynamic type

〈prvalue〉 static type of the prvalue expression 3. 1 1

[defns. ill. formed]

ill-formed program

program that is not well-formed (3.29) 3. 1 2

[defns. impl. defined]

implementation-defined behavior

behavior, for a well-formed program construct and correct data, that depends on the implementation and that each implementation documents 3. 1 3

[defns. impl. limits]

implementation limits

restrictions imposed upon programs by the implementation 3. 1 4

[defns. locale. specific]

locale-specific behavior

behavior that depends on local conventions of nationality, culture, and language that each implementation documents 3. 1 5

[defns. multibyte]

multibyte character

sequence of one or more bytes representing a member of the extended character set of either the source or the execution environment [ Note

1 to entry: The extended character set is a superset of the basic character set (5.3) . — end note ]

3. 1 6

[defns. parameter]

parameter

〈function or catch clause〉 object or reference declared as part of a function declaration or definition or in the catch clause of an exception handler that acquires a value on entry to the function or handler 3. 1 7

[defns. parameter. macro]

parameter

〈function-like macro〉 identifier from the comma-separated list bounded by the parentheses immediately following the macro name 3. 1 8 parameter

〈template〉 member of a

§ 3.18

[defns. parameter. templ]

template-parameter-list

© ISO/IEC 2017 – All rights reserved

4

ISO/IEC 14882: 2017(E)

3. 1 9

[defns. signature]

signature 〈 funct ion〉

[

name,

paramet er typ e list

Note 1 to entry:

(1 1 . 3. 5) ,

and enclosing namesp ace ( if any)

S ignat ures are used as a basis for name mangling and linking.

3. 20

— end note

]

[defns. signature. templ]

signature 〈 funct ion

t emplat e〉

name,

paramet er

typ e

list

(1 1 . 3. 5) ,

enclosing

namespace

( if

any) ,

ret urn

typ e,

and

t emp lat e paramet er list

3. 21

[defns. signature. spec]

signature 〈 funct ion t emplat e sp ecializ at ion〉

signat ure of t he t emp lat e of which it

is a sp ecializ at ion and it s t emplat e

argument s ( whet her explicit ly sp ecified or deduced)

3. 22

[defns. signature. member]

signature 〈 class

cv

memb er

funct ion〉

- qualifiers ( if any) ,

and

name,

paramet er

ref-qualifier

typ e

list

(1 1 . 3. 5) ,

class

of

which

t he

funct ion

is

a

memb er,

( if any)

3. 23

[defns. signature. member. templ]

signature 〈 class memb er funct ion t emplat e〉

cv

- qualifiers ( if any) ,

ref-qualifier

name,

paramet er typ e list ( 1 1 . 3 . 5 ) ,

( if any) ,

ret urn typ e ( if any) ,

3. 24

class of which t he funct ion is a memb er,

and t emplat e paramet er list

[defns. signature. member. spec]

signature 〈 class memb er funct ion t emplat e sp ecializ at ion〉

signat ure of t he memb er funct ion t emplat e of which it

is a

s p ecializ at ion and it s t emplat e argument s ( whet her explicit ly sp ecified or deduced)

3. 25

[defns. static. type]

static type typ e of an expression ( 6 . 9 )

[

Note 1 to entry:

result ing from analysis of t he program wit hout

cons idering execut ion semant ics

The s t at ic typ e of an expres sion dep ends only on t he form of t he program in which t he

expression app ears ,

and do es not

change while t he program is execut ing.

— end note

3. 26

]

[defns. unblock]

unblock s at isfy a condit ion t hat

one or more blo cked t hreads of execut ion are wait ing for

3. 27

[defns. undefined]

undefined behavior b ehavior for which t his do cument

[

Note 1 to entry:

imp oses no requirement s

Undefined b ehavior may b e exp ect ed when t his do cument

b ehavior or when a program uses an erroneous const ruct

or erroneous dat a.

ranges from ignoring t he sit uat ion complet ely wit h unpredict able result s,

omit s any explicit

definit ion of

Permissible undefined b ehavior

t o b ehaving during t ranslat ion or

p rogram execut ion in a do cument ed manner charact erist ic of t he environment

( wit h or wit hout

t he issuance

of a diagnost ic message) , t o t erminat ing a t ranslat ion or execut ion ( wit h t he issuance of a diagnos t ic message) . Many erroneous program const ruct s do not Evaluat ion of a const ant

note

engender undefined b ehavior;

t hey are required t o b e diagnosed.

expression never exhibit s b ehavior explicit ly sp ecified as undefined (8 . 2 0 ) .

— end

]

§ 3. 27

© IS O / IEC

2 0 1 7 – All right s reserved

5

ISO/IEC 14882: 201 7(E)

3. 28

[defns. unspecified]

unspecified behavior b ehavior,

[

for a well- formed program const ruct

Note 1 to entry:

The implement at ion is not

and correct

dat a,

t hat

required t o do cument

p ossible b ehaviors is usually delineat ed by t his do cument .

dep ends on t he implement at ion

which b ehavior o ccurs.

— end note

The range of

]

3. 29

[defns. well. formed]

well-formed program C

++

program const ruct ed according t o t he s ynt ax rules,

diagnosable semant ic rules,

and t he one- definit ion

rule ( 6 . 2 ) .

§ 3. 29

© IS O / IEC

2 0 1 7 – All right s reserved

6

ISO/IEC 14882: 2017(E)

4

General principles

4. 1

Implementation compliance

[intro] [intro. compliance]

1 The set of diagnosable rules consists of all syntactic and semantic rules in this document except for those

rules containing an explicit notation that “no diagnostic is required” or which are described as resulting in “undefined behavior”.

2 Although this document states only requirements on C++ implementations, those requirements are often

easier to understand if they are phrased as requirements on programs, parts of programs, or execution of programs. Such requirements have the following meaning:

(2.1)

— If a program contains no violations of the rules in this document, a conforming implementation shall, within its resource limits, accept and correctly execute2 that program.

(2.2)

— If a program contains a violation of any diagnosable rule or an occurrence of a construct described in this document as “conditionally-supported” when the implementation does not support that construct, a conforming implementation shall issue at least one diagnostic message.

(2.3)

— If a program contains a violation of a rule for which no diagnostic is required, this document places no requirement on implementations with respect to that program. [ Note: During template argument deduction and substitution, certain constructs that in other contexts require a diagnostic are treated differently; see 17.8.2. — end note ]

3 For classes and class templates, the library Clauses specify partial definitions. Private members (Clause 14) are not specified, but each implementation shall supply them to complete the definitions according to the description in the library Clauses.

4 For functions, function templates, obj ects, and values, the library Clauses specify declarations. Implementations shall supply definitions consistent with the descriptions in the library Clauses.

5 The names defined in the library have namespace scope (10.3) . A C++ translation unit (5.2) obtains access to these names by including the appropriate standard library header (19.2) .

6 The templates, classes, functions, and objects in the library have external linkage (6.5) . The implementation

provides definitions for standard library entities, as necessary, while combining translation units to form a complete C++ program (5.2) .

7 Two kinds of implementations are defined: a hosted implementation and a freestanding implementation. For a

hosted implementation, this document defines the set of available libraries. A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementationdefined set of libraries that includes certain language-support libraries (20.5.1.3) .

8 A conforming implementation may have extensions (including additional library functions) , provided they do not alter the behavior of any well-formed program. Implementations are required to diagnose programs that use such extensions that are ill-formed according to this document. Having done so, however, they can compile and execute such programs.

9 Each implementation shall include documentation that identifies all conditionally-supported constructs that it does not support and defines all locale-specific characteristics. 3 2) 3)

“ C orrect execut ion” can include undefined b ehavior, dep ending on t he dat a b eing pro cessed; see C lause 3 and 4. 6. This do cument at ion also defines implement at ion- defined b ehavior; see 4. 6.

§ 4.1

© ISO/IEC 2017 – All rights reserved

7

ISO/IEC 14882: 2017(E)

4. 2

Structure of this document

[intro. structure]

1 Clauses 5 through 19 describe the C++ programming language. That description includes detailed syntactic

specifications in a form described in 4.3. For convenience, Annex A repeats all such syntactic specifications.

2 Clauses 21 through 33 and Annex D (the library clauses) describe the C++ standard library. That description

includes detailed descriptions of the entities and macros that constitute the library, in a form described in Clause 20.

3 Annex B recommends lower bounds on the capacity of conforming implementations. 4 Annex C summarizes the evolution of C++ since its first published description, and explains in detail the differences between C++ and C. Certain features of C++ exist solely for compatibility purposes; Annex D describes those features.

5 Throughout this document, each example is introduced by “[ Example: ” and terminated by “ — end example ] ”. Each note is introduced by “[ Note: ” and terminated by “ — end note ] ”. Examples and notes may be nested. 4. 3

Syntax notation

[syntax]

1 In the syntax notation used in this document, syntactic categories are indicated by italic type, and literal

words and characters in constant width type. Alternatives are listed on separate lines except in a few cases where a long set of alternatives is marked by the phrase “one of”. If the text of an alternative is too long to fit on a line, the text is continued on subsequent lines indented from the first one. An optional terminal or non-terminal symbol is indicated by the subscript “ opt ”, so { expressionopt }

indicates an optional expression enclosed in braces.

2 Names for syntactic categories have generally been chosen according to the following rules: (2.1) — X-name is a use of an identifier in a context that determines its meaning (e.g., class-name , typedef-name ) . (2.2) — X-id is an identifier with no context-dependent meaning (e.g., qualified-id) . (2.3) — X-seq is one or more X’s without intervening delimiters (e.g., declaration-seq is a sequence of declarations) .

(2.4)

— 4. 4

X-list is one or more X’s separated by intervening commas (e.g., identifier-list is a sequence of identifiers separated by commas) . The C

++

memory model

[intro. memory]

1 The fundamental storage unit in the C++ memory model is the byte. A byte is at least large enough to contain

any member of the basic execution character set (5.3) and the eight-bit code units of the Unicode UTF-8 encoding form and is composed of a contiguous sequence of bits, 4 the number of which is implementationdefined. The least significant bit is called the low-order bit; the most significant bit is called the high-order bit. The memory available to a C++ program consists of one or more sequences of contiguous bytes. Every byte has a unique address.

2 [ Note: The representation of types is described in 6.9. — end note ] 3 A memory location is either an obj ect of scalar type or a maximal sequence of adj acent bit-fields all having nonzero width. [ Note: Various features of the language, such as references and virtual functions, might involve

additional memory locations that are not accessible to programs but are managed by the implementation.

— end note ] Two or more threads of execution (4.7) can access separate memory locations without interfering with each other.

4 [ Note: Thus a bit-field and an adj acent non-bit-field are in separate memory locations, and therefore can be

concurrently updated by two threads of execution without interference. The same applies to two bit-fields,

4)

The numb er of bit s in a byt e is rep ort ed by t he macro

§ 4.4

CHAR_BIT

in t he header

.

© ISO/IEC 2017 – All rights reserved

8

ISO/IEC 14882: 2017(E)

if one is declared inside a nested struct declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field declaration. It is not safe to concurrently update two bit-fields in the same struct if all fields between them are also bit-fields of nonzero width. — end note ]

5 [ Example: A structure declared as struct { char a; int b: 5, c: 11, : 0, d: 8; struct {int ee: 8; } e; }

contains four separate memory locations: The member a and bit-fields d and e. ee are each separate memory locations, and can be modified concurrently without interfering with each other. The bit-fields b and c together constitute the fourth memory location. The bit-fields b and c cannot be concurrently modified, but b and a, for example, can be. — end example ] 4. 5

The C

++

obj ect model

[intro. obj ect]

1 The constructs in a C++ program create, destroy, refer to, access, and manipulate obj ects. An object is created by a definition (6.1) , by a new-expression (8.3.4) , when implicitly changing the active member of a

union (12.3) , or when a temporary obj ect is created (7.4, 15.2) . An obj ect occupies a region of storage in its period of construction (15.7) , throughout its lifetime (6.8) , and in its period of destruction (15.7) . [ Note: A function is not an obj ect, regardless of whether or not it occupies storage in the way that obj ects do. — end note ] The properties of an obj ect are determined when the obj ect is created. An obj ect can have a name (Clause 6) . An obj ect has a storage duration (6.7) which influences its lifetime (6.8) . An obj ect has a type (6.9) . Some obj ects are polymorphic (13.3) ; the implementation generates information associated with each such obj ect that makes it possible to determine that obj ect’s type during program execution. For other objects, the interpretation of the values found therein is determined by the type of the expression s (Clause 8) used to access them.

2 Objects can contain other obj ects, called subobjects . A subobj ect can be a member subobject (12.2) , a base class subobject (Clause 13) , or an array element. An obj ect that is not a subobj ect of any other obj ect is called a complete object . If an obj ect is created in storage associated with a member subobj ect or array element e (which may or may not be within its lifetime) , the created obj ect is a subobj ect of e’s containing obj ect if:

(2.1) (2.2) (2.3)

— the lifetime of e’s containing obj ect has begun and not ended, and — the storage for the new obj ect exactly overlays the storage location associated with e, and — the new obj ect is of the same type as

e (ignoring cv-qualification) .

[ Note: If the subobject contains a reference member or a const subobject, the name of the original subobject cannot be used to access the new obj ect (6.8) . — end note ] [ Example: struct X { const int n; }; union U { X x; float f; }; void tong() { U u = {{ 1 }}; u. f = 5. f; X *p = new (&u. x) X {2}; assert(p->n == 2) ; assert(*std: : launder(&u. x. n) == 2) ; assert(u. x. n == 2) ;

§ 4.5

// OK, creates new subobject of u (12.3) // OK, creates new subobject of u // OK // OK // undefined behavior, u. x does not name new subobject

© ISO/IEC 2017 – All rights reserved

9

ISO/IEC 14882: 2017(E)

}

— end example ] 3

If a complete obj ect is created (8.3.4) in storage associated with another obj ect std: : byte ” (21.2.1) , that array provides unsigned char ” or of type “array of obj ect if:

N

(3. 1 )

— the lifetime of e has begun and not ended, and

(3. 2)

— the storage for the new obj ect fits entirely within e, and

(3. 3)

— there is no smaller array obj ect that satisfies these constraints.

e of type “array of N storage for the created

[ Note: If that portion of the array previously provided storage for another obj ect, the lifetime of that obj ect ends because its storage was reused (6.8) . — end note ] [ Example: template struct AlignedUnion { alignas(T. . . ) unsigned char data[max(sizeof(T) . . . ) ] ; }; int f() { AlignedUnion au; int *p = new (au. data) int; // OK, au. data provides storage char *c = new (au. data) char() ; // OK, ends lifetime of *p char *d = new (au. data + 1) char() ; return *c + *d; // OK } struct struct A a; B *b = int *p

4 (4. 1 ) (4. 2) (4. 3) 5

A { unsigned char a[32] ; }; B { unsigned char b[16] ; }; new (a. a + 8) B; // a. a provides storage for *b = new (b->b + 4) int; // b->b provides storage for *p // a. a does not provide storage for *p (directly), // but *p is nested within a (see below)

— end example ] An obj ect a is nested within another obj ect b if: — a is a subobj ect of b, or — b provides storage for a, or — there exists an obj ect c where a is nested within c, and c is nested within b. For every obj ect x, there is some obj ect called the complete object of x , determined as follows:

(5. 1 )

— If x is a complete obj ect, then the complete obj ect of x is itself.

(5. 2)

— Otherwise, the complete obj ect of x is the complete obj ect of the (unique) obj ect that contains x.

6

If a complete obj ect, a data member (12.2) , or an array element is of class type, its type is considered the

most derived class, to distinguish it from the class type of any base class subobj ect; an obj ect of a most derived class type or of a non-class type is called a most derived object . 7

Unless it is a bit-field (12.2.4) , a most derived obj ect shall have a nonzero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size. An object of trivially copyable or standard-layout type (6.9) shall occupy contiguous bytes of storage.

8

Unless an obj ect is a bit-field or a base class subobj ect of zero size, the address of that obj ect is the address of the first byte it occupies. Two obj ects a and b with overlapping lifetimes that are not bit-fields may have the same address if one is nested within the other, or if at least one is a base class subobject of zero size and § 4.5

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they are of different types; otherwise, they have distinct addresses. 5 [ Example: static const char test1 = ’ x’ ; static const char test2 = ’ x’ ; const bool b = &test1 ! = &test2;

// always true

— end example ] 9 [ Note: C++ provides a variety of fundamental types and several ways of composing new types from existing types (6.9) . — end note ] 4. 6

Program execution

[intro. execution]

1 The semantic descriptions in this document define a parameterized nondeterministic abstract machine. This

document places no requirement on the structure of conforming implementations. In particular, they need not copy or emulate the structure of the abstract machine. Rather, conforming implementations are required to emulate (only) the observable behavior of the abstract machine as explained below. 6

2 Certain aspects and operations of the abstract machine are described in this document as implementationdefined (for example, sizeof(int) ) . These constitute the parameters of the abstract machine. Each implementation shall include documentation describing its characteristics and behavior in these respects. 7 Such documentation shall define the instance of the abstract machine that corresponds to that implementation (referred to as the “corresponding instance” below) .

3 Certain other aspects and operations of the abstract machine are described in this document as unspecified (for example, evaluation of expressions in a new-initializer if the allocation function fails to allocate memory (8.3.4) ) . Where possible, this document defines a set of allowable behaviors. These define the nondeterministic aspects of the abstract machine. An instance of the abstract machine can thus have more than one possible execution for a given program and a given input.

4 Certain other operations are described in this document as undefined (for example, the effect of attempting to modify a const object) . [ Note: This document imposes no requirements on the behavior of programs that contain undefined behavior. — end note ] 5 A conforming implementation executing a well-formed program shall produce the same observable behavior as one of the possible executions of the corresponding instance of the abstract machine with the same program and the same input. However, if any such execution contains an undefined operation, this document places no requirement on the implementation executing that program with that input (not even with regard to operations preceding the first undefined operation) .

6 An instance of each obj ect with automatic storage duration (6.7.3) is associated with each entry into its

block. Such an obj ect exists and retains its last-stored value during the execution of the block and while the block is suspended (by a call of a function or receipt of a signal) .

7 The least requirements on a conforming implementation are: (7.1) — Accesses through volatile glvalues are evaluated strictly according to the rules of the abstract machine. (7.2) — At program termination, all data written into files shall be identical to one of the possible results that execution of the program according to the abstract semantics would have produced.

5)

Under t he “ as- if” rule an implement at ion is allowed t o st ore two ob j ect s at t he same machine address or not st ore an ob j ect

at all if t he program cannot observe t he difference ( 4. 6) .

6)

This provision is somet imes called t he “ as- if” rule, b ecause an implement at ion is free t o disregard any requirement of t his

do cument as long as t he result is b ehavior of t he program.

as if

t he requirement had b een ob eyed, as far as can b e det ermined from t he observable

For inst ance, an act ual implement at ion need not evaluat e part of an expression if it can deduce t hat

it s value is not used and t hat no side effect s affect ing t he observable b ehavior of t he program are pro duced.

7)

This do cument at ion also includes condit ionally- supp ort ed const ruct s and lo cale- sp ecific b ehavior.

§ 4.6

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— The input and output dynamics of interactive devices shall take place in such a fashion that prompting output is actually delivered before a program waits for input. What constitutes an interactive device is implementation-defined.

(7. 3)

These collectively are referred to as the observable behavior of the program. [ Note: More stringent correspondences between abstract and actual semantics may be defined by each implementation. — end note ] 8

[ Note: Operators can be regrouped according to the usual mathematical rules only where the operators really are associative or commutative. 8 For example, in the following fragment int a, b; /* . . . */ a = a + 32760 + b + 5;

the expression statement behaves exactly the same as a = (((a + 32760) + b) + 5) ;

due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is next added to b , and that result is then added to 5 which results in the value assigned to a. On a machine in which overflows produce an exception and in which the range of values representable by an int is [-32768, +32767] , the implementation cannot rewrite this expression as a = ((a + b) + 32765) ;

since if the values for a and b were, respectively, -32754 and -15, the sum a + b would produce an exception while the original expression would not; nor can the expression be rewritten either as a = ((a + 32765) + b) ;

or a = (a + (b + 32765) ) ;

since the values for a and b might have been, respectively, 4 and -8 or -17 and 12. However on a machine in which overflows do not produce an exception and in which the results of overflows are reversible, the above expression statement can be rewritten by the implementation in any of the above ways because the same result will occur. — end note ] 9

A

constituent expression is defined as follows:

(9. 1 )

— The constituent expression of an expression is that expression.

(9. 2)

— The constituent expressions of a braced-init-list or of a (possibly parenthesized) constituent expressions of the elements of the respective list.

(9. 3)

— The constituent expressions of a brace-or-equal-initializer of the form = constituent expressions of the initializer-clause .

expression-list are the

initializer-clause are the

[ Example: struct A { int x; }; struct B { int y; struct A a; }; B b = { 5, { 1+1 } };

The constituent expressions of the 10

initializer used for the initialization of b are 5 and 1+1 . — end example ]

immediate subexpressions of an expression e are

The

(10. 1)

— the constituent expressions of e ’s operands (Clause 8) ,

(10. 2)

— any function call that e implicitly invokes,

(10. 3)

— if e is a 8)

lambda-expression (8.1.5) , the initialization of the entities captured by copy and the constituent

O verloaded op erat ors are never assumed t o b e asso ciat ive or commut at ive.

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expressions of the

initializer of the init-captures,

(10. 4)

— if e is a function call (8.2.2) or implicitly invokes a function, the constituent expressions of each default argument (11.3.6) used in the call, or

(10. 5)

— if e creates an aggregate obj ect (11.6.1) , the constituent expressions of each default member initializer (12.2) used in the initialization.

11

A subexpression of an expression e is an immediate subexpression of e or a subexpression of an immediate subexpression of e . [ Note: Expressions appearing in the compound-statement of a lambda-expression are not subexpressions of the lambda-expression . — end note ]

12

A

full-expression is

(12. 1)

— an unevaluated operand (Clause 8) ,

(12. 2)

— a

(12. 3)

constant-expression (8.20) , — an init-declarator (Clause 11) or a mem-initializer (15.6.2) , including the constituent expressions of the initializer,

(12. 4)

— an invocation of a destructor generated at the end of the lifetime of an obj ect other than a temporary obj ect (15.2) , or

(12. 5)

— an expression that is not a subexpression of another expression and that is not otherwise part of a full-expression. If a language construct is defined to produce an implicit call of a function, a use of the language construct is considered to be an expression for the purposes of this definition. Conversions applied to the result of an expression in order to satisfy the requirements of the language construct in which the expression appears are also considered to be part of the full-expression. For an initializer, performing the initialization of the entity (including evaluating default member initializers of an aggregate) is also considered part of the full-expression. [ Example: struct S { S(int i) : I(i) { } // full-expression is initialization of I int& v() { return I; } ~ S() noexcept(false) { } private: int I; }; S s1(1) ; void f() { S s2 = 2; if (S(3) . v() )

{ } bool b = noexcept(S() ) ;

// full-expression is call of S: : S(int) // full-expression is call of S: : S(int) // full-expression includes lvalue-to-rvalue and // int to bool conversions, performed before // temporary is deleted at end of full-expression

// exception specification of destructor of S // considered for noexcept // full-expression is destruction of s2 at end of block

} struct B { B(S = S(0) ) ; }; B b[2] = { B() , B() };

// full-expression is the entire initialization // including the destruction of temporaries

— end example ] § 4.6

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13

[ Note: The evaluation of a full-expression can include the evaluation of subexpressions that are not lexically part of the full-expression. For example, subexpressions involved in evaluating default arguments (11.3.6) are considered to be created in the expression that calls the function, not the expression that defines the default argument. — end note ]

14

Reading an obj ect designated by a volatile glvalue (6.10) , modifying an obj ect, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment. Evaluation of an expression (or a subexpression) in general includes both value computations (including determining the identity of an obj ect for glvalue evaluation and fetching a value previously assigned to an obj ect for prvalue evaluation) and initiation of side effects. When a call to a library I/O function returns or an access through a volatile glvalue is evaluated the side effect is considered complete, even though some external actions implied by the call (such as the I/O itself) or by the volatile access may not have completed yet.

15

Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations executed by a single thread (4.7) , which induces a partial order among those evaluations. Given any two evaluations A and B, if A is sequenced before B (or, equivalently, B is sequenced after A ) , then the execution of A shall precede the execution of B. If A is not sequenced before B and B is not sequenced before A , then A and B are unsequenced. [ Note: The execution of unsequenced evaluations can overlap. — end note ] Evaluations A and B are indeterminately sequenced when either A is sequenced before B or B is sequenced before A , but it is unspecified which. [ Note: Indeterminately sequenced evaluations cannot overlap, but either could be executed first. — end note ] An expression X is said to be sequenced before an expression Y if every value computation and every side effect associated with the expression X is sequenced before every value computation and every side effect associated with the expression Y.

16

Every value computation and side effect associated with a full-expression is sequenced before every value computation and side effect associated with the next full-expression to be evaluated. 9

17

Except where noted, evaluations of operands of individual operators and of subexpressions of individual expressions are unsequenced. [ Note: In an expression that is evaluated more than once during the execution of a program, unsequenced and indeterminately sequenced evaluations of its subexpressions need not be performed consistently in different evaluations. — end note ] The value computations of the operands of an operator are sequenced before the value computation of the result of the operator. If a side effect on a memory location (4.4) is unsequenced relative to either another side effect on the same memory location or a value computation using the value of any obj ect in the same memory location, and they are not potentially concurrent (4.7) , the behavior is undefined. [ Note: The next section imposes similar, but more complex restrictions on potentially concurrent computations. — end note ] [ Example: void g(int i) { i = 7, i++, i++; i = i++ + 1; i = i++ + i; i = i + 1; }

// i becomes 9 // the value of i is incremented // the behavior is undefined // the value of i is incremented

— end example ] 18

When calling a function (whether or not the function is inline) , every value computation and side effect associated with any argument expression, or with the postfix expression designating the called function, is sequenced before execution of every expression or statement in the body of the called function. For each function invocation F, for every evaluation A that occurs within F and every evaluation B that does not 9)

As sp ecified in 1 5 . 2 , aft er a full- expression is evaluat ed, a sequence of zero or more invo cat ions of dest ruct or funct ions for

t emp orary ob j ect s t akes place, usually in reverse order of t he const ruct ion of each t emp orary ob j ect .

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occur within F but is evaluated on the same thread and as part of the same signal handler (if any) , either A is sequenced before B or B is sequenced before A . 10 [ Note: If A and B would not otherwise be sequenced then they are indeterminately sequenced. — end note ] Several contexts in C++ cause evaluation of a function call, even though no corresponding function call syntax appears in the translation unit. [ Example: Evaluation of a new-expression invokes one or more allocation and constructor functions; see 8.3.4. For another example, invocation of a conversion function (15.3.2) can arise in contexts in which no function call syntax appears. — end example ] The sequencing constraints on the execution of the called function (as described above) are features of the function calls as evaluated, whatever the syntax of the expression that calls the function might be.

19 If a signal handler is executed as a result of a call to the std: : raise function, then the execution of the handler is sequenced after the invocation of the std: : raise function and before its return. [ Note: When a

signal is received for another reason, the execution of the signal handler is usually unsequenced with respect to the rest of the program. — end note ]

4. 7

Multi-threaded executions and data races

[intro. multithread]

1 A thread of execution (also known as a thread) is a single flow of control within a program, including the initial

invocation of a specific top-level function, and recursively including every function invocation subsequently executed by the thread. [ Note: When one thread creates another, the initial call to the top-level function of the new thread is executed by the new thread, not by the creating thread. — end note ] Every thread in a program can potentially access every obj ect and function in a program. 11 Under a hosted implementation, a C++ program can have more than one thread running concurrently. The execution of each thread proceeds as defined by the remainder of this document. The execution of the entire program consists of an execution of all of its threads. [ Note: Usually the execution can be viewed as an interleaving of all its threads. However, some kinds of atomic operations, for example, allow executions inconsistent with a simple interleaving, as described below. — end note ] Under a freestanding implementation, it is implementation-defined whether a program can have more than one thread of execution.

2 For a signal handler that is not executed as a result of a call to the std: : raise function, it is unspecified which thread of execution contains the signal handler invocation.

4. 7. 1

Data races

[intro. races]

1 The value of an obj ect visible to a thread T at a particular point is the initial value of the obj ect, a value assigned to the obj ect by T, or a value assigned to the obj ect by another thread, according to the rules below. [ Note: In some cases, there may instead be undefined behavior. Much of this section is motivated by the desire to support atomic operations with explicit and detailed visibility constraints. However, it also implicitly supports a simpler view for more restricted programs. — end note ]

2 Two expression evaluations conflict if one of them modifies a memory location (4.4) and the other one reads or modifies the same memory location.

3 The library defines a number of atomic operations (Clause 32) and operations on mutexes (Clause 33) that are

specially identified as synchronization operations. These operations play a special role in making assignments in one thread visible to another. A synchronization operation on one or more memory locations is either a consume operation, an acquire operation, a release operation, or both an acquire and release operation. A synchronization operation without an associated memory location is a fence and can be either an acquire fence, a release fence, or both an acquire and release fence. In addition, there are relaxed atomic operations, which are not synchronization operations, and atomic read-modify-write operations, which have special characteristics. [ Note: For example, a call that acquires a mutex will perform an acquire operation on the locations comprising the mutex. Correspondingly, a call that releases the same mutex will perform a

10) 11)

In ot her words, funct ion execut ions do not int erleave wit h each ot her. An ob j ect wit h aut omat ic or t hread st orage durat ion ( 6. 7) is asso ciat ed wit h one sp ecific t hread, and can b e accessed by a

different t hread only indirect ly t hrough a p oint er or reference ( 6. 9. 2 ) .

§ 4.7.1

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release operation on those same locations. Informally, performing a release operation on A forces prior side effects on other memory locations to become visible to other threads that later perform a consume or an acquire operation on A . “Relaxed” atomic operations are not synchronization operations even though, like synchronization operations, they cannot contribute to data races. — end note ] 4

All modifications to a particular atomic object M occur in some particular total order, called the modification order of M. [ Note: There is a separate order for each atomic obj ect. There is no requirement that these can be combined into a single total order for all objects. In general this will be impossible since different threads may observe modifications to different obj ects in inconsistent orders. — end note ]

5

A release sequence headed by a release operation A on an atomic obj ect M is a maximal contiguous subsequence of side effects in the modification order of M, where the first operation is A , and every subsequent operation

(5. 1 )

— is performed by the same thread that performed A , or

(5. 2)

— is an atomic read-modify-write operation.

6

Certain library calls synchronize with other library calls performed by another thread. For example, an atomic store-release synchronizes with a load-acquire that takes its value from the store (32.4) . [ Note: Except in the specified cases, reading a later value does not necessarily ensure visibility as described below. Such a requirement would sometimes interfere with efficient implementation. — end note ] [ Note: The specifications of the synchronization operations define when one reads the value written by another. For atomic obj ects, the definition is clear. All operations on a given mutex occur in a single total order. Each mutex acquisition “reads the value written” by the last mutex release. — end note ]

7

An evaluation

(7. 1 ) (7. 1. 1 ) (7. 1. 2)

A carries a dependency to an evaluation B if — the value of A is used as an operand of B, unless: — B is an invocation of any specialization of std: : kill_dependency (32.4) , or — A is the left operand of a built-in logical AND ( && , see 8.14) or logical OR ( | | , see 8.15) operator, or

A is the left operand of a conditional (?: , see 8.16) operator, or — A is the left operand of the built-in comma ( , ) operator (8.19) ; —

(7. 1. 3) (7. 1. 4)

or (7. 2)

(7. 3)

8 (8. 1 )

(8. 2)

9 (9. 1 ) (9. 2)

A writes a scalar obj ect or bit-field M, B reads the value written by A from M, and A is sequenced before B, or — for some evaluation X, A carries a dependency to X, and X carries a dependency to B. [ Note: “Carries a dependency to” is a subset of “is sequenced before”, and is similarly strictly intra-thread. — end note ] An evaluation A is dependency-ordered before an evaluation B if — A performs a release operation on an atomic obj ect M, and, in another thread, B performs a consume operation on M and reads a value written by any side effect in the release sequence headed by A , or — for some evaluation X, A is dependency-ordered before X and X carries a dependency to B. [ Note: The relation “is dependency-ordered before” is analogous to “synchronizes with”, but uses release/consume in place of release/acquire. — end note ] An evaluation A inter-thread happens before an evaluation B if — A synchronizes with B, or — A is dependency-ordered before B, or —

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(9. 3)

— for some evaluation

X

A synchronizes with X and X is sequenced before B, or — A is sequenced before X and X inter-thread happens before B, or — A inter-thread happens before X and X inter-thread happens before B.

—

(9. 3. 1) (9. 3. 2) (9. 3. 3)

[ Note: The “inter-thread happens before” relation describes arbitrary concatenations of “sequenced before”, “synchronizes with” and “dependency-ordered before” relationships, with two exceptions. The first exception is that a concatenation is not permitted to end with “dependency-ordered before” followed by “sequenced before”. The reason for this limitation is that a consume operation participating in a “dependency-ordered before” relationship provides ordering only with respect to operations to which this consume operation actually carries a dependency. The reason that this limitation applies only to the end of such a concatenation is that any subsequent release operation will provide the required ordering for a prior consume operation. The second exception is that a concatenation is not permitted to consist entirely of “sequenced before”. The reasons for this limitation are (1) to permit “inter-thread happens before” to be transitively closed and (2) the “happens before” relation, defined below, provides for relationships consisting entirely of “sequenced before”. — end note ] 10 (10. 1) (10. 2)

A happens before an evaluation B (or, equivalently, B happens after A ) if: — A is sequenced before B, or — A inter-thread happens before B.

An evaluation

The implementation shall ensure that no program execution demonstrates a cycle in the “happens before” relation. [ Note: This cycle would otherwise be possible only through the use of consume operations. — end note ] 11 (11. 1) (11. 2) (11. 3)

12

A strongly happens before an evaluation B if either — A is sequenced before B, or — A synchronizes with B, or — A strongly happens before X and X strongly happens before B. [ Note: In the absence of consume operations, the happens before and strongly happens before relations are identical. Strongly happens before essentially excludes consume operations. — end note ] A visible side effect A on a scalar obj ect or bit-field M with respect to a value computation B of M satisfies An evaluation

the conditions: (12. 1) (12. 2)

—

A happens before B and

X to M such that A happens before X and X happens before B. The value of a non-atomic scalar obj ect or bit-field M, as determined by evaluation B, shall be the value stored by the visible side effect A . [ Note: If there is ambiguity about which side effect to a non-atomic object or bit-field is visible, then the behavior is either unspecified or undefined. — end note ] [ Note: This states — there is no other side effect

that operations on ordinary obj ects are not visibly reordered. This is not actually detectable without data races, but it is necessary to ensure that data races, as defined below, and with suitable restrictions on the use of atomics, correspond to data races in a simple interleaved (sequentially consistent) execution. — end note ]

13

14

The value of an atomic object M, as determined by evaluation B, shall be the value stored by some side effect A that modifies M, where B does not happen before A . [ Note: The set of such side effects is also restricted by the rest of the rules described here, and in particular, by the coherence requirements below. — end note ] If an operation A that modifies an atomic obj ect M happens before an operation B that modifies M, then A shall be earlier than B in the modification order of M. [ Note: This requirement is known as write-write coherence. — end note ]

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15 If a value computation A of an atomic obj ect M happens before a value computation B of M, and A takes its value from a side effect X on M, then the value computed by B shall either be the value stored by X or the value stored by a side effect Y on M, where Y follows X in the modification order of M. [ Note: This requirement is known as read-read coherence. — end note ] 16 If a value computation A of an atomic obj ect M happens before an operation B that modifies M, then A shall take its value from a side effect X on M, where X precedes B in the modification order of M. [ Note: This requirement is known as read-write coherence. — end note ] 17 If a side effect X on an atomic object M happens before a value computation B of M, then the evaluation B shall take its value from X or from a side effect Y that follows X in the modification order of M. [ Note: This requirement is known as write-read coherence. — end note ] 18 [ Note: The four preceding coherence requirements effectively disallow compiler reordering of atomic operations to a single obj ect, even if both operations are relaxed loads. This effectively makes the cache coherence guarantee provided by most hardware available to C++ atomic operations. — end note ]

19 [ Note: The value observed by a load of an atomic depends on the “happens before” relation, which depends

on the values observed by loads of atomics. The intended reading is that there must exist an association of atomic loads with modifications they observe that, together with suitably chosen modification orders and the “happens before” relation derived as described above, satisfy the resulting constraints as imposed here. — end note ]

20 Two actions are potentially concurrent if (20.1) — they are performed by different threads, or (20.2) — they are unsequenced, at least one is performed by a signal handler, and they are not both performed by the same signal handler invocation.

The execution of a program contains a data race if it contains two potentially concurrent conflicting actions, at least one of which is not atomic, and neither happens before the other, except for the special case for signal handlers described below. Any such data race results in undefined behavior. [ Note: It can be shown that programs that correctly use mutexes and memory_order_seq_cst operations to prevent all data races and use no other synchronization operations behave as if the operations executed by their constituent threads were simply interleaved, with each value computation of an object being taken from the last side effect on that object in that interleaving. This is normally referred to as “sequential consistency”. However, this applies only to data-race-free programs, and data-race-free programs cannot observe most program transformations that do not change single-threaded program semantics. In fact, most single-threaded program transformations continue to be allowed, since any program that behaves differently as a result must perform an undefined operation. — end note ]

21 Two accesses to the same obj ect of type volatile std: : sig_atomic_t do not result in a data race if both occur in the same thread, even if one or more occurs in a signal handler. For each signal handler invocation, evaluations performed by the thread invoking a signal handler can be divided into two groups A and B, such that no evaluations in B happen before evaluations in A , and the evaluations of such volatile std: : sig_atomic_t objects take values as though all evaluations in A happened before the execution of the signal handler and the execution of the signal handler happened before all evaluations in B.

22 [ Note: Compiler transformations that introduce assignments to a potentially shared memory location that

would not be modified by the abstract machine are generally precluded by this document, since such an assignment might overwrite another assignment by a different thread in cases in which an abstract machine execution would not have encountered a data race. This includes implementations of data member assignment that overwrite adjacent members in separate memory locations. Reordering of atomic loads in cases in which the atomics in question may alias is also generally precluded, since this may violate the coherence rules. — end note ]

23 [ Note: Transformations that introduce a speculative read of a potentially shared memory location may not § 4.7.1

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preserve the semantics of the C++ program as defined in this document, since they potentially introduce a data race. However, they are typically valid in the context of an optimizing compiler that targets a specific machine with well-defined semantics for data races. They would be invalid for a hypothetical machine that is not tolerant of races or provides hardware race detection. — end note ] 4. 7. 2

Forward progress

[intro. progress]

1 The implementation may assume that any thread will eventually do one of the following: (1.1) — terminate, (1.2) — make a call to a library I/O function, (1.3) — perform an access through a volatile glvalue, or (1.4) — perform a synchronization operation or an atomic operation.

[ Note: This is intended to allow compiler transformations such as removal of empty loops, even when termination cannot be proven. — end note ]

2 Executions of atomic functions that are either defined to be lock-free (32.8) or indicated as lock-free (32.5) are lock-free executions. (2.1) — If there is only one thread that is not blocked (3.6) in a standard library function, a lock-free execution in that thread shall complete. [ Note: Concurrently executing threads may prevent progress of a lock-free (2.2)

execution. For example, this situation can occur with load-locked store-conditional implementations. This property is sometimes termed obstruction-free. — end note ]

— When one or more lock-free executions run concurrently, at least one should complete. [ Note: It is difficult for some implementations to provide absolute guarantees to this effect, since repeated and particularly inopportune interference from other threads may prevent forward progress, e.g., by repeatedly stealing a cache line for unrelated purposes between load-locked and store-conditional instructions. Implementations should ensure that such effects cannot indefinitely delay progress under expected operating conditions, and that such anomalies can therefore safely be ignored by programmers. Outside this document, this property is sometimes termed lock-free. — end note ]

3 During the execution of a thread of execution, each of the following is termed an execution step: (3.1) — termination of the thread of execution, (3.2) — performing an access through a volatile glvalue, or (3.3) — completion of a call to a library I/O function, a synchronization operation, or an atomic operation. 4 An invocation of a standard library function that blocks (3.6) is considered to continuously execute execution steps while waiting for the condition that it blocks on to be satisfied. [ Example: A library I/O function that

blocks until the I/O operation is complete can be considered to continuously check whether the operation is complete. Each such check might consist of one or more execution steps, for example using observable behavior of the abstract machine. — end example ]

5 [ Note: Because of this and the preceding requirement regarding what threads of execution have to perform

eventually, it follows that no thread of execution can execute forever without an execution step occurring.

— end note ]

6 A thread of execution makes progress when an execution step occurs or a lock-free execution does not complete because there are other concurrent threads that are not blocked in a standard library function (see above) .

7 For a thread of execution providing concurrent forward progress guarantees, the implementation ensures that the thread will eventually make progress for as long as it has not terminated. [ Note: This is required

regardless of whether or not other threads of executions (if any) have been or are making progress. To eventually fulfill this requirement means that this will happen in an unspecified but finite amount of time. — end note ] § 4.7.2

© ISO/IEC 2017 – All rights reserved

19

ISO/IEC 1 4882:201 7(E)

8 It is implementation-defined whether the implementation-created thread of execution that executes main (6.6.1)

and the threads of execution created by std: : thread (33.3.2) provide concurrent forward progress guarantees. [ Note: General-purpose implementations should provide these guarantees. — end note ]

9 For a thread of execution providing parallel forward progress guarantees, the implementation is not required

to ensure that the thread will eventually make progress if it has not yet executed any execution step; once this thread has executed a step, it provides concurrent forward progress guarantees.

10 [ Note: This does not specify a requirement for when to start this thread of execution, which will typically be

specified by the entity that creates this thread of execution. For example, a thread of execution that provides concurrent forward progress guarantees and executes tasks from a set of tasks in an arbitrary order, one after the other, satisfies the requirements of parallel forward progress for these tasks. — end note ]

11 For a thread of execution providing weakly parallel forward progress guarantees, the implementation does not ensure that the thread will eventually make progress.

12 [ Note: Threads of execution providing weakly parallel forward progress guarantees cannot be expected to make progress regardless of whether other threads make progress or not; however, blocking with forward progress guarantee delegation, as defined below, can be used to ensure that such threads of execution make progress eventually. — end note ]

13 Concurrent forward progress guarantees are stronger than parallel forward progress guarantees, which in turn are stronger than weakly parallel forward progress guarantees. [ Note: For example, some kinds of

synchronization between threads of execution may only make progress if the respective threads of execution provide parallel forward progress guarantees, but will fail to make progress under weakly parallel guarantees. — end note ]

14 When a thread of execution P is specified to block with forward progress guarantee delegation on the completion of a set S of threads of execution, then throughout the whole time of P being blocked on S, the implementation shall ensure that the forward progress guarantees provided by at least one thread of execution in S is at least as strong as P’s forward progress guarantees. [ Note: It is unspecified which thread or threads of execution in S are chosen and for which number of execution steps. The strengthening is not permanent and not necessarily in place for the rest of the lifetime of the affected thread of execution. As long as P is blocked, the implementation has to eventually select and potentially strengthen a thread of execution in S. — end note ] Once a thread of execution in S terminates, it is removed from S. Once S is empty, P is unblocked. 15 [ Note: A thread of execution B thus can temporarily provide an effectively stronger forward progress guarantee for a certain amount of time, due to a second thread of execution A being blocked on it with forward progress guarantee delegation. In turn, if B then blocks with forward progress guarantee delegation on C, this may also temporarily provide a stronger forward progress guarantee to C. — end note ] 16 [ Note: If all threads of execution in S finish executing (e.g., they terminate and do not use blocking synchronization incorrectly) , then P ’s execution of the operation that blocks with forward progress guarantee delegation will not result in P’s progress guarantee being effectively weakened. — end note ] 17 [ Note: This does not remove any constraints regarding blocking synchronization for threads of execution providing parallel or weakly parallel forward progress guarantees because the implementation is not required to strengthen a particular thread of execution whose too-weak progress guarantee is preventing overall progress. — end note ]

18 An implementation should ensure that the last value (in modification order) assigned by an atomic or synchronization operation will become visible to all other threads in a finite period of time.

4. 8

Acknowledgments

[intro. ack]

1 The C++ programming language as described in this document is based on the language as described in Chapter R (Reference Manual) of Stroustrup: The C++ Programming Language (second edition, AddisonWesley Publishing Company, ISBN 0-201-53992-6, copyright ©1991 AT&T) . That, in turn, is based on the C § 4.8

© ISO/IEC 2017 – All rights reserved

20

ISO/IEC 14882: 2017(E)

programming language as described in Appendix A of Kernighan and Ritchie: (Prentice-Hall, 1978, ISBN 0-13-110163-3, copyright ©1978 AT&T) . 2

The C Programming Language

Portions of the library Clauses of this document are based on work by P.J. Plauger, which was published as

The Draft Standard C++ Library (Prentice-Hall, ISBN 0-13-117003-1, copyright ©1995 P.J. Plauger) .

3

POSIX® is a registered trademark of the Institute of Electrical and Electronic Engineers, Inc.

4

ECMAScript® is a registered trademark of Ecma International.

5

All rights in these originals are reserved.

§ 4.8

© ISO/IEC 2017 – All rights reserved

21

ISO/IEC 14882: 2017(E)

5

Lexical conventions

5. 1

[lex]

Separate translation

[lex. separate]

1 The text of the program is kept in units called source files in this document. A source file together with

all the headers (20.5.1.2) and source files included (19.2) via the preprocessing directive #include , less any source lines skipped by any of the conditional inclusion (19.1) preprocessing directives, is called a translation unit. [ Note: A C++ program need not all be translated at the same time. — end note ]

2 [ Note: Previously translated translation units and instantiation units can be preserved individually or in

libraries. The separate translation units of a program communicate (6.5) by (for example) calls to functions whose identifiers have external linkage, manipulation of obj ects whose identifiers have external linkage, or manipulation of data files. Translation units can be separately translated and then later linked to produce an executable program (6.5) . — end note ] 5. 2

Phases of translation

1 The precedence among the syntax rules of translation is specified by the following phases. 12

[lex. phases]

1. Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. The set of physical source file characters accepted is implementation-defined. Any source file character not in the basic source character set (5.3) is replaced by the universal-character-name that designates that character. An implementation may use any internal encoding, so long as an actual extended character encountered in the source file, and the same extended character expressed in the source file as a universal-charactername (e.g., using the \uXXXX notation) , are handled equivalently except where this replacement is reverted (5.4) in a raw string literal. 2. Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. Except for splices reverted in a raw string literal, if a splice results in a character sequence that matches the syntax of a universal-character-name , the behavior is undefined. A source file that is not empty and that does not end in a new-line character, or that ends in a new-line character immediately preceded by a backslash character before any such splicing takes place, shall be processed as if an additional new-line character were appended to the file. 3. The source file is decomposed into preprocessing tokens (5.4) and sequences of white-space characters (including comments) . A source file shall not end in a partial preprocessing token or in a partial comment. 13 Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is unspecified. The process of dividing a source file’s characters into preprocessing tokens is context-dependent. [ Example: See the handling of < within a #include preprocessing directive. — end example ] 4. Preprocessing directives are executed, macro invocations are expanded, and _Pragma unary operator expressions are executed. If a character sequence that matches the syntax of a universal-character-name is produced by token concatenation (19.3.3) , the behavior is undefined. A #include preprocessing

12)

Implement at ions must b ehave as if t hese separat e phases o ccur, alt hough in pract ice different phases might b e folded

t oget her.

13)

A part ial prepro cessing t oken would arise from a source file ending in t he first p ort ion of a mult i- charact er t oken t hat

requires a t erminat ing sequence of charact ers, such as a would arise from a source file ending wit h an unclosed

§ 5.2

/*

header-name

t hat is missing t he closing

"

or

>.

A part ial comment

comment .

© ISO/IEC 2017 – All rights reserved

22

ISO/IEC 14882: 2017(E)

directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. All preprocessing directives are then deleted. 5. Each source character set member in a character literal or a string literal, as well as each escape sequence and universal-character-name in a character literal or a non-raw string literal, is converted to the corresponding member of the execution character set (5.13.3, 5.13.5) ; if there is no corresponding member, it is converted to an implementation-defined member other than the null (wide) character. 1 4 6. Adj acent string literal tokens are concatenated. 7. White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token (5.6) . The resulting tokens are syntactically and semantically analyzed and translated as a translation unit. [ Note: The process of analyzing and translating the tokens may occasionally result in one token being replaced by a sequence of other tokens (17.2) . — end note ] [ Note: Source files, translation units and translated translation units need not necessarily be stored as files, nor need there be any one-to-one correspondence between these entities and any external representation. The description is conceptual only, and does not specify any particular implementation. — end note ] 8. Translated translation units and instantiation units are combined as follows: [ Note: Some or all of these may be supplied from a library. — end note ] Each translated translation unit is examined to produce a list of required instantiations. [ Note: This may include instantiations which have been explicitly requested (17.7.2) . — end note ] The definitions of the required templates are located. It is implementation-defined whether the source of the translation units containing these definitions is required to be available. [ Note: An implementation could encode sufficient information into the translated translation unit so as to ensure the source is not required here. — end note ] All the required instantiations are performed to produce instantiation units. [ Note: These are similar to translated translation units, but contain no references to uninstantiated templates and no template definitions. — end note ] The program is ill-formed if any instantiation fails. 9. All external entity references are resolved. Library components are linked to satisfy external references to entities not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment. 5. 3 1

a A 0 _ 2

Character sets

[lex. charset]

The basic source character set consists of 96 characters: the space character, the control characters representing horizontal tab, vertical tab, form feed, and new-line, plus the following 91 graphical characters: 15

The

b B 1 {

c C 2 }

d D 3 [

e E 4 ]

f F 5 #

g G 6 (

h H 7 )

i I 8
% : ; . ? * + - / ^ & | ~ ! = , \ " ’

universal-character-name construct provides a way to name other characters. hex-quad : hexadecimal-digit hexadecimal-digit hexadecimal-digit hexadecimal-digit universal-character-name : \u hex-quad \U hex-quad hex-quad

The character designated by the universal-character-name \UNNNNNNNN is that character whose character short name in ISO/IEC 10646 is NNNNNNNN ; the character designated by the universal-character-name \uNNNN 14) 15)

An implement at ion need not convert all non- corresp onding source charact ers t o t he same execut ion charact er. The glyphs for t he memb ers of t he basic source charact er set are int ended t o ident ify charact ers from t he subset of IS O / IEC

1 0646 which corresp onds t o t he AS C I I charact er set .

However, b ecause t he mapping from source file charact ers t o t he source

charact er set ( describ ed in t ranslat ion phase 1 ) is sp ecified as implement at ion- defined, an implement at ion is required t o do cument how t he basic source charact ers are represent ed in source files.

§ 5.3

© ISO/IEC 2017 – All rights reserved

23

ISO/IEC 14882: 2017(E)

is that character whose character short name in ISO/IEC 10646 is 0000NNNN . If the hexadecimal value for a

universal-character-name corresponds to a surrogate code point (in the range 0xD800–0xDFFF, inclusive) , the program is ill-formed. Additionally, if the hexadecimal value for a universal-character-name outside the c-char-sequence, s-char-sequence, or r-char-sequence of a character or string literal corresponds to a control character (in either of the ranges 0x00–0x1F or 0x7F–0x9F, both inclusive) or to a character in the basic source character set, the program is ill-formed. 16

3

The basic execution character set and the basic execution wide-character set shall each contain all the members of the basic source character set, plus control characters representing alert, backspace, and carriage return, plus a null character (respectively, null wide character ) , whose value is 0. For each basic execution character set, the values of the members shall be non-negative and distinct from one another. In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous. The execution character set and the execution wide-character set are implementation-defined supersets of the basic execution character set and the basic execution wide-character set, respectively. The values of the members of the execution character sets and the sets of additional members are locale-specific. 5. 4

Preprocessing tokens

[lex. pptoken]

preprocessing-token : header-name identifier pp-number character-literal user-defined-character-literal string-literal user-defined-string-literal preprocessing-op-or-punc each non- whit e- space charact er t hat

cannot

b e one of t he ab ove

1

Each preprocessing token that is converted to a token (5.6) shall have the lexical form of a keyword, an identifier, a literal, an operator, or a punctuator.

2

A preprocessing token is the minimal lexical element of the language in translation phases 3 through 6. The categories of preprocessing token are: header names, identifiers, preprocessing numbers, character literals (including user-defined character literals) , string literals (including user-defined string literals) , preprocessing operators and punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories. If a ’ or a " character matches the last category, the behavior is undefined. Preprocessing tokens can be separated by white space; this consists of comments (5.7) , or white-space characters (space, horizontal tab, new-line, vertical tab, and form-feed) , or both. As described in Clause 19, in certain circumstances during translation phase 4, white space (or the absence thereof) serves as more than preprocessing token separation. White space can appear within a preprocessing token only as part of a header name or between the quotation characters in a character literal or string literal.

3

If the input stream has been parsed into preprocessing tokens up to a given character:

(3. 1 )

— If the next character begins a sequence of characters that could be the prefix and initial double quote of a raw string literal, such as R" , the next preprocessing token shall be a raw string literal. Between the initial and final double quote characters of the raw string, any transformations performed in phases 1 and 2 ( universal-character-name s and line splicing) are reverted; this reversion shall apply before any d-char, r-char, or delimiting parenthesis is identified. The raw string literal is defined as the shortest sequence of characters that matches the raw-string pattern encoding-prefixopt R raw-string 16)

A sequence of charact ers resembling a

character-name.

§ 5.4

universal-character-name

in an

r-char-sequence

© ISO/IEC 2017 – All rights reserved

( 5. 1 3. 5 ) do es not form a

universal-

24

ISO/IEC 14882: 2017(E)

(3. 2)

—

O t herwise,

if t he next

t hree charact ers are

except

new- line and

"

q-char-sequence : q-char q-char-sequence q-char q-char: any memb er of t he source charact er set

1

[ Note: Header name preprocessing tokens only appear within a #include preprocessing directive (see 5.4) . — end note ] The sequences in both forms of header-names are mapped in an implementation-defined manner to headers or to external source file names as specified in 19.2.

2

The appearance of either of the characters ’ or \ or of either of the character sequences /* or // in a

q-char-sequence or an h-char-sequence is conditionally-supported with implementation-defined semantics, as is the appearance of the character " in an h-char-sequence . 20 5. 9

Preprocessing numbers pp-number: digit . digit pp-number pp-number pp-number pp-number pp-number pp-number pp-number pp-number pp-number

20)

[lex. ppnumber]

digit identifier-nondigit ’ digit ’ nondigit e sign E sign p sign P sign .

Thus, a sequence of charact ers t hat resembles an escap e sequence might result in an error, b e int erpret ed as t he charact er

corresp onding t o t he escap e sequence, or have a complet ely different meaning, dep ending on t he implement at ion.

§ 5.9

© ISO/IEC 2017 – All rights reserved

26

ISO/IEC 14882: 2017(E)

1

Preprocessing number tokens lexically include all integer literal tokens (5.13.2) and all floating literal tokens (5.13.4) .

2

A preprocessing number does not have a type or a value; it acquires both after a successful conversion to an integer literal token or a floating literal token. 5. 1 0

Identifiers

[lex. name]

identifier: identifier-nondigit identifier identifier-nondigit identifier digit identifier-nondigit : nondigit universal-character-name nondigit : one of a b c d e n o p q r A B C D E N O P Q R digit :

f s F S

g t G T

h u H U

i v I V

j w J W

k x K X

l y L Y

m z M Z _

one of

0 1 2 3 4 5 6 7 8 9 1

An identifier is an arbitrarily long sequence of letters and digits. Each universal-character-name in an identifier shall designate a character whose encoding in ISO 10646 falls into one of the ranges specified in Table 2. The initial element shall not be a universal-character-name designating a character whose encoding falls into one of the ranges specified in Table 3. Upper- and lower-case letters are different. All characters are significant. 21 Table 2 — Ranges of characters allowed

00A8 00B7-00BA 0100-167F 200B-200D 2070-218F 3004-3007 F900-FD3D 10000-1FFFD 60000-6FFFD B0000-BFFFD

00AA 00BC-00BE 1681-180D 202A-202E 2460-24FF 3021-302F FD40-FDCF 20000-2FFFD 70000-7FFFD C0000-CFFFD

00AD 00C0-00D6 180F-1FFF 203F-2040 2776-2793 3031-D7FF FDF0-FE44 30000-3FFFD 80000-8FFFD D0000-DFFFD

00AF 00D8-00F6

00B2-00B5 00F8-00FF

2054 2C00-2DFF

2060-206F 2E80-2FFF

FE47-FFFD 40000-4FFFD 90000-9FFFD E0000-EFFFD

50000-5FFFD A0000-AFFFD

Table 3 — Ranges of characters disallowed initially (combining characters)

0300-036F 21 )

1DC0-1DFF

20D0-20FF

FE20-FE2F

O n syst ems in which linkers cannot accept ext ended charact ers, an enco ding of t he

in forming valid ext ernal ident ifiers. enco de t he

\u

in a

universal-character-name

may b e used

For example, some ot herwise unused charact er or sequence of charact ers may b e used t o

universal-character-name.

Ext ended charact ers may pro duce a long ext ernal ident ifier, but C

place a t ranslat ion limit on significant charact ers for ext ernal ident ifiers.

In C

++ ,

++

do es not

upp er- and lower- case let t ers are considered

different for all ident ifiers, including ext ernal ident ifiers.

§ 5.10

© ISO/IEC 2017 – All rights reserved

27

ISO/IEC 14882: 2017(E)

2 The identifiers in Table 4 have a special meaning when appearing in a certain context. When referred to in the grammar, these identifiers are used explicitly rather than using the identifier grammar production. Unless otherwise specified, any ambiguity as to whether a given identifier has a special meaning is resolved to interpret the token as a regular identifier . Table 4 — Identifiers with special meaning

override

final

3 In addition, some identifiers are reserved for use by C++ implementations and shall not be used otherwise; no diagnostic is required.

(3.1)

— Each identifier that contains a double underscore _ _ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use.

(3.2)

— Each identifier that begins with an underscore is reserved to the implementation for use as a name in the global namespace. 5. 1 1

Keywords

[lex. key]

1 The identifiers shown in Table 5 are reserved for use as keywords (that is, they are unconditionally treated as keywords in phase 7) except in an attribute-token (10.6.1) : Table 5 — Keywords

alignas alignof asm auto bool break case catch char char16_t char32_t class const constexpr const_cast

continue decltype default delete do double dynamic_cast else enum explicit export extern false float for

friend goto if inline int long mutable namespace new noexcept nullptr operator private protected public

register reinterpret_cast return short signed sizeof static static_assert static_cast struct switch template this thread_local throw

true try typedef typeid typename union unsigned using virtual void volatile wchar_t while

[ Note: The export and register keywords are unused but are reserved for future use. — end

note ]

2 Furthermore, the alternative representations shown in Table 6 for certain operators and punctuators (5.5) are reserved and shall not be used otherwise:

Table 6 — Alternative representations

and not_eq

§ 5.11

and_eq or

bitand or_eq

bitor xor

compl xor_eq

© ISO/IEC 2017 – All rights reserved

not

28

ISO/IEC 14882: 2017(E)

1

5. 1 2

Operators and punctuators

++

The lexical rep resent at ion of C

[lex. operators]

programs includes a numb er of prepro cessing t okens which are us ed in t he

s ynt ax of t he prepro cess or or are convert ed int o t okens for op erat ors and punct uat ors:

preprocessing-op-or-punc : one { } [

:: / > > ++ compl

## %: %: .* ^ -= >>= -not

( ;

) :

& *=

is convert ed t o a single t oken in t ranslat ion phase 7 (5 . 2 ) .

Literals

5. 1 3. 1

...

Kinds of literals

There are several kinds of lit erals.

[lex. literal] [lex. literal. kinds]

22

literal : integer-literal character-literal floating-literal string-literal boolean-literal pointer-literal user-defined-literal

5. 1 3. 2

Integer literals

[lex. icon]

integer-literal : binary-literal integer-suffixopt octal-literal integer-suffixopt decimal-literal integer-suffixopt hexadecimal-literal integer-suffixopt binary-literal : 0b binary-digit 0B binary-digit binary-literal ’ opt binary-digit octal-literal : 0 octal-literal ’ opt octal-digit decimal-literal : nonzero-digit decimal-literal ’ opt digit hexadecimal-literal : hexadecimal-prefix hexadecimal-digit-sequence binary-digit : 0 1

22)

The t erm “ lit eral” generally designat es, in t his do cument , t hose t okens t hat are called “ const ant s” in IS O C .

§ 5. 1 3. 2

© IS O / IEC

2 0 1 7 – All right s reserved

29

ISO/IEC 14882: 2017(E)

octal-digit :

one of

0 1 2 3 4 5 6 7

nonzero-digit :

one of

1 2 3 4 5 6 7 8 9

hexadecimal-prefix :

one of

0x 0X

hexadecimal-digit-sequence : hexadecimal-digit hexadecimal-digit-sequence ’ opt hexadecimal-digit hexadecimal-digit :

one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

integer-suffix : unsigned-suffix long-suffixopt unsigned-suffix long-long-suffixopt long-suffix unsigned-suffixopt long-long-suffix unsigned-suffixopt unsigned-suffix :

one of

u U

long-suffix :

one of

l L

long-long-suffix :

one of

ll LL

1

An

integer literal

is a sequence of digit s t hat

has no p erio d or exp onent

quot es t hat are ignored when det ermining it s value. and a s uffix t hat

sp ecifies it s typ e.

binary integer literal integer literal integer literal hexadecimal integer literal

( base two)

digit s,

( base

eight )

( base

t en)

The lexically firs t

b egins wit h

b egins

b egins

wit h

t he

sixt een)

b egins

[

Example:

,

typ e

,

of an

int eger

,

lit eral

is

t he

first

,

and

of t he

wit h opt ional s eparat ing single

of t he sequence of digit s is t he most

significant .

and consis t s of a sequence of binary digit s.

and

cons ist s

0 0x a

t han

wit h

and

or

of a sequence consist s

0X

and

consist s

and

,

,

of o ct al

of a sequence

f A 12 014 0XC 0’ 004’ 000’ 000 t hrough

The numb er twelve can b e writ t en

1048576 1’ 048’ 576 0X100000 0x10’ 0000 The

0

ot her

digit

0B

which include t he decimal digit s and t he let t ers

t hrough fift een.

2

or

digit

wit h a digit

( base

0b

part ,

An int eger lit eral may have a prefix t hat s p ecifies it s base

or

F 0b1100

list

in

Table

7

A

digit s.

A

of hexadecimal

wit h decimal values t en .

The int eger lit erals

all have t he same value.

corresp onding

23

of decimal

of a sequence

t hrough ,

digit s.

A

octal decimal

An

in

which

— end example it s

value

can

]

be

represent ed.

3

If an int eger lit eral cannot represent it s value, are signed, unsigned,

b e rep resent ed by any typ e in it s list

it may have t hat ext ended int eger typ e.

t he ext ended int eger typ e shall b e signed.

cont ains an int eger lit eral t hat

23)

The digit s

§ 5. 1 3. 2

If all of t he typ es in t he list

t he ext ended int eger typ e shall b e unsigned.

t he ext ended int eger typ e may b e signed or unsigned.

8

and

9

cannot

and an ext ended int eger typ e (6 . 9 . 1 )

can

If all of t he typ es in t he list for t he int eger lit eral

If t he list

for t he int eger lit eral are

cont ains b ot h signed and unsigned typ es,

A program is ill- formed if one of it s t ranslat ion unit s

b e represent ed by any of t he allowed typ es.

are not o ct al digit s.

© IS O / IEC

2 0 1 7 – All right s reserved

30

ISO/IEC 14882: 2017(E)

Table 7 — Typ es of int eger lit erals

Suffix

Decimal literal

u

l

or

or

U

ll

5. 1 3. 3

unsigned int unsigned long int unsigned long long int long int long long int

L

B ot h and

Binary, octal, or hexadecimal literal

int long int long long int

none

l

or

u

or

or

U

unsigned long int unsigned long long int long long int

U LL

unsigned long long int

L

LL

u ll

B ot h

or

and

or

int unsigned int long int unsigned long long long int unsigned long unsigned int unsigned long unsigned long long int unsigned long long long int unsigned long unsigned long unsigned long long long int unsigned long unsigned long

int long int int long int int long int int long int long int long int

Character literals

[lex. ccon]

character-literal : encoding-prefixopt ’ c-char-sequence ’ encoding-prefix : u8

u

one of

U

L

c-char-sequence : c-char c-char-sequence c-char c-char: any memb er of t he source charact er set t he single- quot e

’,

backslash

escape-sequence universal-character-name

\,

except

or new- line charact er

escape-sequence : simple-escape-sequence octal-escape-sequence hexadecimal-escape-sequence simple-escape-sequence : \’ \a

\" \b

\? \f

one of

\\ \n

\r

\t

\v

octal-escape-sequence : \ octal-digit \ octal-digit octal-digit \ octal-digit octal-digit octal-digit hexadecimal-escape-sequence : \x hexadecimal-digit hexadecimal-escape-sequence hexadecimal-digit § 5. 1 3. 3

© IS O / IEC

2 0 1 7 – All right s reserved

31

ISO/IEC 14882: 2017(E)

1

A character literal is one or more characters enclosed in single quotes, as in ’ x’ , optionally preceded by u8 , u, U, or L , as in u8’ w’ , u’ x’ , U’ y’ , or L’ z’ , respectively.

2

A character literal that does not begin with u8 , u, U, or L is an ordinary character literal . An ordinary character literal that contains a single c-char representable in the execution character set has type char , with value equal to the numerical value of the encoding of the c-char in the execution character set. An ordinary character literal that contains more than one c-char is a multicharacter literal. A multicharacter literal, or an ordinary character literal containing a single c-char not representable in the execution character set, is conditionally-supported, has type int , and has an implementation-defined value.

3

A character literal that begins with u8 , such as u8’ w’ , is a character literal of type char , known as a UTF-8 character literal. The value of a UTF-8 character literal is equal to its ISO 10646 code point value, provided

that the code point value is representable with a single UTF-8 code unit (that is, provided it is in the C0 Controls and Basic Latin Unicode block) . If the value is not representable with a single UTF-8 code unit, the program is ill-formed. A UTF-8 character literal containing multiple c-char s is ill-formed.

4

A character literal that begins with the letter u, such as u’ x’ , is a character literal of type char16_t . The value of a char16_t character literal containing a single c-char is equal to its ISO 10646 code point value, provided that the code point is representable with a single 16-bit code unit. (That is, provided it is a basic multi-lingual plane code point.) If the value is not representable within 16 bits, the program is ill-formed. A char16_t character literal containing multiple c-chars is ill-formed.

5

A character literal that begins with the letter U, such as U’ y’ , is a character literal of type char32_t . The value of a char32_t character literal containing a single c-char is equal to its ISO 10646 code point value. A char32_t character literal containing multiple c-chars is ill-formed.

6

A character literal that begins with the letter L , such as L’ z’ , is a wide-character literal . A wide-character literal has type wchar_t . 24 The value of a wide-character literal containing a single c-char has value equal to the numerical value of the encoding of the c-char in the execution wide-character set, unless the c-char has no representation in the execution wide-character set, in which case the value is implementation-defined. [ Note: The type wchar_t is able to represent all members of the execution wide-character set (see 6.9.1) . — end note ] The value of a wide-character literal containing multiple c-chars is implementation-defined.

7

Certain non-graphic characters, the single quote ’ , the double quote " , the question mark ? , 25 and the backslash \ , can be represented according to Table 8. The double quote " and the question mark ? , can be represented as themselves or by the escape sequences \" and \? respectively, but the single quote ’ and the backslash \ shall be represented by the escape sequences \’ and \\ respectively. Escape sequences in which the character following the backslash is not listed in Table 8 are conditionally-supported, with implementation-defined semantics. An escape sequence specifies a single character.

8

The escape \ ooo consists of the backslash followed by one, two, or three octal digits that are taken to specify the value of the desired character. The escape \xhhh consists of the backslash followed by x followed by one or more hexadecimal digits that are taken to specify the value of the desired character. There is no limit to the number of digits in a hexadecimal sequence. A sequence of octal or hexadecimal digits is terminated by the first character that is not an octal digit or a hexadecimal digit, respectively. The value of a character literal is implementation-defined if it falls outside of the implementation-defined range defined for char (for character literals with no prefix) or wchar_t (for character literals prefixed by L ) . [ Note: If the value of a character literal prefixed by u, u8 , or U is outside the range defined for its type, the program is ill-formed. — end note ]

9

A universal-character-name is translated to the encoding, in the appropriate execution character set, of the character named. If there is no such encoding, the universal-character-name is translated to an implementationdefined encoding. [ Note: In translation phase 1, a universal-character-name is introduced whenever an actual 24) 25)

They are int ended for charact er set s where a charact er do es not fit int o a single byt e. Using an escap e sequence for a quest ion mark is supp ort ed for compat ibility wit h IS O C

§ 5.13.3

© ISO/IEC 2017 – All rights reserved

++

201 4 and IS O C .

32

ISO/IEC 14882: 2017(E)

Table 8 — Escape sequences new-line horizontal tab vertical tab backspace carriage return form feed alert backslash question mark single quote double quote octal number hex number

NL(LF) HT VT BS CR FF BEL \ ?

’ " ooo hhh

\n \t \v \b \r \f \a \\ \? \’ \" \ ooo \xhhh

extended character is encountered in the source text. Therefore, all extended characters are described in terms of universal-character-name s. However, the actual compiler implementation may use its own native character set, so long as the same results are obtained. — end note ] 5. 1 3. 4

Floating literals

[lex. fcon]

floating-literal : decimal-floating-literal hexadecimal-floating-literal decimal-floating-literal : fractional-constant exponent-partopt floating-suffixopt digit-sequence exponent-part floating-suffixopt hexadecimal-floating-literal : hexadecimal-prefix hexadecimal-fractional-constant binary-exponent-part floating-suffixopt hexadecimal-prefix hexadecimal-digit-sequence binary-exponent-part floating-suffixopt fractional-constant : digit-sequenceopt . digit-sequence digit-sequence . hexadecimal-fractional-constant : hexadecimal-digit-sequenceopt . hexadecimal-digit-sequence hexadecimal-digit-sequence . exponent-part : e signopt digit-sequence E signopt digit-sequence binary-exponent-part : p signopt digit-sequence P signopt digit-sequence sign :

one of

+ -

digit-sequence : digit digit-sequence ’ opt digit

§ 5.13.4

© ISO/IEC 2017 – All rights reserved

33

ISO/IEC 14882: 2017(E)

floating-suffix :

one of

f l F L 1

A floating literal consists of an optional prefix specifying a base, an integer part, a radix point, a fraction part, an e , E, p or P , an optionally signed integer exponent, and an optional type suffix. The integer and fraction parts both consist of a sequence of decimal (base ten) digits if there is no prefix, or hexadecimal (base sixteen) digits if the prefix is 0x or 0X . The floating literal is a decimal floating literal in the former case and a hexadecimal floating literal in the latter case. Optional separating single quotes in a digit-sequence or hexadecimal-digit-sequence are ignored when determining its value. [ Example: The floating literals 1. 602’ 176’ 565e-19 and 1. 602176565e-19 have the same value. — end example ] Either the integer part or the fraction part (not both) can be omitted. Either the radix point or the letter e or E and the exponent (not both) can be omitted from a decimal floating literal. The radix point (but not the exponent) can be omitted from a hexadecimal floating literal. The integer part, the optional radix point, and the optional fraction part, form the significand of the floating literal. In a decimal floating literal, the exponent, if present, indicates the power of 10 by which the significand is to be scaled. In a hexadecimal floating literal, the exponent indicates the power of 2 by which the significand is to be scaled. [ Example: The floating literals 49. 625 and 0xC. 68p+2 have the same value. — end example ] If the scaled value is in the range of representable values for its type, the result is the scaled value if representable, else the larger or smaller representable value nearest the scaled value, chosen in an implementation-defined manner. The type of a floating literal is double unless explicitly specified by a suffix. The suffixes f and F specify float , the suffixes l and L specify long double . If the scaled value is not in the range of representable values for its type, the program is ill-formed. 5. 1 3. 5

String literals

[lex. string]

string-literal : encoding-prefixopt " s-char-sequenceopt " encoding-prefixopt R raw-string s-char-sequence : s-char s-char-sequence s-char s-char: any memb er of t he source charact er set t he double- quot e

",

backslash

escape-sequence universal-character-name

\,

except

or new- line charact er

raw-string : " d-char-sequenceopt ( r-char-sequenceopt ) d-char-sequenceopt " r-char-sequence : r-char r-char-sequence r-char r-char: any memb er of t he source charact er set , a right

parent hesis

)

except

followed by t he init ial

( which may b e empty)

d-char-sequence

followed by a double quot e

".

d-char-sequence : d-char d-char-sequence d-char d-char: any memb er of t he basic source charact er set space,

t he left

parent hesis

(,

t he right

except :

parent hesis

),

t he backslash

\,

and t he cont rol charact ers represent ing horiz ont al t ab, vert ical t ab,

§ 5.13.5

form feed,

and newline.

© ISO/IEC 2017 – All rights reserved

34

ISO/IEC 14882: 2017(E)

1

string-literal

A

is

a sequence of charact ers

( as

defined in 5 . 1 3 . 3 )

surrounded by double quot es,

opt ionally

R u8 u8R u uR U UR L LR " . . . " R" (. . . ) " u8" . . . " u8R" **(. . . ) **" u" . . . " uR" * ~ (. . . ) * ~ " U" . . . " UR" zzz(. . . ) zzz" L" . . . " LR" (. . . ) "

prefixed

by

,

,

,

,

2

,

,

,

as

in

,

,

,

or

,

in t he prefix is a of a

shall consist

The charact ers

is equivalent

[

or

,

t hat has an

A

4

,

,

,

,

resp ect ively.

string-literal R raw string literal d-char-sequence d-char-sequence raw-string d-char-sequence d-char-sequence Note: ’ (’ ’)’ raw-string R" delimiter((a| b) ) delimiter" " (a| b) " — end note Note:

A

[

,

,

t erminat ing

3

,

A

lit eral.

of at

and

to

.

The

s erves as a delimit er.

The

is t he same s equence of charact ers as t he init ial

most

are p ermit t ed in a

.

.

1 6 charact ers.

.

Thus,

]

source- file new- line

in a raw st ring lit eral result s

Assuming no whit espace at

in a new- line

in t he

res ult ing execut ion st ring

t he b eginning of lines in t he following example,

t he assert

will succeed:

const char* p = R" (a\ b c) " ; assert(std: : strcmp(p, " a\\\nb\nc" ) == 0) ;

5

— end note Example:

[

]

The raw st ring

R" a( )\ a" ) a" is equivalent

to

" \n) \\\na\" \n"

.

The raw st ring

R" (x = " \" y\" " ) " is equivalent

6

8

" x = \" \\\" y\\\" \" " — end example .

Aft er t rans lat ion phase 6 ,

literal string-literal ,

7

to

string-literal

a

t hat

do es not

]

b egin wit h an

encoding-prefix

is an

ordinary string

and is init ializ ed wit h t he given charact ers.

A

O rdinary

t hat

u8

b egins wit h

st ring lit erals

and

,

such as

UTF - 8

s t ring lit eral has typ e “ array of

u8" asdf"

st ring lit erals

n const char

”,

,

are

where

is a

also

n

UTF-8 string literal referred

to

as

.

narrow s t ring lit erals.

A

is t he siz e of t he st ring as defined b elow,

narrow and has

s t at ic st orage durat ion ( 6 . 7) .

9

For a UTF - 8

st ring lit eral,

corresp onding co de unit

10

A

string-literal

has

t hat

typ e “ array of

each successive element

of t he ob j ect

represent at ion (6 . 9 )

has t he value of t he

of t he UTF - 8 enco ding of t he st ring.

b egins wit h

u

,

such as

n const char16_t c-char ”,

wit h t he given charact ers.

u" asdf"

where

A single

n

,

is

is a

char16_t

t he s iz e of t he

st ring lit eral. st ring as

may pro duce more t han one

A

char16_t

defined b elow;

char16_t

it

st ring lit eral is

init ializ ed

charact er in t he form of

s urrogat e pairs .

11

A

string-literal

has

t hat

typ e “ array of

b egins wit h

U

,

such as

n const char32_t

”,

U" asdf"

where

n

,

is

is a

char32_t

t he s iz e of t he

s t ring lit eral. st ring as

A

char32_t

defined b elow;

it

st ring lit eral is

init ializ ed

wit h t he given charact ers.

12

A

string-literal n const wchar_t t hat

b egins wit h

“ array of

”,

L

,

such as

n

where

L" asdf"

,

is a

wide string literal

.

A wide st ring lit eral has typ e

is t he s iz e of t he st ring as defined b elow;

it is init ializ ed wit h t he given

charact ers.

13

In t ranslat ion phase 6 ( 5 . 2 ) ,

encoding-prefix encoding-prefix ,

no

UTF - 8

§ 5. 1 3. 5

string-literal

adj acent

string-literal encoding-prefix string-literal encoding-prefix

s are concat enat ed.

If b ot h

t he result ing concat enat ed st ring lit eral has t hat ,

it

is

t reat ed

as

a

string-literal

st ring lit eral t oken is adj acent

of t he

same

t o a wide st ring lit eral t oken,

© IS O / IEC

s have t he same

.

as

If one

t he

ot her

op erand.

t he program is ill- formed.

2 0 1 7 – All right s reserved

has If a

Any ot her

35

ISO/IEC 14882: 2017(E)

concat enat ions are condit ionally- supp ort ed wit h implement at ion- defined b ehavior. is

an

int erpret at ion,

not

a

conversion.

B ecause

t he

int erpret at ion

happ ens

in

[

Note:

This concat enat ion

t ranslat ion

phase

6

( aft er

each charact er from a st ring lit eral has b een t ranslat ed int o a value from t he appropriat e charact er set ) ,

string-literal — end note ]

’s

init ial

rawness

has

no

effect

on

t he

int erpret at ion

or

well- formedness

of t he

a

concat enat ion.

Tab le 9 has some examples of valid concat enat ions.

Table 9 — S t ring lit eral concat enat ions

S ource

u" a" u" a" " a"

Means

u" b" " b" u" b"

S ource

U" a" U" a" " a"

u" ab" u" ab" u" ab"

C haract ers in concat enat ed st rings are kept

[

Means

U" b" " b" U" b"

S ource

L" a" L" a" " a"

U" ab" U" ab" U" ab"

Means

L" b" " b" L" b"

L" ab" L" ab" L" ab"

dist inct .

Example: " \xA" " B"

’ \xA’

cont ains t he two charact ers

’ \xAB’ 14

— end example

and

p rograms t hat

Escap e

sequences

a

\’ char16_t

,

aft er concat enat ion ( and not

and

in t ranslat ion phase 7 (5 . 2 ) ,

universal-character-name except

t hat

and t he double quot e

"

t erminat ing

char

element

pair,

U’ \0’

or

L’ \0’

universal-character-name

s equences, plus

one

co de unit s,

in

non- raw

not

for

t he

t erminat ing

.

16

s,

\

siz e

.

t he

given

a

charact ers

is unsp ecified.

5. 1 3. 6

[

sp ecified

in

a

of a

and except

char16_t

Note: — end note [

The

s t ring

ab ove.

,

.

lit eral

Whet her

The effect

t hat a

]

to

ob j ect all

s,

s ame

meaning

as

in

in

a

char32_t

or wide st ring

and ot her charact ers,

lit eral

char16_t char32_t 0x10FFFF of a

Wit hin

0x0

t he

universal-character-name universal-character-name

The siz e of a

st ring

is

t he

.

plus at

’ \0’

t ot al

numb er

plus one of escap e

lit eral

char16_t

is

t he

numb er

st ring lit erals,

of

any

The siz e of a narrow st ring lit eral is

least

one for t he mult ibyt e enco ding of

.

wit h

st ring

st ring

and

st at ic

lit erals

and whet her success ive evaluat ions of a

Note:

have

plus one for each charact er requiring a surrogat e

s iz e

plus one for t he t erminat ing

result s

as

nonoverlapp ing ob j ect s) ob j ect

,

lit erals

multibyte encoding universal-character-name

and ot her charact ers,

u’ \0’

st ring

In a narrow st ring lit eral,

s shall b e wit hin t he range

universal-character-name string-literal

Evaluat ing

is app ended t o every st ring lit eral so

is represent able eit her by it s elf or by t he escap e

t he t ot al numb er of escap e sequences and ot her charact ers, each

’ \0’

due t o

The

t he numb er of charact ers.

universal-character-name

’

shall b e preceded by a

lit eral is t he t ot al numb er of escap e sequences, t he

s

t he single quot e

st ring lit eral may yield a surrogat e pair.

may map t o more t han one

for

t he single hexadecimal charact er

scan a st ring can find it s end.

charact er lit erals ( 5 . 1 3 . 3 ) , s equence

’ B’

]

Aft er any necessary concat enat ion, t hat

15

).

st orage

are

durat ion,

dis t inct

string-literal

( t hat

init ializ ed is,

from

st ored

in

yield t he same or a different

of at t empt ing t o mo dify a st ring lit eral is undefined.

Boolean literals

are

— end note

]

[lex. bool]

boolean-literal : false true 1

The B o olean lit erals are t he keywords

5. 1 3. 7

false

and

true

.

S uch lit erals are prvalues and have typ e

Pointer literals

bool

.

[lex. nullptr]

pointer-literal : nullptr

§ 5. 1 3. 7

© IS O / IEC

2 0 1 7 – All right s reserved

36

ISO/IEC 14882: 2017(E)

1

The pointer literal is the keyword nullptr . It is a prvalue of type std: : nullptr_t . [ Note: std: : nullptr_t is a distinct type that is neither a pointer type nor a pointer to member type; rather, a prvalue of this type is a null pointer constant and can be converted to a null pointer value or null member pointer value. See 7.11 and 7.12. — end note ] 5. 1 3. 8

User-defined literals

[lex. ext]

user-defined-literal : user-defined-integer-literal user-defined-floating-literal user-defined-string-literal user-defined-character-literal user-defined-integer-literal : decimal-literal ud-suffix octal-literal ud-suffix hexadecimal-literal ud-suffix binary-literal ud-suffix user-defined-floating-literal : fractional-constant exponent-partopt ud-suffix digit-sequence exponent-part ud-suffix hexadecimal-prefix hexadecimal-fractional-constant binary-exponent-part ud-suffix hexadecimal-prefix hexadecimal-digit-sequence binary-exponent-part ud-suffix user-defined-string-literal : string-literal ud-suffix user-defined-character-literal : character-literal ud-suffix ud-suffix : identifier 1

If a token matches both user-defined-literal and another literal kind, it is treated as the latter. [ Example: 123_km is a user-defined-literal , but 12LL is an integer-literal . — end example ] The syntactic non-terminal preceding the ud-suffix in a user-defined-literal is taken to be the longest sequence of characters that could match that non-terminal.

2

A user-defined-literal is treated as a call to a literal operator or literal operator template (16.5.8) . To determine the form of this call for a given user-defined-literal L with ud-suffix X, the literal-operator-id whose literal suffix identifier is X is looked up in the context of L using the rules for unqualified name lookup (6.4.1) . Let S be the set of declarations found by this lookup. S shall not be empty.

3

If L is a user-defined-integer-literal , let n be the literal without its ud-suffix . If S contains a literal operator with parameter type unsigned long long, the literal L is treated as a call of the form operator " "

X ( n ULL)

Otherwise, S shall contain a raw literal operator or a literal operator template (16.5.8) but not both. If contains a raw literal operator, the literal L is treated as a call of the form operator " "

X (" n " )

Otherwise ( S contains a literal operator template) , operator " "

S

X ()

where n is the source character sequence c1 c2 ...ck . [ Note: The sequence c1 c2 ...ck can only contain characters from the basic source character set. — end note ]

§ 5.13.8

© ISO/IEC 2017 – All rights reserved

37

ISO/IEC 14882: 2017(E)

4

If

L

is a

user-defined-floating-literal

wit h paramet er typ e

S

f

literal L

t he

If

L

is a

ud-suffix

.

If

S

cont ains a lit eral op erat or

b ut

not

b ot h.

If

S

is t reat ed as a call of t he form

X ()

from t he basic source charact er set .

5

it s

is t reat ed as a call of t he form

cont ains a lit eral op erat or t emplat e) ,

operator " " where

L

X (" f " )

operator " "

S

b e t he lit eral wit hout

shall cont ain a raw lit eral op erat or or a lit eral op erat or t emplat e (1 6 . 5 . 8 )

cont ains a raw lit eral op erat or,

O t herwise (

f

let

t he lit eral

X ( f L)

operator " " O t herwise,

,

long double ,

let

[

Note:

The sequence

c1 c2 ...ck

can only cont ain charact ers

]

b e t he lit eral wit hout

it s

ud-suffix

it s lengt h excluding t he t erminat ing null charact er) .

and let

len L

The lit eral

b e t he numb er of is t reat ed as a call

of t he form

operator " " 6

If

L

is

a

X ( str , len )

user-defined-character-literal

op erat or ( 1 6 . 5 . 8 )

operator " " 7

[

,

let

ch

be

t he

lit eral

whose only paramet er has t he typ e of

ch

wit hout

ud-suffix S L

it s

.

and t he lit eral

shall cont ain a lit eral

is t reat ed as a call of t he form

X ( ch )

Example: long double operator " " _w(long double) ; std: : string operator " " _w(const char16_t*, std: : size_t) ; unsigned operator " " _w(const char*) ; int main() { 1. 2_w; // calls operator " " _w(1. 2L) u" one" _w; // calls operator " " _w(u" one" , 3) 12_w; // calls operator " " _w(" 12" ) " two" _w; // error: no applicable literal operator }

— end example 8

In

t ranslat ion

]

phas e

6

(5. 2) ,

adj acent

st ring

considered st ring lit erals for t hat purp ose.

lit erals

concat enat ion pro ces s o ccurs as describ ed in 5 . 1 3 . 5 . concat enat ion involving at leas t one s hall have t he same

9

[

ud-suffix

are

concat enat ed

D uring concat enat ion, At

,

user-defined-string-literal

are

if a st ring lit eral is t he result of a

all t he part icipat ing

suffix is applied t o t he res ult

s

es are removed and ignored and t he

t he end of phase 6 ,

user-defined-string-literal

and t hat

and

ud-suffix

user-defined-string-literal

s

of t he concat enat ion.

Example: int main() { L" A" " B" " C" _x; // OK: same as L" ABC" _x " P" _x " Q" " R" _y; // error: two different ud-suffixes }

— end example

§ 5. 1 3. 8

]

© IS O / IEC

2 0 1 7 – All right s reserved

38

ISO/IEC 1 4882:201 7(E)

6

Basic concepts

[basic]

1 [ Note: This Clause presents the basic concepts of the C++ language. It explains the difference between an

obj ect and a name and how they relate to the value categories for expressions. It introduces the concepts of a declaration and a definition and presents C++ ’s notion of type, scope, linkage, and storage duration. The mechanisms for starting and terminating a program are discussed. Finally, this Clause presents the fundamental types of the language and lists the ways of constructing compound types from these. — end note ]

2 [ Note: This Clause does not cover concepts that affect only a single part of the language. Such concepts are discussed in the relevant Clauses. — end note ] 3 An entity is a value, object, reference, function, enumerator, type, class member, bit-field, template, template specialization, namespace, or parameter pack.

4 A name is a use of an identifier (5.10) , operator-function-id (16.5) , literal-operator-id (16.5.8) , conversionfunction-id (15.3.2) , or template-id (17.2) that denotes an entity or label (9.6.4, 9.1) . 5 Every name that denotes an entity is introduced by a declaration. Every name that denotes a label is introduced either by a goto statement (9.6.4) or a labeled-statement (9.1) . 6 A variable is introduced by the declaration of a reference other than a non-static data member or of an obj ect. The variable’s name, if any, denotes the reference or obj ect.

7 Some names denote types or templates. In general, whenever a name is encountered it is necessary to determine whether that name denotes one of these entities before continuing to parse the program that contains it. The process that determines this is called name lookup (6.4) .

8 Two names are the same if (8.1) — they are identifier s composed of the same character sequence, or (8.2) — they are operator-function-id s formed with the same operator, or (8.3) — they are conversion-function-id s formed with the same type, or (8.4) — they are template-id s that refer to the same class, function, or variable (17.4) , or (8.5) — they are the names of literal operators (16.5.8) formed with the same literal suffix identifier. 9 A name used in more than one translation unit can potentially refer to the same entity in these translation units depending on the linkage (6.5) of the name specified in each translation unit. 6. 1

Declarations and definitions

[basic. def]

1 A declaration (Clause 10) may introduce one or more names into a translation unit or redeclare names

introduced by previous declarations. If so, the declaration specifies the interpretation and attributes of these names. A declaration may also have effects including:

(1.1) — a static assertion (Clause 10) , (1.2) — controlling template instantiation (17.7.2) , (1.3) — guiding template argument deduction for constructors (17.9) , (1.4) — use of attributes (Clause 10) , and (1.5) — nothing (in the case of an empty-declaration ) . 2 A declaration is a definition unless § 6.1

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(2. 1)

— it declares a function without specifying the function’s body (11.4) ,

(2. 2)

— it contains the extern specifier (10.1.1) or a a function-body,

(2. 3)

— it declares a non-inline static data member in a class definition (12.2, 12.2.3) ,

(2. 4)

— it declares a static data member outside a class definition and the variable was defined within the class with the constexpr specifier (this usage is deprecated; see D.1) ,

(2. 5)

— it is a class name declaration (12.1) ,

(2. 6)

— it is an

(2. 7) (2. 8)

(2. 9)

linkage-specification 26 (10.5) and neither an initializer nor

opaque-enum-declaration (10.2) , — it is a template-parameter (17.1) , — it is a parameter-declaration (11.3.5) in a function declarator that is not the declarator of a functiondefinition, — it is a typedef declaration (10.1.3) ,

alias-declaration (10.1.3) , a using-declaration (10.3.3) , a deduction-guide (17.9) , a static_assert-declaration (Clause 10) , an attribute-declaration (Clause 10) , an empty-declaration (Clause 10) , a using-directive (10.3.4) ,

(2. 10)

— it is an

(2. 11 )

— it is

(2. 12)

— it is

(2. 13)

— it is

(2. 14)

— it is

(2. 15)

— it is

(2. 16)

— it is

(2. 17)

— it is an explicit instantiation declaration (17.7.2) , or

(2. 18)

— it is an explicit specialization (17.7.3) whose

declaration is not a definition.

[ Example: All but one of the following are definitions: // defines a // defines c // defines f and defines x // defines S , S: : a, and S: : b // defines X // defines non-static data member x // declares static data member y // defines a constructor of X

int a; extern const int c = 1; int f(int x) { return x+a; } struct S { int a; int b; }; struct X { int x; static int y; X() : x(0) { } }; int X: : y = 1; enum { up, down }; namespace N { int d; } namespace N1 = N; X anX;

// defines // defines // defines // defines // defines

X: : y up and down N and N: : d N1 anX

whereas these are j ust declarations: // declares // declares // declares // declares // declares

extern int a; extern const int c; int f(int) ; struct S; typedef int Int; 26)

App earing inside t he braced- enclosed

a c f S Int

declaration-seq

in a

linkage-specification

do es not affect whet her a declarat ion is a

definit ion.

§ 6.1

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extern X anotherX; using N: : d;

// declares anotherX // declares d

— end example ] 3 [ Note: In some circumstances, C++ implementations implicitly define the default constructor (15.1) , copy

constructor (15.8) , move constructor (15.8) , copy assignment operator (15.8) , move assignment operator (15.8) , or destructor (15.4) member functions. — end note ] [ Example: Given #include struct C { std: : string s; }; int C C b }

// std: : string is the standard library class (Clause 24)

main() { a; b = a; = a;

the implementation will implicitly define functions to make the definition of C equivalent to struct C { std: : string s; C() : s() { } C(const C& x) : s(x. s) { } C(C&& x) : s(static_cast(x. s) ) { } // : s(std: : move(x. s) ) { } C& operator=(const C& x) { s = x. s; return *this; } C& operator=(C&& x) { s = static_cast(x. s) ; return *this; } // { s = std: : move(x. s) ; return *this; } ~ C() { } };

— end example ] 4 [ Note: A class name can also be implicitly declared by an elaborated-type-specifier (10.1.7.3) . — end note ] 5 A program is ill-formed if the definition of any obj ect gives the obj ect an incomplete type (6.9) . 6. 2

One-definition rule

[basic. def. odr]

1 No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template.

2 An expression is potentially evaluated unless it is an unevaluated operand (Clause 8) or a subexpression thereof. The set of potential results of an expression e is defined as follows: (2.1) — If e is an id-expression (8.1.4) , the set contains only e . (2.2) — If e is a subscripting operation (8.2.1) with an array operand, the set contains the potential results of that operand.

(2.3)

— If e is a class member access expression (8.2.5) , the set contains the potential results of the obj ect expression.

(2.4)

— If e is a pointer-to-member expression (8.5) whose second operand is a constant expression, the set contains the potential results of the obj ect expression.

(2.5) (2.6)

— If e has the form (e1) , the set contains the potential results of e1 . — If e is a glvalue conditional expression (8.16) , the set is the union of the sets of potential results of the § 6.2

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second and third operands. (2. 7)

— If e is a comma expression (8.19) , the set contains the potential results of the right operand.

(2. 8)

— Otherwise, the set is empty. [ Note: This set is a (possibly-empty) set of id-expression s, each of which is either e or a subexpression of e . [ Example: In the following example, the set of potential results of the initializer of n contains the first S: : x subexpression, but not the second S: : x subexpression. struct S { static const int x = 0; }; const int &f(const int &r) ; int n = b ? (1, S: : x) // S: : x is not odr-used here : f(S: : x) ; // S: : x is odr-used here, so a definition is required

— end example ] — end note ] 3

A variable x whose name appears as a potentially-evaluated expression ex is odr-used by ex unless applying the lvalue-to-rvalue conversion (7.1) to x yields a constant expression (8.20) that does not invoke any non-trivial functions and, if x is an obj ect, ex is an element of the set of potential results of an expression e , where either the lvalue-to-rvalue conversion (7.1) is applied to e , or e is a discarded-value expression (Clause 8) . this is odr-used if it appears as a potentially-evaluated expression (including as the result of the implicit transformation in the body of a non-static member function (12.2.2) ) . A virtual member function is odr-used if it is not pure. A function whose name appears as a potentially-evaluated expression is odr-used if it is the unique lookup result or the selected member of a set of overloaded functions (6.4, 16.3, 16.4) , unless it is a pure virtual function and either its name is not explicitly qualified or the expression forms a pointer to member (8.3.1) . [ Note: This covers calls to named functions (8.2.2) , operator overloading (Clause 16) , user-defined conversions (15.3.2) , allocation functions for placement new-expression s (8.3.4) , as well as non-default initialization (11.6) . A constructor selected to copy or move an obj ect of class type is odr-used even if the call is actually elided by the implementation (15.8) . — end note ] An allocation or deallocation function for a class is odr-used by a new-expression appearing in a potentially-evaluated expression as specified in 8.3.4 and 15.5. A deallocation function for a class is odr-used by a delete expression appearing in a potentially-evaluated expression as specified in 8.3.5 and 15.5. A non-placement allocation or deallocation function for a class is odr-used by the definition of a constructor of that class. A non-placement deallocation function for a class is odr-used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor (15.4) . 27 An assignment operator function in a class is odr-used by an implicitly-defined copy-assignment or move-assignment function for another class as specified in 15.8. A constructor for a class is odr-used as specified in 11.6. A destructor for a class is odr-used if it is potentially invoked (15.4) .

4

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (9.4.1) ; no diagnostic required. The definition can appear explicitly in the program, it can be found in the standard or a user-defined library, or (when appropriate) it is implicitly defined (see 15.1, 15.4 and 15.8) . An inline function or variable shall be defined in every translation unit in which it is odr-used outside of a discarded statement.

5

Exactly one definition of a class is required in a translation unit if the class is used in a way that requires the class type to be complete. [ Example: The following complete translation unit is well-formed, even though it never defines X: struct X; struct X* x1; X* x2;

// declare X as a struct type // use X in pointer formation // use X in pointer formation

— end example ] [ Note: The rules for declarations and expressions describe in which contexts complete class 27)

An implement at ion is not required t o call allo cat ion and deallo cat ion funct ions from const ruct ors or dest ruct ors; however,

t his is a p ermissible implement at ion t echnique.

§ 6.2

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types are required. A class type T must be complete if: (5. 1)

— an obj ect of type T is defined (6.1) , or

(5. 2)

— a non-static class data member of type T is declared (12.2) , or

(5. 3)

— T is used as the allocated type or array element type in a

(5. 4)

— an lvalue-to-rvalue conversion is applied to a glvalue referring to an obj ect of type T (7.1) , or

(5. 5)

— an expression is converted (either implicitly or explicitly) to type T (Clause 7, 8.2.3, 8.2.7, 8.2.9, 8.4) , or

(5. 6)

— an expression that is not a null pointer constant, and has type other than cv void* , is converted to the type pointer to T or reference to T using a standard conversion (Clause 7) , a dynamic_cast (8.2.7) or a static_cast (8.2.9) , or

(5. 7)

— a class member access operator is applied to an expression of type T (8.2.5) , or

(5. 8)

— the typeid operator (8.2.8) or the sizeof operator (8.3.3) is applied to an operand of type T, or

(5. 9)

— a function with a return type or argument type of type T is defined (6.1) or called (8.2.2) , or

new-expression (8.3.4) , or

(5. 10)

— a class with a base class of type T is defined (Clause 13) , or

(5. 11 )

— an lvalue of type T is assigned to (8.18) , or

(5. 12)

— the type T is the subj ect of an alignof expression (8.3.6) , or

(5. 13)

6

exception-declaration has type T, reference to T, or pointer to T (18.3) . — end note ] — an

There can be more than one definition of a class type (Clause 12) , enumeration type (10.2) , inline function with external linkage (10.1.6) , inline variable with external linkage (10.1.6) , class template (Clause 17) , non-static function template (17.5.6) , static data member of a class template (17.5.1.3) , member function of a class template (17.5.1.1) , or template specialization for which some template parameters are not specified (17.7, 17.5.5) in a program provided that each definition appears in a different translation unit, and provided the definitions satisfy the following requirements. Given such an entity named D defined in more than one translation unit, then

(6. 1 )

— each definition of D shall consist of the same sequence of tokens; and

(6. 2)

— in each definition of D , corresponding names, looked up according to 6.4, shall refer to an entity defined within the definition of D , or shall refer to the same entity, after overload resolution (16.3) and after matching of partial template specialization (17.8.3) , except that a name can refer to — a non-volatile const obj ect with internal or no linkage if the obj ect

(6. 2. 1 ) (6. 2. 1. 1)

— has the same literal type in all definitions of D ,

(6. 2. 1. 2)

— is initialized with a constant expression (8.20) ,

(6. 2. 1. 3)

— is not odr-used in any definition of D , and

(6. 2. 1. 4)

— has the same value in all definitions of D , or — a reference with internal or no linkage initialized with a constant expression such that the reference refers to the same entity in all definitions of D ;

(6. 2. 2)

and (6. 3)

— in each definition of D , corresponding entities shall have the same language linkage; and

(6. 4)

— in each definition of D , the overloaded operators referred to, the implicit calls to conversion functions, constructors, operator new functions and operator delete functions, shall refer to the same function, or to a function defined within the definition of D ; and § 6.2

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(6. 5)

— in each definition of D , a default argument used by an (implicit or explicit) function call is treated as if its token sequence were present in the definition of D ; that is, the default argument is subj ect to the requirements described in this paragraph (and, if the default argument has subexpressions with default arguments, this requirement applies recursively) 28 ; and

(6. 6)

— if D is a class with an implicitly-declared constructor (15.1) , it is as if the constructor was implicitly defined in every translation unit where it is odr-used, and the implicit definition in every translation unit shall call the same constructor for a subobj ect of D . [ Example: // translation unit 1: struct X { X(int, int) ; X(int, int, int) ; }; X: : X(int, int = 0) { } class D { X x = 0; }; D d1;

// X(int, int) called by D()

// translation unit 2: struct X { X(int, int) ; X(int, int, int) ; }; X: : X(int, int = 0, int = 0) { } class D { X x = 0; }; D d2; // X(int, int, int) called by D() ; // D() ’s implicit definition violates the ODR

— end example ] If D is a template and is defined in more than one translation unit, then the preceding requirements shall apply both to names from the template’s enclosing scope used in the template definition (17.6.3) , and also to dependent names at the point of instantiation (17.6.2) . If the definitions of D satisfy all these requirements, then the behavior is as if there were a single definition of D . If the definitions of D do not satisfy these requirements, then the behavior is undefined. 6. 3

Scope

6. 3. 1

[basic. scope]

Declarative regions and scopes

[basic. scope. declarative]

1

Every name is introduced in some portion of program text called a declarative region, which is the largest part of the program in which that name is valid , that is, in which that name may be used as an unqualified name to refer to the same entity. In general, each particular name is valid only within some possibly discontiguous portion of program text called its scope. To determine the scope of a declaration, it is sometimes convenient to refer to the potential scope of a declaration. The scope of a declaration is the same as its potential scope unless the potential scope contains another declaration of the same name. In that case, the potential scope of the declaration in the inner (contained) declarative region is excluded from the scope of the declaration in the outer (containing) declarative region.

2

[ Example: In int j = 24; 28)

1 1 . 3. 6 describ es how default argument names are lo oked up.

§ 6.3.1

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int main() { int i = j , j ; j = 42; }

the identifier j is declared twice as a name (and used twice) . The declarative region of the first j includes the entire example. The potential scope of the first j begins immediately after that j and extends to the end of the program, but its (actual) scope excludes the text between the , and the } . The declarative region of the second declaration of j (the j immediately before the semicolon) includes all the text between { and } , but its potential scope excludes the declaration of i . The scope of the second declaration of j is the same as its potential scope. — end example ] 3

The names declared by a declaration are introduced into the scope in which the declaration occurs, except that the presence of a friend specifier (14.3) , certain uses of the elaborated-type-specifier (10.1.7.3) , and using-directives (10.3.4) alter this general behavior.

4

Given a set of declarations in a single declarative region, each of which specifies the same unqualified name,

(4. 1 )

— they shall all refer to the same entity, or all refer to functions and function templates; or

(4. 2)

— exactly one declaration shall declare a class name or enumeration name that is not a typedef name and the other declarations shall all refer to the same variable, non-static data member, or enumerator, or all refer to functions and function templates; in this case the class name or enumeration name is hidden (6.3.10) . [ Note: A namespace name or a class template name must be unique in its declarative region (10.3.2, Clause 17) . — end note ] [ Note: These restrictions apply to the declarative region into which a name is introduced, which is not necessarily the same as the region in which the declaration occurs. In particular, elaborated-type-specifier s (10.1.7.3) and friend declarations (14.3) may introduce a (possibly not visible) name into an enclosing namespace; these restrictions apply to that region. Local extern declarations (6.5) may introduce a name into the declarative region where the declaration appears and also introduce a (possibly not visible) name into an enclosing namespace; these restrictions apply to both regions. — end note ]

5

[ Note: The name lookup rules are summarized in 6.4. 6. 3. 2

1

— end note ]

Point of declaration

[basic. scope. pdecl]

The point of declaration for a name is immediately after its complete declarator (Clause 11) and before its initializer (if any) , except as noted below. [ Example: unsigned char x = 12; { unsigned char x = x; }

Here the second x is initialized with its own (indeterminate) value. 2

— end example ]

[ Note: A name from an outer scope remains visible up to the point of declaration of the name that hides it. [ Example: const int i = 2; { int i[i] ; }

— end example ] — end note ] The point of declaration for a class or class template first declared by a class-specifier is immediately after the identifier or simple-template-id (if any) in its class-head (Clause 12) . The point of declaration for an enumeration is immediately after the identifier (if any) in either its enum-specifier (10.2) or its first opaque-enum-declaration (10.2) , whichever comes first. The point of declaration of an alias or alias template immediately follows the type-id to which the alias refers. The point of declaration of a using-declarator that does not name a constructor is immediately after the using-declarator (10.3.3) . declares a block-scope array of two integers.

3

4

§ 6.3.2

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5

The point of declaration for an enumerator is immediately after its

enumerator-definition. [ Example:

const int x = 12; { enum { x = x }; }

Here, the enumerator x is initialized with the value of the constant x , namely 12. 6

— end example ]

After the point of declaration of a class member, the member name can be looked up in the scope of its class. [ Note: This is true even if the class is an incomplete class. For example, struct X { enum E { z = 16 }; int b[X: : z] ; // OK };

— end note ] 7 (7. 1 )

The point of declaration of a class first declared in an

elaborated-type-specifier is as follows:

— for a declaration of the form class-key attribute-specifier-seqopt identifier ;

identifier is declared to be a class-name in the scope that contains the declaration, otherwise — for an elaborated-type-specifier of the form the

(7. 2)

class-key identifier

if the elaborated-type-specifier is used in the decl-specifier-seq or parameter-declaration-clause of a function defined in namespace scope, the identifier is declared as a class-name in the namespace that contains the declaration; otherwise, except as a friend declaration, the identifier is declared in the smallest namespace or block scope that contains the declaration. [ Note: These rules also apply within templates. — end note ] [ Note: Other forms of elaborated-type-specifier do not declare a new name, and therefore must refer to an existing type-name . See 6.4.4 and 10.1.7.3. — end note ] 8

The point of declaration for an injected-class-name (Clause 12) is immediately following the opening brace of the class definition.

9

The point of declaration for a function-local predefined variable (11.4) is immediately before the of a function definition.

10

The point of declaration for a template parameter is immediately after its complete [ Example:

function-body

template-parameter.

typedef unsigned char T; template struct A { };

11

— end example ] [ Note: Friend declarations refer to functions or classes that are members of the nearest enclosing namespace, but they do not introduce new names into that namespace (10.3.1.2) . Function declarations at block scope and variable declarations with the extern specifier at block scope refer to declarations that are members of an enclosing namespace, but they do not introduce new names into that scope. — end note ]

12

[ Note: For point of instantiation of a template, see 17.6.4.1. — end 6. 3. 3

1

note ]

Block scope

[basic. scope. block]

A name declared in a block (9.3) is local to that block; it has block scope. Its potential scope begins at its point of declaration (6.3.2) and ends at the end of its block. A variable declared at block scope is a local variable. § 6.3.3

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2

The potential scope of a function parameter name (including one appearing in a lambda-declarator) or of a function-local predefined variable in a function definition (11.4) begins at its point of declaration. If the function has a function-try-block the potential scope of a parameter or of a function-local predefined variable ends at the end of the last associated handler, otherwise it ends at the end of the outermost block of the function definition. A parameter name shall not be redeclared in the outermost block of the function definition nor in the outermost block of any handler associated with a function-try-block .

3

The name declared in an exception-declaration is local to the outermost block of the handler .

4

Names declared in the init-statement , the for-range-declaration , and in the condition of if , while , for , and switch statements are local to the if , while , for , or switch statement (including the controlled statement) , and shall not be redeclared in a subsequent condition of that statement nor in the outermost block (or, for the if statement, any of the outermost blocks) of the controlled statement; see 9.4. 6. 3. 4

handler and shall not be redeclared in the

Function prototype scope

[basic. scope. proto]

1

In a function declaration, or in any function declarator except the declarator of a function definition (11.4) , names of parameters (if supplied) have function prototype scope, which terminates at the end of the nearest enclosing function declarator.

1

Labels (9.1) have function scope and may be used anywhere in the function in which they are declared. Only labels have function scope.

6. 3. 5

6. 3. 6 1

Function scope

[basic. funscope]

Namespace scope

[basic. scope. namespace]

The declarative region of a namespace-definition is its namespace-body . Entities declared in a namespace-body are said to be members of the namespace, and names introduced by these declarations into the declarative region of the namespace are said to be member names of the namespace. A namespace member name has namespace scope. Its potential scope includes its namespace from the name’s point of declaration (6.3.2) onwards; and for each using-directive (10.3.4) that nominates the member’s namespace, the member’s potential scope includes that portion of the potential scope of the using-directive that follows the member’s point of declaration. [ Example: namespace N { int i; int g(int a) { return a; } int j () ; void q() ; } namespace { int l=1; } // the potential scope of l is from its point of declaration to the end of the translation unit namespace N { int g(char a) { return l+a; }

// overloads N: : g(int) // l is from unnamed namespace

int i; int j () ;

// error: duplicate definition // OK: duplicate function declaration

int j () { return g(i) ; } int q() ;

// OK: definition of N: : j () // calls N: : g(int)

§ 6.3.6

// error: different return type

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}

— end example ] 2

A namespace member can also be referred to after the : : scope resolution operator (8.1) applied to the name of its namespace or the name of a namespace which nominates the member’s namespace in a using-directive ; see 6.4.3.2.

3

The outermost declarative region of a translation unit is also a namespace, called the global namespace . A name declared in the global namespace has global namespace scope (also called global scope) . The potential scope of such a name begins at its point of declaration (6.3.2) and ends at the end of the translation unit that is its declarative region. A name with global namespace scope is said to be a global name. 6. 3. 7

Class scope

[basic. scope. class]

1

The potential scope of a name declared in a class consists not only of the declarative region following the name’s point of declaration, but also of all function bodies, default arguments, noexcept-specifier s, and brace-or-equal-initializers of non-static data members in that class (including such things in nested classes) .

2

A name N used in a class S shall refer to the same declaration in its context and when re-evaluated in the completed scope of S . No diagnostic is required for a violation of this rule.

3

A name declared within a member function hides a declaration of the same name whose scope extends to or past the end of the member function’s class.

4

The potential scope of a declaration that extends to or past the end of a class definition also extends to the regions defined by its member definitions, even if the members are defined lexically outside the class (this includes static data member definitions, nested class definitions, and member function definitions, including the member function body and any portion of the declarator part of such definitions which follows the declarator-id, including a parameter-declaration-clause and any default arguments (11.3.6) ) .

5

[ Example: typedef int c; enum { i = 1 }; class X { char v[i] ; int f() { return sizeof(c) ; } char c; enum { i = 2 }; }; typedef char* T; struct Y { T a; typedef long T; T b; }; typedef int I; class D { typedef I I; };

// error: i refers to : : i but when reevaluated is X: : i // OK: X: : c

// error: T refers to : : T but when reevaluated is Y: : T

// error, even though no reordering involved

— end example ] 6 (6. 1 )

The name of a class member shall only be used as follows: — in the scope of its class (as described above) or a class derived (Clause 13) from its class, § 6.3.7

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(6. 2)

— after the . operator applied to an expression of the type of its class (8.2.5) or a class derived from its class,

(6. 3)

— after the -> operator applied to a pointer to an obj ect of its class (8.2.5) or a class derived from its class,

(6. 4)

— after the : : scope resolution operator (8.1) applied to the name of its class or a class derived from its class. 6. 3. 8

1

[basic. scope. enum]

The name of a scoped enumerator (10.2) has enumeration scope . Its potential scope begins at its point of declaration and terminates at the end of the enum-specifier . 6. 3. 9

1

Enumeration scope

Template parameter scope

[basic. scope. temp]

The declarative region of the name of a template parameter of a template

template-parameter-list in which the name was introduced. 2

template-parameter is the smallest

The declarative region of the name of a template parameter of a template is the smallest template-declaration in which the name was introduced. Only template parameter names belong to this declarative region; any other kind of name introduced by the declaration of a template-declaration is instead introduced into the same declarative region where it would be introduced as a result of a non-template declaration of the same name. [ Example: namespace N { template struct A { }; template void f(U) { } struct B { template friend int g(struct C*) ; }; }

// #1 // #2 // #3

The declarative regions of T , U and V are the template-declaration s on lines #1, #2, and #3, respectively. But the names A , f , g and C all belong to the same declarative region — namely, the namespace-body of N . ( g is still considered to belong to this declarative region in spite of its being hidden during qualified and unqualified name lookup.) — end example ] 3

The potential scope of a template parameter name begins at its point of declaration (6.3.2) and ends at the end of its declarative region. [ Note: This implies that a template-parameter can be used in the declaration of subsequent template-parameter s and their default arguments but cannot be used in preceding template-parameters or their default arguments. For example, template class X { /* . . . */ }; template void f(T* p = new T) ;

This also implies that a

template-parameter can be used in the specification of base classes. For example,

template class X : public Array { /* . . . */ }; template class Y : public T { /* . . . */ };

The use of a template parameter as a base class implies that a class used as a template argument must be defined and not j ust declared when the class template is instantiated. — end note ] 4

The declarative region of the name of a template parameter is nested within the immediately-enclosing declarative region. [ Note: As a result, a template-parameter hides any entity with the same name in an enclosing scope (6.3.10) . [ Example: typedef int N; template struct A;

§ 6.3.9

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Here, X is a non-type template parameter of type int and Y is a non-type template parameter of the same type as the second template parameter of A. — end example ] — end note ] 5

[ Note: Because the name of a template parameter cannot be redeclared within its potential scope (17.6.1) , a template parameter’s scope is often its potential scope. However, it is still possible for a template parameter name to be hidden; see 17.6.1. — end note ] 6. 3. 1 0

Name hiding

[basic. scope. hiding]

1

A name can be hidden by an explicit declaration of that same name in a nested declarative region or derived class (13.2) .

2

A class name (12.1) or enumeration name (10.2) can be hidden by the name of a variable, data member, function, or enumerator declared in the same scope. If a class or enumeration name and a variable, data member, function, or enumerator are declared in the same scope (in any order) with the same name, the class or enumeration name is hidden wherever the variable, data member, function, or enumerator name is visible.

3

In a member function definition, the declaration of a name at block scope hides the declaration of a member of the class with the same name; see 6.3.7. The declaration of a member in a derived class (Clause 13) hides the declaration of a member of a base class of the same name; see 13.2.

4

During the lookup of a name qualified by a namespace name, declarations that would otherwise be made visible by a using-directive can be hidden by declarations with the same name in the namespace containing the using-directive ; see 6.4.3.2.

5

If a name is in scope and is not hidden it is said to be

1

The name lookup rules apply uniformly to all names (including typedef-name s (10.1.3) , namespace-name s (10.3) , and class-name s (12.1) ) wherever the grammar allows such names in the context discussed by a particular rule. Name lookup associates the use of a name with a set of declarations (6.1) of that name. The declarations found by name lookup shall either all declare the same entity or shall all declare functions; in the latter case, the declarations are said to form a set of overloaded functions (16.1) . Overload resolution (16.3) takes place after name lookup has succeeded. The access rules (Clause 14) are considered only once name lookup and function overload resolution (if applicable) have succeeded. Only after name lookup, function overload resolution (if applicable) and access checking have succeeded are the attributes introduced by the name’s declaration used further in expression processing (Clause 8) .

2

A name “looked up in the context of an expression” is looked up as an unqualified name in the scope where the expression is found.

3

The injected-class-name of a class (Clause 12) is also considered to be a member of that class for the purposes of name hiding and lookup.

4

[ Note: 6.5 discusses linkage issues. The notions of scope, point of declaration and name hiding are discussed in 6.3. — end note ]

6. 4

6. 4. 1

visible.

Name lookup

[basic. lookup]

Unqualified name lookup

[basic. lookup. unqual]

1

In all the cases listed in 6.4.1, the scopes are searched for a declaration in the order listed in each of the respective categories; name lookup ends as soon as a declaration is found for the name. If no declaration is found, the program is ill-formed.

2

The declarations from the namespace nominated by a using-directive become visible in a namespace enclosing the using-directive ; see 10.3.4. For the purpose of the unqualified name lookup rules described in 6.4.1, the declarations from the namespace nominated by the using-directive are considered members of that enclosing namespace.

§ 6.4.1

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3

The lookup for an unqualified name used as the postfix-expression of a function call is described in 6.4.2. [ Note: For purposes of determining (during parsing) whether an expression is a postfix-expression for a function call, the usual name lookup rules apply. The rules in 6.4.2 have no effect on the syntactic interpretation of an expression. For example, typedef int f; namespace N { struct A { friend void f(A &) ; operator int() ; void g(A a) { int i = f(a) ; // f is the typedef, not the friend function: equivalent to int(a) } }; }

Because the expression is not a function call, the argument-dependent name lookup (6.4.2) does not apply and the friend function f is not found. — end note ] 4

A name used in global scope, outside of any function, class or user-declared namespace, shall be declared before its use in global scope.

5

A name used in a user-declared namespace outside of the definition of any function or class shall be declared before its use in that namespace or before its use in a namespace enclosing its namespace.

6

In the definition of a function that is a member of namespace N , a name used after the function’s declarator-id 29 shall be declared before its use in the block in which it is used or in one of its enclosing blocks (9.3) or shall be declared before its use in namespace N or, if N is a nested namespace, shall be declared before its use in one of N’s enclosing namespaces. [ Example: namespace A { namespace N { void f() ; } } void A: : N: : f() { i = 5; // The following scopes are searched for a declaration of i : // 1) outermost block scope of A: : N: : f , before the use of i // 2) scope of namespace N // 3) scope of namespace A // 4) global scope, before the definition of A: : N: : f }

— end example ] 7

A name used in the definition of a class X outside of a member function body, default argument, noexceptspecifier, brace-or-equal-initializer of a non-static data member, or nested class definition30 shall be declared in one of the following ways:

(7. 1 )

— before its use in class X or be a member of a base class of X (13.2) , or

(7. 2)

— if X is a nested class of class Y (12.2.5) , before the definition of X in Y, or shall be a member of a base class of Y (this lookup applies in turn to Y’s enclosing classes, starting with the innermost enclosing 29)

This refers t o unqualified names t hat o ccur, for inst ance, in a typ e or default argument in t he

or used in t he funct ion b o dy.

30)

This refers t o unqualified names following t he class name; such a name may b e used in t he

parameter-declaration-clause

base-clause

or may b e used in

t he class definit ion.

§ 6.4.1

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class) , 31 or (7. 3)

— if X is a local class (12.4) or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or

(7. 4)

— if X is a member of namespace N , or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N, before the definition of class X in namespace N or in one of N ’s enclosing namespaces. [ Example: namespace M { class B { }; } namespace class Y class int }; }; }

N { : public M: : B { X { a[i] ;

// The following scopes are searched for a declaration of i : // 1) scope of class N: : Y: : X , before the use of i // 2) scope of class N: : Y, before the definition of N: : Y: : X // 3) scope of N: : Y’s base class M: : B // 4) scope of namespace N, before the definition of N: : Y // 5) global scope, before the definition of N

— end example ] [ Note: When looking for a prior declaration of a class or function introduced by a friend declaration, scopes outside of the innermost enclosing namespace scope are not considered; see 10.3.1.2. — end note ] [ Note: 6.3.7 further describes the restrictions on the use of names in a class definition. 12.2.5 further describes the restrictions on the use of names in nested class definitions. 12.4 further describes the restrictions on the use of names in local class definitions. — end note ] 8

For the members of a class X , a name used in a member function body, in a default argument, in a noexceptspecifier, in the brace-or-equal-initializer of a non-static data member (12.2) , or in the definition of a class member outside of the definition of X, following the member’s declarator-id 32 , shall be declared in one of the following ways:

(8. 1 )

— before its use in the block in which it is used or in an enclosing block (9.3) , or

(8. 2)

— shall be a member of class X or be a member of a base class of X (13.2) , or

(8. 3)

— if X is a nested class of class Y (12.2.5) , shall be a member of Y, or shall be a member of a base class of Y (this lookup applies in turn to Y’s enclosing classes, starting with the innermost enclosing class) , 33 or

(8. 4)

— if X is a local class (12.4) or is a nested class of a local class, before the definition of class X in a block enclosing the definition of class X, or

(8. 5)

— if X is a member of namespace N , or is a nested class of a class that is a member of N, or is a local class or a nested class within a local class of a function that is a member of N , before the use of the name, in namespace N or in one of N’s enclosing namespaces. 31 )

This lo okup applies whet her t he definit ion of

namespace scop e enclosing

Y’ s

X

is nest ed wit hin

Y’ s

definit ion or whet her

definit ion ( 1 2 . 2. 5 ) .

That is, an unqualified name t hat o ccurs, for inst ance, in a typ e in t he

33)

This lo okup applies whet her t he memb er funct ion is defined wit hin t he definit ion of class

is defined in a namespace scop e enclosing

§ 6.4.1

X’ s

definit ion app ears in a

parameter-declaration-clause

32)

specifier.

X’ s

X

or in t he

noexcept-

or whet her t he memb er funct ion

definit ion.

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[ Example: class B { }; namespace M { namespace N { class X : public B { void f() ; }; } } void M: : N: : X: : f() { i = 16; }

// The following scopes are searched for a declaration of i : // 1) outermost block scope of M: : N: : X: : f , before the use of i // 2) scope of class M: : N: : X // 3) scope of M: : N: : X ’s base class B // 4) scope of namespace M: : N // 5) scope of namespace M // 6) global scope, before the definition of M: : N: : X: : f

— end example ] [ Note: 12.2.1 and 12.2.3 further describe the restrictions on the use of names in member function definitions. 12.2.5 further describes the restrictions on the use of names in the scope of nested classes. 12.4 further describes the restrictions on the use of names in local class definitions. — end note ] 9

10

Name lookup for a name used in the definition of a friend function (14.3) defined inline in the class granting friendship shall proceed as described for lookup in member function definitions. If the friend function is not defined in the class granting friendship, name lookup in the friend function definition shall proceed as described for lookup in namespace member function definitions. In a friend declaration naming a member function, a name used in the function declarator and not part of a

template-argument in the declarator-id is first looked up in the scope of the member function’s class (13.2) . If it is not found, or if the name is part of a template-argument in the declarator-id , the look up is as described for unqualified names in the definition of the class granting friendship. [ Example: struct A { typedef int AT; void f1(AT) ; void f2(float) ; template void f3() ; }; struct B { typedef char AT; typedef float BT; friend void A: : f1(AT) ; // parameter type is A: : AT friend void A: : f2(BT) ; // parameter type is B: : BT friend void A: : f3() ; // template argument is B: : AT };

— end example ] 11

During the lookup for a name used as a default argument (11.3.6) in a function parameter-declaration-clause or used in the expression of a mem-initializer for a constructor (15.6.2) , the function parameter names are visible and hide the names of entities declared in the block, class or namespace scopes containing the function declaration. [ Note: 11.3.6 further describes the restrictions on the use of names in default arguments. 15.6.2 further describes the restrictions on the use of names in a ctor-initializer . — end note ]

§ 6.4.1

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12

13

14

During the lookup of a name used in the constant-expression of an enumerator-definition , previously declared enumerators of the enumeration are visible and hide the names of entities declared in the block, class, or namespace scopes containing the enum-specifier . A name used in the definition of a static data member of class X (12.2.3.2) (after the qualified-id of the static member) is looked up as if the name was used in a member function of X . [ Note: 12.2.3.2 further describes the restrictions on the use of names in the definition of a static data member. — end note ] If a variable member of a namespace is defined outside of the scope of its namespace then any name that appears in the definition of the member (after the declarator-id) is looked up as if the definition of the member occurred in its namespace. [ Example: namespace N { int i = 4; extern int j ; } int i = 2; int N: : j = i;

// N: : j == 4

— end example ] 15

A name used in the handler for a function-try-block (Clause 18) is looked up as if the name was used in the outermost block of the function definition. In particular, the function parameter names shall not be redeclared in the exception-declaration nor in the outermost block of a handler for the function-try-block . Names declared in the outermost block of the function definition are not found when looked up in the scope of a handler for the function-try-block . [ Note: But function parameter names are found. — end note ]

16

[ Note: The rules for name lookup in template definitions are described in 17.6.

— end note ]

6. 4. 2

[basic. lookup. argdep]

1

Argument-dependent name lookup

When the postfix-expression in a function call (8.2.2) is an unqualified-id, other namespaces not considered during the usual unqualified lookup (6.4.1) may be searched, and in those namespaces, namespace-scope friend function or function template declarations (14.3) not otherwise visible may be found. These modifications to the search depend on the types of the arguments (and for template template arguments, the namespace of the template argument) . [ Example: namespace N { struct S { }; void f(S) ; } void g() { N: : S s; f(s) ; (f) (s) ; }

// OK: calls N: : f // error: N: : f not considered; parentheses prevent argument-dependent lookup

— end example ] 2

(2. 1 )

For each argument type T in the function call, there is a set of zero or more associated namespaces and a set of zero or more associated classes to be considered. The sets of namespaces and classes are determined entirely by the types of the function arguments (and the namespace of any template template argument) . Typedef names and using-declaration s used to specify the types do not contribute to this set. The sets of namespaces and classes are determined in the following way: — If T is a fundamental type, its associated sets of namespaces and classes are both empty. § 6.4.2

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(2. 2)

— If T is a class type (including unions) , its associated classes are: the class itself; the class of which it is a member, if any; and its direct and indirect base classes. Its associated namespaces are the innermost enclosing namespaces of its associated classes. Furthermore, if T is a class template specialization, its associated namespaces and classes also include: the namespaces and classes associated with the types of the template arguments provided for template type parameters (excluding template template parameters) ; the namespaces of which any template template arguments are members; and the classes of which any member templates used as template template arguments are members. [ Note: Non-type template arguments do not contribute to the set of associated namespaces. — end note ]

(2. 3)

— If T is an enumeration type, its associated namespace is the innermost enclosing namespace of its declaration. If it is a class member, its associated class is the member’s class; else it has no associated class.

(2. 4)

— If T is a pointer to U or an array of U, its associated namespaces and classes are those associated with U.

(2. 5)

— If T is a function type, its associated namespaces and classes are those associated with the function parameter types and those associated with the return type.

(2. 6)

— If T is a pointer to a member function of a class X, its associated namespaces and classes are those associated with the function parameter types and return type, together with those associated with X .

(2. 7)

— If T is a pointer to a data member of class X, its associated namespaces and classes are those associated with the member type together with those associated with X. If an associated namespace is an inline namespace (10.3.1) , its enclosing namespace is also included in the set. If an associated namespace directly contains inline namespaces, those inline namespaces are also included in the set. In addition, if the argument is the name or address of a set of overloaded functions and/or function templates, its associated classes and namespaces are the union of those associated with each of the members of the set, i.e., the classes and namespaces associated with its parameter types and return type. Additionally, if the aforementioned set of overloaded functions is named with a template-id, its associated classes and namespaces also include those of its type template-argument s and its template template-argument s.

3

Let X be the lookup set produced by unqualified lookup (6.4.1) and let argument dependent lookup (defined as follows) . If X contains

(3. 1 )

— a declaration of a class member, or

(3. 2)

— a block-scope function declaration that is not a

(3. 3)

— a declaration that is neither a function nor a function template

Y be the lookup set produced by

using-declaration, or

then Y is empty. Otherwise Y is the set of declarations found in the namespaces associated with the argument types as described below. The set of declarations found by the lookup of the name is the union of X and Y. [ Note: The namespaces and classes associated with the argument types can include namespaces and classes already considered by the ordinary unqualified lookup. — end note ] [ Example: namespace NS { class T { }; void f(T) ; void g(T, int) ; } NS: : T parm; void g(NS: : T, float) ; int main() { f(parm) ; extern void g(NS: : T, float) ; g(parm, 1) ; }

// OK: calls NS: : f // OK: calls g(NS: : T, float)

— end example ] § 6.4.2

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4

When considering an associated namespace, the lookup is the same as the lookup performed when the associated namespace is used as a qualifier (6.4.3.2) except that:

using-directives in the associated namespace are ignored.

(4. 1 )

— Any

(4. 2)

— Any namespace-scope friend functions or friend function templates declared in associated classes are visible within their respective namespaces even if they are not visible during an ordinary lookup (14.3) .

(4. 3)

— All names except those of (possibly overloaded) functions and function templates are ignored. 6. 4. 3

1

Qualified name lookup

The name of a class or namespace member or enumerator can be referred to after the : : scope resolution operator (8.1) applied to a nested-name-specifier that denotes its class, namespace, or enumeration. If a : : scope resolution operator in a nested-name-specifier is not preceded by a decltype-specifier, lookup of the name preceding that : : considers only namespaces, types, and templates whose specializations are types. If the name found does not designate a namespace or a class, enumeration, or dependent type, the program is ill-formed. [ Example: class A { public: static int n; }; int main() { int A; A: : n = 42; A b; }

2

3

[basic. lookup. qual]

// OK // ill-formed: A does not name a type

— end example ] [ Note: Multiply qualified names, such as N1: : N2: : N3: : n, can be used to refer to members of nested classes (12.2.5) or members of nested namespaces. — end note ] In a declaration in which the declarator-id is a qualified-id , names used before the qualified-id being declared are looked up in the defining namespace scope; names following the qualified-id are looked up in the scope of the member’s class or namespace. [ Example: class X { }; class C { class X { }; static const int number = 50; static X arr[number] ; }; X C: : arr[number] ; // ill-formed: // equivalent to : : X C: : arr[C: : number] ; // and not to C: : X C: : arr[C: : number] ;

— end example ] 4

A name prefixed by the unary scope operator : : (8.1) is looked up in global scope, in the translation unit where it is used. The name shall be declared in global namespace scope or shall be a name whose declaration is visible in global scope because of a using-directive (6.4.3.2) . The use of : : allows a global name to be referred to even if its identifier has been hidden (6.3.10) .

5

A name prefixed by a nested-name-specifier that nominates an enumeration type shall represent an enumerator of that enumeration.

6

If a pseudo-destructor-name (8.2.4) contains a nested-name-specifier , the type-name s are looked up as types in the scope designated by the nested-name-specifier . Similarly, in a qualified-id of the form:

§ 6.4.3

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nested-name-specifieropt class-name : : ~ class-name

the second

class-name is looked up in the same scope as the first. [ Example:

struct C { typedef int I; }; typedef int I1, I2; extern int* p; extern int* q; // I is looked up in the scope of C p->C: : I: : ~ I() ; // I2 is looked up in the scope of the postfix-expression q->I1: : ~ I2() ; struct A { ~ A() ; }; typedef A AB; int main() { AB* p; p->AB: : ~ AB() ; }

// explicitly calls the destructor for A

— end example ] [ Note: 6.4.5 describes how name lookup proceeds after the . and -> operators. — end note ] 6. 4. 3. 1 1

Class members

[class. qual]

nested-name-specifier of a qualified-id nominates a class, the name specified after the nested-namespecifier is looked up in the scope of the class (13.2) , except for the cases listed below. The name shall represent one or more members of that class or of one of its base classes (Clause 13) . [ Note: A class member can be referred to using a qualified-id at any point in its potential scope (6.3.7) . — end note ] The exceptions If the

to the name lookup rule above are the following:

(1. 1 )

— the lookup for a destructor is as specified in 6.4.3;

(1. 2)

— a conversion-type-id of a conversion-function-id is looked up in the same manner as a in a class member access (see 6.4.5) ;

(1. 3)

— the names in a

(1. 4)

conversion-type-id

template-argument of a template-id are looked up in the context in which the entire postfix-expression occurs. — the lookup for a name specified in a using-declaration (10.3.3) also finds class or enumeration names hidden within the same scope (6.3.10) .

2

In a lookup in which function names are not ignored34 and the

nested-name-specifier nominates a class C :

nested-name-specifier, when looked up in C , is the injected-class-name of

(2. 1 )

— if the name specified after the C (Clause 12) , or

(2. 2)

— in a using-declarator of a using-declaration (10.3.3) that is a member-declaration , if the name specified after the nested-name-specifier is the same as the identifier or the simple-template-id’s template-name in the last component of the nested-name-specifier , the name is instead considered to name the constructor of class C . [ Note: For example, the constructor is not an acceptable lookup result in an elaborated-type-specifier so the constructor would not be used in place of the inj ected-class-name. — end note ] Such a constructor name shall be used only in the declarator-id of a declaration that names a constructor or in a using-declaration . [ Example: struct A { A() ; }; struct B: public A { B() ; }; 34) or a

Lo okups in which funct ion names are ignored include names app earing in a

base-specifier.

§ 6.4.3.1

nested-name-specifier,

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an

elaborated-type-specifier ,

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A: : A() { } B: : B() { }

// object of type A // error, A: : A is not a type name // object of type A

B: : A ba; A: : A a; struct A: : A a2;

— end example ] 3

A class member name hidden by a name in a nested declarative region or by the name of a derived class member can still be found if qualified by the name of its class followed by the : : operator. 6. 4. 3. 2

Namespace members

[namespace. qual]

1

If the nested-name-specifier of a qualified-id nominates a namespace (including the case where the nestedname-specifier is : : , i.e., nominating the global namespace) , the name specified after the nested-name-specifier is looked up in the scope of the namespace. The names in a template-argument of a template-id are looked up in the context in which the entire postfix-expression occurs.

2

For a namespace X and name m, the namespace-qualified lookup set ( ) is defined as follows: Let 0 ( ) be the set of all declarations of m in X and the inline namespace set of X (10.3.1) . If 0 ( ) is not empty, ( ) is 0 ( ) ; otherwise, ( ) is the union of ( i ) for all namespaces i nominated by using-directive s in X and its inline namespace set.

3

S X, m S X, m

S X, m

S X, m

S X, m S N,m

S X, m N

Given X: : m (where X is a user-declared namespace) , or given : : m (where X is the global namespace) , if ( ) is the empty set, the program is ill-formed. Otherwise, if ( ) has exactly one member, or if the context of the reference is a using-declaration (10.3.3) , ( ) is the required set of declarations of m. Otherwise if the use of m is not one that allows a unique declaration to be chosen from ( ), the program is ill-formed. [ Example:

S X, m

S X, m

S X, m

S X, m

int x; namespace Y { void f(float) ; void h(int) ; } namespace Z { void h(double) ; } namespace A { using namespace Y; void f(int) ; void g(int) ; int i; } namespace B { using namespace Z; void f(char) ; int i; } namespace AB { using namespace A; using namespace B; void g() ;

§ 6.4.3.2

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} void h() { AB: : g() ; AB: : f(1) ;

// f is not declared directly in AB so the rules are applied recursively to A and B ; // namespace Y is not searched and Y: : f(float) is not considered; // S is { A: : f(int) , B: : f(char) } and overload resolution chooses A: : f(int)

AB: : f(’ c’ ) ;

// as above but resolution chooses B: : f(char)

AB: : x++;

// x is not declared directly in AB , and is not declared in A or B , so the rules // are applied recursively to Y and Z , S is {} so the program is ill-formed

AB: : i++;

// i is not declared directly in AB so the rules are applied recursively to A and B , // S is { A: : i , B: : i } so the use is ambiguous and the program is ill-formed

AB: : h(16. 8) ;

// h is not declared directly in AB and not declared directly in A or B so the rules // are applied recursively to Y and Z , S is { Y: : h(int) , Z: : h(double) } and // overload resolution chooses Z: : h(double)

} 4

// g is declared directly in AB , therefore S is { AB: : g() } and AB: : g() is chosen

— end example ] [ Note: The same declaration found more than once is not an ambiguity (because it is still a unique declaration) . [ Example: namespace A { int a; } namespace B { using namespace A; } namespace C { using namespace A; } namespace BC { using namespace B; using namespace C; } void f() { BC: : a++; }

// OK: S is { A: : a, A: : a}

namespace D { using A: : a; } namespace BD { using namespace B; using namespace D;

§ 6.4.3.2

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} void g() { BD: : a++; } 5

// OK: S is { A: : a, A: : a}

— end example ] — end note ] [ Example: Because each referenced namespace is searched at most once, the following is well-defined: namespace B { int b; } namespace A { using namespace B; int a; } namespace B { using namespace A; } void f() { A: : a++; B: : a++; A: : b++; B: : b++; }

// OK: // OK: // OK: // OK:

a declared directly in A , S is { A: : a} both A and B searched (once), S is { A: : a} both A and B searched (once), S is { B: : b } b declared directly in B , S is { B: : b }

— end example ] 6

During the lookup of a qualified namespace member name, if the lookup finds more than one declaration of the member, and if one declaration introduces a class name or enumeration name and the other declarations either introduce the same variable, the same enumerator or a set of functions, the non-type name hides the class or enumeration name if and only if the declarations are from the same namespace; otherwise (the declarations are from different namespaces) , the program is ill-formed. [ Example: namespace A { struct x { }; int x; int y; } namespace B { struct y { }; } namespace C { using namespace A; using namespace B; int i = C: : x; // OK, A: : x (of type int ) int j = C: : y; // ambiguous, A: : y or B: : y }

— end example ] § 6.4.3.2

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7

In a declaration for a namespace member in which the for the namespace member has the form

declarator-id is a qualified-id, given that the qualified-id

nested-name-specifier unqualified-id

the unqualified-id shall name a member of the namespace designated by the element of the inline namespace set (10.3.1) of that namespace. [ Example:

nested-name-specifier or of an

namespace A { namespace B { void f1(int) ; } using namespace B; } void A: : f1(int) { } // ill-formed, f1 is not a member of A

— end example ] However, in such namespace member declarations, the nested-name-specifier may rely on using-directives to implicitly provide the initial part of the nested-name-specifier. [ Example: namespace A { namespace B { void f1(int) ; } } namespace C { namespace D { void f1(int) ; } } using namespace A; using namespace C: : D; void B: : f1(int) { } // OK, defines A: : B: : f1(int)

— end example ] 6. 4. 4

Elaborated type specifiers

[basic. lookup. elab]

1

An elaborated-type-specifier (10.1.7.3) may be used to refer to a previously declared even though the name has been hidden by a non-type declaration (6.3.10) .

2

If the elaborated-type-specifier has no nested-name-specifier , and unless the in a declaration with the following form:

class-name or enum-name

elaborated-type-specifier appears

class-key attribute-specifier-seqopt identifier ;

the identifier is looked up according to 6.4.1 but ignoring any non-type names that have been declared. If the elaborated-type-specifier is introduced by the enum keyword and this lookup does not find a previously declared type-name , the elaborated-type-specifier is ill-formed. If the elaborated-type-specifier is introduced by the class-key and this lookup does not find a previously declared type-name , or if the elaborated-type-specifier appears in a declaration with the form: class-key attribute-specifier-seqopt identifier ;

elaborated-type-specifier is a declaration that introduces the class-name as described in 6.3.2. If the elaborated-type-specifier has a nested-name-specifier , qualified name lookup is performed, as described the

3

in 6.4.3, but ignoring any non-type names that have been declared. If the name lookup does not find a previously declared type-name , the elaborated-type-specifier is ill-formed. [ Example: struct Node {

§ 6.4.4

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struct Node* Next; struct Data* Data; }; struct Data { struct Node* Node; friend struct : : Glob; friend struct Glob; /* . . . */ };

// OK: Refers to Node at global scope // OK: Declares type Data // at global scope and member Data

// OK: Refers to Node at global scope // error: Glob is not declared, cannot introduce a qualified type (10.1.7.3) // OK: Refers to (as yet) undeclared Glob at global scope.

struct Base { struct Data; struct : : Data* thatData; struct Base: : Data* thisData; friend class : : Data; friend class Data; struct Data { /* . . . */ }; };

// OK: Declares nested Data // OK: Refers to : : Data // OK: Refers to nested Data // OK: global Data is a friend // OK: nested Data is a friend // Defines nested Data

struct struct struct struct struct

// OK: Redeclares Data at global scope // error: cannot introduce a qualified type (10.1.7.3) // error: cannot introduce a qualified type (10.1.7.3) // error: Datum undefined // OK: refers to nested Data

Data; : : Data; Base: : Data; Base: : Datum; Base: : Data* pBase;

— end example ] 6. 4. 5

Class member access

[basic. lookup. classref]

1

In a class member access expression (8.2.5) , if the . or -> token is immediately followed by an identifier followed by a < , the identifier must be looked up to determine whether the < is the beginning of a template argument list (17.2) or a less-than operator. The identifier is first looked up in the class of the obj ect expression. If the identifier is not found, it is then looked up in the context of the entire postfix-expression and shall name a class template.

2

If the id-expression in a class member access (8.2.5) is an unqualified-id , and the type of the object expression is of a class type C , the unqualified-id is looked up in the scope of class C . For a pseudo-destructor call (8.2.4) , the unqualified-id is looked up in the context of the complete postfix-expression .

3

If the unqualified-id is ~ type-name , the type-name is looked up in the context of the entire postfix-expression . If the type T of the object expression is of a class type C , the type-name is also looked up in the scope of class C . At least one of the lookups shall find a name that refers to cv T . [ Example: struct A { }; struct B { struct A { }; void f(: : A* a) ; }; void B: : f(: : A* a) { a-> ~ A() ; }

// OK: lookup in *a finds the injected-class-name

— end example ] § 6.4.5

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4

id-expression in a class member access is a qualified-id of the form

If the

class-name-or-namespace-name: : . . .

the class-name-or-namespace-name following the . or -> operator is first looked up in the class of the obj ect expression and the name, if found, is used. Otherwise it is looked up in the context of the entire postfix-expression. [ Note: See 6.4.3, which describes the lookup of a name before : : , which will only find a type or namespace name. — end note ] 5

qualified-id has the form

If the

: : class-name-or-namespace-name: : . . .

class-name-or-namespace-name is looked up in global scope as a class-name or namespace-name. If the nested-name-specifier contains a simple-template-id (17.2) , the names in its template-argument s are looked up in the context in which the entire postfix-expression occurs. If the id-expression is a conversion-function-id , its conversion-type-id is first looked up in the class of the

the 6

7

obj ect expression and the name, if found, is used. Otherwise it is looked up in the context of the entire postfix-expression. In each of these lookups, only names that denote types or templates whose specializations are types are considered. [ Example: struct A { }; namespace N { struct A { void g() { } template operator T() ; }; } int main() { N: : A a; a. operator A() ; }

// calls N: : A: : operator N: : A

— end example ] 6. 4. 6 1

In a using-directive or namespace-alias-definition , during the lookup for a a nested-name-specifier only namespace names are considered. 6. 5

1

Using-directives and namespace aliases

[basic. lookup. udir]

namespace-name or for a name in

Program and linkage

A program consists of one or more a sequence of declarations.

[basic. link]

translation units (Clause 5) linked together. A translation unit consists of

translation-unit : declaration-seqopt 2

A name is said to have linkage when it might denote the same obj ect, reference, function, type, template, namespace or value as a name introduced by a declaration in another scope:

(2. 1 )

— When a name has external linkage , the entity it denotes can be referred to by names from scopes of other translation units or from other scopes of the same translation unit.

(2. 2)

— When a name has internal linkage , the entity it denotes can be referred to by names from other scopes in the same translation unit.

(2. 3)

— When a name has

3

no linkage, the entity it denotes cannot be referred to by names from other scopes.

A name having namespace scope (6.3.6) has internal linkage if it is the name of § 6.5

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(3. 1)

— a variable, function or function template that is explicitly declared static ; or,

(3. 2)

— a non-inline variable of non-volatile const-qualified type that is neither explicitly declared extern nor previously declared to have external linkage; or

(3. 3)

— a data member of an anonymous union.

4

An unnamed namespace or a namespace declared directly or indirectly within an unnamed namespace has internal linkage. All other namespaces have external linkage. A name having namespace scope that has not been given internal linkage above has the same linkage as the enclosing namespace if it is the name of

(4. 1 )

— a variable; or

(4. 2)

— a function; or

(4. 3)

— a named class (Clause 12) , or an unnamed class defined in a typedef declaration in which the class has the typedef name for linkage purposes (10.1.3) ; or

(4. 4)

— a named enumeration (10.2) , or an unnamed enumeration defined in a typedef declaration in which the enumeration has the typedef name for linkage purposes (10.1.3) ; or

(4. 5)

— a template.

5

In addition, a member function, static data member, a named class or enumeration of class scope, or an unnamed class or enumeration defined in a class-scope typedef declaration such that the class or enumeration has the typedef name for linkage purposes (10.1.3) , has the same linkage, if any, as the name of the class of which it is a member.

6

The name of a function declared in block scope and the name of a variable declared by a block scope extern declaration have linkage. If there is a visible declaration of an entity with linkage having the same name and type, ignoring entities declared outside the innermost enclosing namespace scope, the block scope declaration declares that same entity and receives the linkage of the previous declaration. If there is more than one such matching entity, the program is ill-formed. Otherwise, if no matching entity is found, the block scope entity receives external linkage. If, within a translation unit, the same entity is declared with both internal and external linkage, the program is ill-formed. [ Example: static void f() ; static int i = 0; void g() { extern void f() ; int i; { extern void f() ; extern int i; } }

// #1 // internal linkage // #2: i has no linkage // internal linkage // #3: external linkage, ill-formed

Without the declaration at line #2, the declaration at line #3 would link with the declaration at line #1. Because the declaration with internal linkage is hidden, however, #3 is given external linkage, making the program ill-formed. — end example ] 7

When a block scope declaration of an entity with linkage is not found to refer to some other declaration, then that entity is a member of the innermost enclosing namespace. However such a declaration does not introduce the member name in its namespace scope. [ Example: namespace X { void p() { q() ; extern void q() ; }

§ 6.5

// error: q not yet declared // q is a member of namespace X

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void middle() { q() ; }

// error: q not yet declared

void q() { /* . . . */ }

// definition of X: : q

}

// some other, unrelated q

void q() { /* . . . */ }

— end example 8

Names

not

covered

s cop e ( 6 . 3 . 3 )

(8. 1 )

—

it

]

by

t hese

has no linkage.

—

it

have

no

is a clas s or enumerat ion typ e t hat

name has linkage;

(8. 2)

rules

linkage.

M oreover,

—

it

is a sp ecializ at ion of a class t emplat e ( C lause 1 7)

(8. 4)

—

it

is a fundament al typ e ( 6 . 9 . 1 ) ;

(8. 5)

—

it

is a comp ound typ e ( 6 . 9 . 2 )

(8. 6)

—

it

35

;

ot her t han a class or enumerat ion,

—

t he ent ity is declared wit hin an unnamed namespace (1 0 . 3 . 1 ) ,

(8. 9)

—

t he ent ity is not

language linkage (1 0 . 5 ) ,

o dr- used ( 6 . 2 )

[

comp ounded exclusively from typ es

or

or

or is defined in t he same t ranslat ion unit .

a typ e wit hout

linkage cont ains a class or enumerat ion t hat

cannot

An ent ity wit h ext ernal linkage declared us ing such a typ e could not

ot her ent ity in anot her t ranslat ion unit

t hat

or

b e used as t he typ e of a variable or funct ion wit h ext ernal linkage unless

(8. 8)

is o dr- used.

and t he

has linkage.

t he ent ity has C

In ot her words,

b lo ck

or

—

Note:

at

or

version of a typ e t hat

linkage shall not

it s t ranslat ion unit .

declared

is a memb er of a class wit h linkage;

(8. 7)

[

a name

or

is a cv- qualified ( 6 . 9 . 3 )

A typ e wit hout

not ed,

is named ( or has a name for linkage purp oses (1 0 . 1 . 3 ) )

is an unnamed clas s or unnamed enumerat ion t hat

have linkage;

as

or

(8. 3)

t hat

except

A typ e is said t o have linkage if and only if:

Als o not e t hat

of t he program and t hus must

b e named out side corresp ond t o any

b e defined in t he t rans lat ion unit

classes wit h linkage may cont ain memb ers whose typ es do not

typ edef names are ignored in t he det erminat ion of whet her a typ e has linkage.

have linkage,

— end note

if it and

]

Example: template struct B { void g(T) { } void h(T) ; friend void i(B, T) { } }; void f() { struct A { int x; }; A a = { 1 }; B ba; ba. g(a) ; ba. h(a) ; i(ba, a) ; }

— end example 9

Two

35)

names

// no linkage // declares B: : g(A) and B: : h(A) // OK // error: B: : h(A) not defined in the translation unit // OK

]

t hat

are

t he

same

( C lause

6)

and

t hat

are

declared

in

different

scop es

shall

denot e

t he

same

A class t emplat e has t he linkage of t he innermost enclosing class or namespace in which it is declared.

§ 6. 5

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variable, function, type, template or namespace if

(9.1)

— both names have external linkage or else both names have internal linkage and are declared in the same translation unit; and

(9.2)

— both names refer to members of the same namespace or to members, not by inheritance, of the same class; and

(9.3) — when both names denote functions, the parameter-type-lists of the functions (11.3.5) are identical; and (9.4) — when both names denote function templates, the signatures (17.5.6.1) are the same. 10 After all adj ustments of types (during which typedefs (10.1.3) are replaced by their definitions) , the types

specified by all declarations referring to a given variable or function shall be identical, except that declarations for an array object can specify array types that differ by the presence or absence of a major array bound (11.3.4) . A violation of this rule on type identity does not require a diagnostic.

11 [ Note: Linkage to non-C++ declarations can be achieved using a linkage-specification (10.5) . — end note ] 6. 6

Start and termination

[basic. start]

6. 6. 1 main function [basic. start. main] 1 A program shall contain a global function called main. Executing a program starts a main thread of

execution (4.7, 33.3) in which the main function is invoked, and in which variables of static storage duration might be initialized (6.6.2) and destroyed (6.6.4) . It is implementation-defined whether a program in a freestanding environment is required to define a main function. [ Note: In a freestanding environment, start-up and termination is implementation-defined; start-up contains the execution of constructors for obj ects of namespace scope with static storage duration; termination contains the execution of destructors for obj ects with static storage duration. — end note ]

2 An implementation shall not predefine the main function. This function shall not be overloaded. Its type shall have C++ language linkage and it shall have a declared return type of type int , but otherwise its type is implementation-defined. An implementation shall allow both

(2.1) (2.2)

— a function of () returning int and — a function of (int , pointer to pointer to char) returning int as the type of main (11.3.5) . In the latter form, for purposes of exposition, the first function parameter is called argc and the second function parameter is called argv , where argc shall be the number of arguments passed to the program from the environment in which the program is run. If argc is nonzero these arguments shall be supplied in argv[0] through argv[argc-1] as pointers to the initial characters of null-terminated multibyte strings ( ntmbs s) (20.4.2.1.5.2) and argv[0] shall be the pointer to the initial character of a ntmbs that represents the name used to invoke the program or " " . The value of argc shall be non-negative. The value of argv[argc] shall be 0. [ Note: It is recommended that any further (optional) parameters be added after argv . — end note ]

3 The function main shall not be used within a program. The linkage (6.5) of main is implementation-defined. A

program that defines main as deleted or that declares main to be inline , static , or constexpr is ill-formed. The main function shall not be declared with a linkage-specification (10.5) . A program that declares a variable main at global scope or that declares the name main with C language linkage (in any namespace) is ill-formed. The name main is not otherwise reserved. [ Example: Member functions, classes, and enumerations can be called main, as can entities in other namespaces. — end example ]

4 Terminating the program without leaving the current block (e.g., by calling the function std: : exit(int)

(21.5) ) does not destroy any obj ects with automatic storage duration (15.4) . If std: : exit is called to end a program during the destruction of an object with static or thread storage duration, the program has undefined behavior.

§ 6.6.1

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5

A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std: : exit with the return value as the argument. If control flows off the end of the compound-statement of main, the effect is equivalent to a return with operand 0 (see also 18.3) .

1

Variables with static storage duration are initialized as a consequence of program initiation. Variables with thread storage duration are initialized as a consequence of thread execution. Within each of these phases of initiation, initialization occurs as follows.

2

A constant initializer for a variable or temporary object o is an initializer whose full-expression is a constant expression, except that if o is an obj ect, such an initializer may also invoke constexpr constructors for o and its subobj ects even if those obj ects are of non-literal class types. [ Note: Such a class may have a non-trivial destructor. — end note ] Constant initialization is performed if a variable or temporary obj ect with static or thread storage duration is initialized by a constant initializer for the entity. If constant initialization is not performed, a variable with static storage duration (6.7.1) or thread storage duration (6.7.2) is zero-initialized (11.6) . Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization . All static initialization strongly happens before (4.7.1) any dynamic initialization. [ Note: The dynamic initialization of non-local variables is described in 6.6.3; that of local static variables is described in 9.7. — end note ]

3

An implementation is permitted to perform the initialization of a variable with static or thread storage duration as a static initialization even if such initialization is not required to be done statically, provided that

(3. 1 )

— the dynamic version of the initialization does not change the value of any other obj ect of static or thread storage duration prior to its initialization, and

(3. 2)

— the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically.

6. 6. 2

Static initialization

[basic. start. static]

[ Note: As a consequence, if the initialization of an obj ect obj 1 refers to an obj ect obj 2 of namespace scope potentially requiring dynamic initialization and defined later in the same translation unit, it is unspecified whether the value of obj 2 used will be the value of the fully initialized obj 2 (because obj 2 was statically initialized) or will be the value of obj 2 merely zero-initialized. For example, inline double fd() { return 1. 0; } extern double d1; double d2 = d1; // unspecified: // may be statically initialized to 0. 0 or // dynamically initialized to 0. 0 if d1 is // dynamically initialized, or 1. 0 otherwise double d1 = fd() ; // may be initialized statically or dynamically to 1. 0

— end note ] 6. 6. 3

Dynamic initialization of non-local variables

[basic. start. dynamic]

1

Dynamic initialization of a non-local variable with static storage duration is unordered if the variable is an implicitly or explicitly instantiated specialization, is partially-ordered if the variable is an inline variable that is not an implicitly or explicitly instantiated specialization, and otherwise is ordered. [ Note: An explicitly specialized non-inline static data member or variable template specialization has ordered initialization. — end note ]

2

Dynamic initialization of non-local variables V and W with static storage duration are ordered as follows:

(2. 1 )

— If V and W have ordered initialization and V is defined before W within a single translation unit, the initialization of V is sequenced before the initialization of W.

§ 6.6.3

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(2. 2)

— If V has partially-ordered initialization, W does not have unordered initialization, and V is defined before W in every translation unit in which W is defined, then

(2. 2. 1 )

— if the program starts a thread (4.7) other than the main thread (6.6.1) , the initialization of V strongly happens before the initialization of W;

(2. 2. 2)

— otherwise, the initialization of V is sequenced before the initialization of W.

(2. 3)

— Otherwise, if the program starts a thread other than the main thread before either V or W is initialized, it is unspecified in which threads the initializations of V and W occur; the initializations are unsequenced if they occur in the same thread.

(2. 4)

— Otherwise, the initializations of V and W are indeterminately sequenced. [ Note: This definition permits initialization of a sequence of ordered variables concurrently with another sequence. — end note ]

3

A non-initialization odr-use is an odr-use (6.2) not caused directly or indirectly by the initialization of a non-local static or thread storage duration variable.

4

It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized. 36 It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs. [ Note: Such points should be chosen in a way that allows the programmer to avoid deadlocks. — end note ] [ Example: // - File 1 #include " a. h" #include " b. h" B b; A: : A() { b. Use() ; } // - File 2 #include " a. h" A a; // - File 3 #include " a. h" #include " b. h" extern A a; extern B b; int main() { a. Use() ; b. Use() ; }

It is implementation-defined whether either a or b is initialized before main is entered or whether the initializations are delayed until a is first odr-used in main. In particular, if a is initialized before main is entered, it is not guaranteed that b will be initialized before it is odr-used by the initialization of a, that is, before A: : A is called. If, however, a is initialized at some point after the first statement of main, b will be initialized prior to its use in A: : A . — end example ] 36)

A non- lo cal variable wit h st at ic st orage durat ion having init ializat ion wit h side effect s is init ialized in t his case, even if it is

not it self o dr- used ( 6. 2 , 6. 7. 1 ) .

§ 6.6.3

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5

It is implementation-defined whether the dynamic initialization of a non-local inline variable with static storage duration is sequenced before the first statement of main or is deferred. If it is deferred, it strongly happens before any non-initialization odr-use of that variable. It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.

6

It is implementation-defined whether the dynamic initialization of a non-local non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred. If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration defined in the same translation unit as the variable to be initialized. It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.

7

If the initialization of a non-local variable with static or thread storage duration exits via an exception, std: : terminate is called (18.5.1) . 6. 6. 4

Termination

[basic. start. term]

1

Destructors (15.4) for initialized obj ects (that is, obj ects whose lifetime (6.8) has begun) with static storage duration, and functions registered with std: : atexit , are called as part of a call to std: : exit (21.5) . The call to std: : exit is sequenced before the invocations of the destructors and the registered functions. [ Note: Returning from main invokes std: : exit (6.6.1) . — end note ]

2

Destructors for initialized obj ects with thread storage duration within a given thread are called as a result of returning from the initial function of that thread and as a result of that thread calling std: : exit . The completions of the destructors for all initialized obj ects with thread storage duration within that thread strongly happen before the initiation of the destructors of any obj ect with static storage duration.

3

If the completion of the constructor or dynamic initialization of an obj ect with static storage duration strongly happens before that of another, the completion of the destructor of the second is sequenced before the initiation of the destructor of the first. If the completion of the constructor or dynamic initialization of an obj ect with thread storage duration is sequenced before that of another, the completion of the destructor of the second is sequenced before the initiation of the destructor of the first. If an object is initialized statically, the obj ect is destroyed in the same order as if the obj ect was dynamically initialized. For an obj ect of array or class type, all subobj ects of that obj ect are destroyed before any block-scope obj ect with static storage duration initialized during the construction of the subobj ects is destroyed. If the destruction of an obj ect with static or thread storage duration exits via an exception, std: : terminate is called (18.5.1) .

4

If a function contains a block-scope object of static or thread storage duration that has been destroyed and the function is called during the destruction of an object with static or thread storage duration, the program has undefined behavior if the flow of control passes through the definition of the previously destroyed block-scope object. Likewise, the behavior is undefined if the block-scope object is used indirectly (i.e., through a pointer) after its destruction.

5

If the completion of the initialization of an object with static storage duration strongly happens before a call to std: : atexit (see , 21.5) , the call to the function passed to std: : atexit is sequenced before the call to the destructor for the obj ect. If a call to std: : atexit strongly happens before the completion of the initialization of an object with static storage duration, the call to the destructor for the object is sequenced before the call to the function passed to std: : atexit . If a call to std: : atexit strongly happens before another call to std: : atexit , the call to the function passed to the second std: : atexit call is sequenced before the call to the function passed to the first std: : atexit call.

6

If there is a use of a standard library obj ect or function not permitted within signal handlers (21.10) that does not happen before (4.7) completion of destruction of objects with static storage duration and execution of std: : atexit registered functions (21.5) , the program has undefined behavior. [ Note: If there is a use of an obj ect with static storage duration that does not happen before the obj ect’s destruction, the program has undefined behavior. Terminating every thread before a call to std: : exit or the exit from main is § 6.6.4

© ISO/IEC 2017 – All rights reserved

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sufficient, but not necessary, to satisfy these requirements. These requirements permit thread managers as static-storage-duration obj ects. — end note ] 7

Calling the function std: : abort() declared in terminates the program without executing any destructors and without calling the functions passed to std: : atexit() or std: : at_quick_exit() . 6. 7

1

Storage duration

[basic. stc]

The storage duration is the property of an obj ect that defines the minimum potential lifetime of the storage containing the obj ect. The storage duration is determined by the construct used to create the obj ect and is one of the following:

(1. 1 )

— static storage duration

(1. 2)

— thread storage duration

(1. 3)

— automatic storage duration

(1. 4)

— dynamic storage duration

2

Static, thread, and automatic storage durations are associated with obj ects introduced by declarations (6.1) and implicitly created by the implementation (15.2) . The dynamic storage duration is associated with objects created by a new-expression (8.3.4) .

3

The storage duration categories apply to references as well.

4

When the end of the duration of a region of storage is reached, the values of all pointers representing the address of any part of that region of storage become invalid pointer values (6.9.2) . Indirection through an invalid pointer value and passing an invalid pointer value to a deallocation function have undefined behavior. Any other use of an invalid pointer value has implementation-defined behavior. 37

1

All variables which do not have dynamic storage duration, do not have thread storage duration, and are not local have static storage duration. The storage for these entities shall last for the duration of the program (6.6.2, 6.6.4) .

2

If a variable with static storage duration has initialization or a destructor with side effects, it shall not be eliminated even if it appears to be unused, except that a class obj ect or its copy/move may be eliminated as specified in 15.8.

3

The keyword static can be used to declare a local variable with static storage duration. [ Note: 9.7 describes the initialization of local static variables; 6.6.4 describes the destruction of local static variables. — end note ]

4

The keyword static applied to a class data member in a class definition gives the data member static storage duration.

6. 7. 1

6. 7. 2

Static storage duration

Thread storage duration

[basic. stc. static]

[basic. stc. thread]

1

All variables declared with the thread_local keyword have thread storage duration. The storage for these entities shall last for the duration of the thread in which they are created. There is a distinct obj ect or reference per thread, and use of the declared name refers to the entity associated with the current thread.

2

A variable with thread storage duration shall be initialized before its first odr-use (6.2) and, if constructed, shall be destroyed on thread exit. 37)

S ome implement at ions might define t hat copying an invalid p oint er value causes a syst em- generat ed runt ime fault .

§ 6.7.2

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6. 7. 3

Automatic storage duration

[basic. stc. auto]

1 Block-scope variables not explicitly declared static , thread_local , or extern have automatic storage duration. The storage for these entities lasts until the block in which they are created exits. 2 [ Note: These variables are initialized and destroyed as described in 9.7. — end note ] 3 If a variable with automatic storage duration has initialization or a destructor with side effects, an implementation shall not destroy it before the end of its block nor eliminate it as an optimization, even if it appears to be unused, except that a class obj ect or its copy/move may be eliminated as specified in 15.8.

6. 7. 4

Dynamic storage duration

[basic. stc. dynamic]

1 Obj ects can be created dynamically during program execution (4.6) , using new-expressions (8.3.4) , and destroyed using delete-expression s (8.3.5) . A C++ implementation provides access to, and management of, dynamic storage via the global allocation functions operator new and operator new[] and the global deallocation functions operator delete and operator delete[] . [ Note: The non-allocating forms described in 21.6.2.3 do not perform allocation or deallocation. — end note ] 2 The library provides default definitions for the global allocation and deallocation functions. Some global allocation and deallocation functions are replaceable (21.6.2) . A C ++ program shall provide at most one definition of a replaceable allocation or deallocation function. Any such function definition replaces the default version provided in the library (20.5.4.6) . The following allocation and deallocation functions (21.6) are implicitly declared in global scope in each translation unit of a program. void* operator new(std: : size_t) ; void* operator new(std: : size_t, std: : align_val_t) ; void void void void

operator operator operator operator

delete(void*) delete(void*, delete(void*, delete(void*,

noexcept; std: : size_t) noexcept; std: : align_val_t) noexcept; std: : size_t, std: : align_val_t) noexcept;

void* operator new[] (std: : size_t) ; void* operator new[] (std: : size_t, std: : align_val_t) ; void void void void

operator operator operator operator

delete[] (void*) delete[] (void*, delete[] (void*, delete[] (void*,

noexcept; std: : size_t) noexcept; std: : align_val_t) noexcept; std: : size_t, std: : align_val_t) noexcept;

These implicit declarations introduce only the function names operator new, operator new[] , operator delete , and operator delete[] . [ Note: The implicit declarations do not introduce the names std, std: : size_t , std: : align_val_t , or any other names that the library uses to declare these names. Thus, a new-expression, delete-expression or function call that refers to one of these functions without including the header is well-formed. However, referring to std or std: : size_t or std: : align_val_t is ill-formed unless the name has been declared by including the appropriate header. — end note ] Allocation and/or deallocation functions may also be declared and defined for any class (15.5) .

3 Any allocation and/or deallocation functions defined in a C++ program, including the default versions in the library, shall conform to the semantics specified in 6.7.4.1 and 6.7.4.2.

6. 7. 4. 1

Allocation functions

[basic.stc. dynamic.allocation]

1 An allocation function shall be a class member function or a global function; a program is ill-formed if an

allocation function is declared in a namespace scope other than global scope or declared static in global scope. The return type shall be void* . The first parameter shall have type std: : size_t (21.2) . The first parameter shall not have an associated default argument (11.3.6) . The value of the first parameter shall be § 6.7.4.1

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interpreted as the requested size of the allocation. An allocation function can be a function template. Such a template shall declare its return type and first parameter as specified above (that is, template parameter types shall not be used in the return type and first parameter type) . Template allocation functions shall have two or more parameters.

2 The allocation function attempts to allocate the requested amount of storage. If it is successful, it shall return

the address of the start of a block of storage whose length in bytes shall be at least as large as the requested size. There are no constraints on the contents of the allocated storage on return from the allocation function. The order, contiguity, and initial value of storage allocated by successive calls to an allocation function are unspecified. The pointer returned shall be suitably aligned so that it can be converted to a pointer to any suitable complete obj ect type (21.6.2.1) and then used to access the obj ect or array in the storage allocated (until the storage is explicitly deallocated by a call to a corresponding deallocation function) . Even if the size of the space requested is zero, the request can fail. If the request succeeds, the value returned shall be a non-null pointer value (7.11) p0 different from any previously returned value p1 , unless that value p1 was subsequently passed to an operator delete . Furthermore, for the library allocation functions in 21.6.2.1 and 21.6.2.2, p0 shall represent the address of a block of storage disj oint from the storage for any other object accessible to the caller. The effect of indirecting through a pointer returned as a request for zero size is undefined. 38

3 An allocation function that fails to allocate storage can invoke the currently installed new-handler function (21.6.3.3) , if any. [ Note: A program-supplied allocation function can obtain the address of the currently installed new_handler using the std: : get_new_handler function (21.6.3.4) . — end note ] If an allocation function that has a non-throwing exception specification (18.4) fails to allocate storage, it shall return a null pointer. Any other allocation function that fails to allocate storage shall indicate failure only by throwing an exception (18.1) of a type that would match a handler (18.3) of type std: : bad_alloc (21.6.3.1) .

4 A global allocation function is only called as the result of a new expression (8.3.4) , or called directly using the function call syntax (8.2.2) , or called indirectly through calls to the functions in the C++ standard library. [ Note: In particular, a global allocation function is not called to allocate storage for obj ects with static storage duration (6.7.1) , for obj ects or references with thread storage duration (6.7.2) , for obj ects of type std: : type_info (8.2.8) , or for an exception obj ect (18.1) . — end note ] 6. 7. 4. 2

Deallocation functions

[basic. stc.dynamic. deallocation]

1 Deallocation functions shall be class member functions or global functions; a program is ill-formed if deallocation functions are declared in a namespace scope other than global scope or declared static in global scope.

2 Each deallocation function shall return void and its first parameter shall be void* . A deallocation function may have more than one parameter. A usual deallocation function is a deallocation function that has: (2.1) — exactly one parameter; or (2.2) — exactly two parameters, the type of the second being either std: : align_val_t or std: : size_t 39 ; or (2.3) — exactly three parameters, the type of the second being std: : size_t and the type of the third being std: : align_val_t .

A deallocation function may be an instance of a function template. Neither the first parameter nor the return type shall depend on a template parameter. [ Note: That is, a deallocation function template shall have a first parameter of type void* and a return type of void (as specified above) . — end note ] A deallocation function template shall have two or more function parameters. A template instance is never a usual deallocation function, regardless of its signature.

38)

The int ent is t o have

t ially t he same.

C

++

operator new()

implement able by calling

39) The global operator delete( void*, std: : size_t) precludes t, std: : size_t) as a placement allo cat ion funct ion ( C . 3. 2 ) .

§ 6.7.4.2

std: : malloc()

or

std: : calloc() ,

so t he rules are subst an-

differs from C in requiring a zero request t o ret urn a non- null p oint er. use of an allo cat ion funct ion

© ISO/IEC 2017 – All rights reserved

void operator new(std: : size_-

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3 If a deallocation function terminates by throwing an exception, the behavior is undefined. The value of the

first argument supplied to a deallocation function may be a null pointer value; if so, and if the deallocation function is one supplied in the standard library, the call has no effect.

4 If the argument given to a deallocation function in the standard library is a pointer that is not the null

pointer value (7.11) , the deallocation function shall deallocate the storage referenced by the pointer, ending the duration of the region of storage.

6. 7. 4. 3

Safely-derived pointers

[basic. stc.dynamic. safety]

1 A traceable pointer object is (1.1) — an obj ect of an obj ect pointer type (6.9.2) , or (1.2) — an obj ect of an integral type that is at least as large as std: : intptr_t , or (1.3) — a sequence of elements in an array of narrow character type (6.9.1) , where the size and alignment of the sequence match those of some obj ect pointer type.

2 A pointer value is a safely-derived pointer to a dynamic obj ect only if it has an obj ect pointer type and it is one of the following:

(2.1) (2.2) (2.3) (2.4) (2.5) (2.6) (2.7)

— the value returned by a call to the C++ standard library implementation of : : operator new(std: : size_t) or : : operator new(std: : size_t, std: : align_val_t) ; 40 — the result of taking the address of an object (or one of its subobjects) designated by an lvalue resulting from indirection through a safely-derived pointer value; — the result of well-defined pointer arithmetic (8.7) using a safely-derived pointer value; — the result of a well-defined pointer conversion (7.11, 8.4) of a safely-derived pointer value; — the result of a reinterpret_cast of a safely-derived pointer value; — the result of a reinterpret_cast of an integer representation of a safely-derived pointer value; — the value of an object whose value was copied from a traceable pointer object, where at the time of the copy the source obj ect contained a copy of a safely-derived pointer value.

3 An integer value is an integer representation of a safely-derived pointer only if its type is at least as large as (3.1) (3.2) (3.3) (3.4)

std: : intptr_t and it is one of the following:

— the result of a reinterpret_cast of a safely-derived pointer value; — the result of a valid conversion of an integer representation of a safely-derived pointer value; — the value of an object whose value was copied from a traceable pointer object, where at the time of the copy the source obj ect contained an integer representation of a safely-derived pointer value; — the result of an additive or bitwise operation, one of whose operands is an integer representation of a safely-derived pointer value P , if that result converted by reinterpret_cast would compare equal to a safely-derived pointer computable from reinterpret_cast(P) .

4 An implementation may have relaxed pointer safety, in which case the validity of a pointer value does not depend on whether it is a safely-derived pointer value. Alternatively, an implementation may have strict pointer safety, in which case a pointer value referring to an object with dynamic storage duration that is not

a safely-derived pointer value is an invalid pointer value unless the referenced complete object has previously been declared reachable (23.10.4) . [ Note: The effect of using an invalid pointer value (including passing it to a deallocation function) is undefined, see 6.7. This is true even if the unsafely-derived pointer value might

40)

This sect ion do es not imp ose rest rict ions on indirect ion t hrough p oint ers t o memory not allo cat ed by

This maint ains t he ability of many C

++

part icular, t his applies t o C binaries, b ecause indirect ion t hrough p oint ers t o memory allo cat ed by

§ 6.7.4.3

: : operator new.

implement at ions t o use binary libraries and comp onent s writ t en in ot her languages.

© ISO/IEC 2017 – All rights reserved

std: : malloc

In

is not rest rict ed.

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compare equal to some safely-derived pointer value. implementation has relaxed or strict pointer safety. 6. 7. 5 1

Duration of subobj ects

[basic. stc. inherit]

The storage duration of subobj ects and reference members is that of their complete obj ect (4.5) . 6. 8

1

— end note ] It is implementation-defined whether an

Obj ect lifetime

[basic. life]

lifetime of an object or reference is a runtime property of the object or reference. An object is said to have non-vacuous initialization if it is of a class or aggregate type and it or one of its subobjects is initialized by a constructor other than a trivial default constructor. [ Note: Initialization by a trivial copy/move constructor is non-vacuous initialization. — end note ] The lifetime of an obj ect of type T begins when: The

(1. 1 )

— storage with the proper alignment and size for type T is obtained, and

(1. 2)

— if the obj ect has non-vacuous initialization, its initialization is complete, except that if the object is a union member or subobject thereof, its lifetime only begins if that union member is the initialized member in the union (11.6.1, 15.6.2) , or as described in 12.3. The lifetime of an obj ect o of type T ends when:

(1. 3)

— if T is a class type with a non-trivial destructor (15.4) , the destructor call starts, or

(1. 4)

— the storage which the obj ect occupies is released, or is reused by an obj ect that is not nested within o (4.5) .

2

The lifetime of a reference begins when its initialization is complete. The lifetime of a reference ends as if it were a scalar obj ect.

3

[ Note: 15.6.2 describes the lifetime of base and member subobj ects.

4

The properties ascribed to obj ects and references throughout this document apply for a given obj ect or reference only during its lifetime. [ Note: In particular, before the lifetime of an obj ect starts and after its lifetime ends there are significant restrictions on the use of the obj ect, as described below, in 15.6.2 and in 15.7. Also, the behavior of an obj ect under construction and destruction might not be the same as the behavior of an obj ect whose lifetime has started and not ended. 15.6.2 and 15.7 describe the behavior of obj ects during the construction and destruction phases. — end note ]

5

A program may end the lifetime of any object by reusing the storage which the object occupies or by explicitly calling the destructor for an object of a class type with a non-trivial destructor. For an object of a class type with a non-trivial destructor, the program is not required to call the destructor explicitly before the storage which the obj ect occupies is reused or released; however, if there is no explicit call to the destructor or if a delete-expression (8.3.5) is not used to release the storage, the destructor shall not be implicitly called and any program that depends on the side effects produced by the destructor has undefined behavior.

6

Before the lifetime of an obj ect has started but after the storage which the obj ect will occupy has been allocated 41 or, after the lifetime of an obj ect has ended and before the storage which the obj ect occupied is reused or released, any pointer that represents the address of the storage location where the object will be or was located may be used but only in limited ways. For an obj ect under construction or destruction, see 15.7. Otherwise, such a pointer refers to allocated storage (6.7.4.2) , and using the pointer as if the pointer were of type void* , is well-defined. Indirection through such a pointer is permitted but the resulting lvalue may only be used in limited ways, as described below. The program has undefined behavior if:

(6. 1 )

— the obj ect will be or was of a class type with a non-trivial destructor and the pointer is used as the operand of a delete-expression ,

(6. 2)

— the pointer is used to access a non-static data member or call a non-static member function of the obj ect, or 41 )

— end note ]

For example, b efore t he const ruct ion of a global ob j ect of non- P O D class typ e (1 5. 7) .

§ 6.8

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(6. 3)

— the pointer is implicitly converted (7.11) to a pointer to a virtual base class, or

(6. 4)

— the pointer is used as the operand of a static_cast (8.2.9) , except when the conversion is to pointer to cv void, or to pointer to cv void and subsequently to pointer to cv char , cv unsigned char , or cv std: : byte (21.2.1) , or

(6. 5)

— the pointer is used as the operand of a dynamic_cast (8.2.7) . [ Example: #include struct B { virtual void f() ; void mutate() ; virtual ~ B() ; }; struct D1 : B { void f() ; }; struct D2 : B { void f() ; }; void B: : mutate() { new (this) D2; f() ; . . . = this; }

// reuses storage — ends the lifetime of *this // undefined behavior // OK, this points to valid memory

void g() { void* p = std: : malloc(sizeof(D1) + sizeof(D2) ) ; B* pb = new (p) D1; pb->mutate() ; *pb; // OK: pb points to valid memory void* q = pb; // OK: pb points to valid memory pb->f() ; // undefined behavior, lifetime of *pb has ended }

— end example ] 7

Similarly, before the lifetime of an obj ect has started but after the storage which the obj ect will occupy has been allocated or, after the lifetime of an obj ect has ended and before the storage which the obj ect occupied is reused or released, any glvalue that refers to the original obj ect may be used but only in limited ways. For an object under construction or destruction, see 15.7. Otherwise, such a glvalue refers to allocated storage (6.7.4.2) , and using the properties of the glvalue that do not depend on its value is well-defined. The program has undefined behavior if:

(7. 1 )

— the glvalue is used to access the obj ect, or

(7. 2)

— the glvalue is used to call a non-static member function of the obj ect, or

(7. 3)

— the glvalue is bound to a reference to a virtual base class (11.6.3) , or

(7. 4)

— the glvalue is used as the operand of a dynamic_cast (8.2.7) or as the operand of typeid.

8

If, after the lifetime of an obj ect has ended and before the storage which the obj ect occupied is reused or released, a new obj ect is created at the storage location which the original obj ect occupied, a pointer that pointed to the original obj ect, a reference that referred to the original obj ect, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new obj ect, if:

(8. 1 )

— the storage for the new obj ect exactly overlays the storage location which the original obj ect occupied, § 6.8

© ISO/IEC 2017 – All rights reserved

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ISO/IEC 14882: 2017(E)

and (8. 2)

— the new obj ect is of the same type as the original obj ect (ignoring the top-level cv-qualifiers) , and

(8. 3)

— the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type, and

(8. 4)

— the original obj ect was a most derived obj ect (4.5) of type T and the new obj ect is a most derived obj ect of type T (that is, they are not base class subobj ects) . [ Example: struct C { int i; void f() ; const C& operator=( const C& ) ; }; const C& C: : operator=( const C& other) { if ( this ! = &other ) { // lifetime of *this ends this-> ~ C() ; new (this) C(other) ; // new object of type C created f() ; // well-defined } return *this; } C c1; C c2; c1 = c2; c1. f() ;

// well-defined // well-defined; c1 refers to a new object of type C

— end example ] [ Note: If these conditions are not met, a pointer to the new object can be obtained from a pointer that represents the address of its storage by calling std: : launder (21.6) . — end note ] 9

If a program ends the lifetime of an obj ect of type T with static (6.7.1) , thread (6.7.2) , or automatic (6.7.3) storage duration and if T has a non-trivial destructor, 42 the program must ensure that an obj ect of the original type occupies that same storage location when the implicit destructor call takes place; otherwise the behavior of the program is undefined. This is true even if the block is exited with an exception. [ Example: class T { }; struct B { ~ B() ; }; void h() { B b; new (&b) T; }

// undefined behavior at block exit

— end example ] 10

Creating a new obj ect within the storage that a const complete obj ect with static, thread, or automatic storage duration occupies, or within the storage that such a const obj ect used to occupy before its lifetime ended, results in undefined behavior. [ Example: struct B { 42)

That is, an ob j ect for which a dest ruct or will b e called implicit ly—up on exit from t he blo ck for an ob j ect wit h aut omat ic

st orage durat ion, up on exit from t he t hread for an ob j ect wit h t hread st orage durat ion, or up on exit from t he program for an ob j ect wit h st at ic st orage durat ion.

§ 6.8

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};

B() ; ~ B() ;

const B b; void h() { b. ~ B() ; new (const_cast(&b) ) const B; }

// undefined behavior

— end example ] 11

In this section, “before” and “after” refer to the “happens before” relation (4.7) . [ Note: Therefore, undefined behavior results if an obj ect that is being constructed in one thread is referenced from another thread without adequate synchronization. — end note ] 6. 9

Types

[basic. types]

1

[ Note: 6.9 and the subclauses thereof impose requirements on implementations regarding the representation of types. There are two kinds of types: fundamental types and compound types. Types describe obj ects (4.5) , references (11.3.2) , or functions (11.3.5) . — end note ]

2

For any obj ect (other than a base-class subobj ect) of trivially copyable type T , whether or not the obj ect holds a valid value of type T , the underlying bytes (4.4) making up the obj ect can be copied into an array of char , unsigned char , or std: : byte (21.2.1) . 43 If the content of that array is copied back into the obj ect, the obj ect shall subsequently hold its original value. [ Example: #define N sizeof(T) char buf[N] ; T obj ; std: : memcpy(buf, &obj , N) ; std: : memcpy(&obj , buf, N) ;

// obj initialized to its original value // between these two calls to std: : memcpy, obj might be modified // at this point, each subobject of obj of scalar type holds its original value

— end example ] 3

For any trivially copyable type T , if two pointers to T point to distinct T objects obj 1 and obj 2 , where neither obj 1 nor obj 2 is a base-class subobject, if the underlying bytes (4.4) making up obj 1 are copied into obj 2 , 44 obj 2 shall subsequently hold the same value as obj 1 . [ Example: T* t1p; T* t2p; // provided that t2p points to an initialized object ... std: : memcpy(t1p, t2p, sizeof(T) ) ; // at this point, every subobject of trivially copyable type in *t1p contains // the same value as the corresponding subobject in *t2p

4

— end example ] The object representation of an object of type T is the sequence of N unsigned char objects taken up by the obj ect of type T , where N equals sizeof(T) . The value representation of an obj ect is the set of bits that

hold the value of type T . For trivially copyable types, the value representation is a set of bits in the obj ect representation that determines a value , which is one discrete element of an implementation-defined set of values. 45 5

A class that has been declared but not defined, an enumeration type in certain contexts (10.2) , or an array of 43) 44) 45)

B y using, for example, t he library funct ions ( 20. 5. 1 . 2 ) B y using, for example, t he library funct ions ( 20. 5. 1 . 2 ) The int ent is t hat t he memory mo del of C

§ 6.9

++

std: : memcpy std: : memcpy

or or

std: : memmove . std: : memmove .

is compat ible wit h t hat of IS O / IEC 9899 P rogramming Language C .

© ISO/IEC 2017 – All rights reserved

77

ISO/IEC 14882: 2017(E)

unknown b ound or of incomp let e element ob j ect

cv void

typ es and

typ e,

incomplete types

are

is an

incompletely-defined object type 46 .

( 6. 9. 1 ) .

O b j ect s shall not

Incomplet ely- defined

b e defined t o have an incomp let e

typ e.

6

class X class X

A class typ e ( such as “ on; an

t he

typ e

array

“

”

of incomplet e

t ranslat ion unit ,

is

t he

class

”)

typ e

in a t ranslat ion unit

b ound of

T

”

and

at

b ot h

t herefore

might

NT typedef

”)

one p oint

p oint s.

The

incomplet e;

in a t ranslat ion unit

declared

if t he

class

t he array typ e at

typ e

is

and complet e lat er

array

ob j ect

complet ed

might

lat er

on

in

t hos e two p oint s is t he same typ e.

are different

t he array typ es at

typ es .

be t he

The one

t hose two p oint s ( “ array of unknown

The typ e of a p oint er t o array of unknown b ound, cannot

b e complet ed.

[

or of

Example:

// X is an incomplete type // xp is a pointer to an incomplete type // the type of arr is incomplete // UNKA is an incomplete type // arrp is a pointer to an incomplete type

void foo() { xp++; arrp++; arrpp++; }

// ill-formed: X is incomplete // ill-formed: incomplete type // OK: sizeof UNKA* is known

struct X { int i; }; int arr[10] ;

// now X is a complete type // now the type of arr is complete

X x; void bar() { xp = &x; arrp = &arr; xp++; arrp++; }

[

of an

declarat ion t o b e an array of unknown b ound,

class X; extern X* xp; extern int arr[] ; typedef int UNKA[] ; UNKA* arrp; UNKA** arrpp;

— end example Note: — end note object type cv void

typ e

b e an array of unknown b ound and t herefore b e incomplet e at

and complet e lat er on;

and “ array of

a typ e defined by a

7

b e incomplet e at

typ e

t he array typ e b ecomes complet e;

declared typ e of an array ob j ect p oint

might

same

// OK; type is “pointer to X ” // ill-formed: different types // OK: X is complete // ill-formed: UNKA can’t be completed

]

The rules for declarat ions and expres sions describ e in which cont ext s incomplet e typ es are prohibit ed. ]

8

An

is

a ( p ossibly

cv- qualified)

typ e

t hat

is

not

a funct ion

typ e,

not

a reference

typ e,

and

not

.

9

Arit hmet ic typ es ( 6 . 9 . 1 ) ,

t

,

and

cv- qualified

classes ( C lause 1 2 ) ,

types

.

enumerat ion typ es ,

(6. 9. 3)

versions

p oint er typ es,

of t hese

typ es

are

C v- unqualified

typ es ( C lause 1 2 ) , .

S calar typ es,

s calar

typ es,

A typ e is a

46)

scalar types

.

std: : nullptr_-

S calar

typ es,

t rivially

copyable

class

typ es

( C lause

1 2) ,

trivially copyable types

arrays .

of such

POD

S calar typ es,

st andard- layout

literal type

if it

class typ es ( C lause 1 2 ) ,

standard-layout types

POD

typ es ,

and

t rivial class

arrays of such typ es and cv- qualified versions of t hese typ es are collect ively called

t hese typ es are collect ively called

10

called

arrays of such typ es and cv- qualified versions of t hese typ es are collect ively called

cv- qualified versions of t hese typ es are collect ively called

types

p oint er t o memb er typ es (6 . 9 . 2 ) ,

collect ively

trivial

arrays of such typ es and cv- qualified vers ions of

.

is:

The size and layout of an inst ance of an incomplet ely- defined ob j ect typ e is unknown.

§ 6. 9

© IS O / IEC

2 0 1 7 – All right s reserved

78

ISO/IEC 14882: 2017(E)

(10. 1)

— possibly cv-qualified void; or

(10. 2)

— a scalar type; or

(10. 3)

— a reference type; or

(10. 4)

— an array of literal type; or

(10. 5)

— a possibly cv-qualified class type (Clause 12) that has all of the following properties:

(10. 5. 1)

— it has a trivial destructor,

(10. 5. 2)

— it is either a closure type (8.1.5.1) , an aggregate type (11.6.1) , or has at least one constexpr constructor or constructor template (possibly inherited (10.3.3) from a base class) that is not a copy or move constructor,

(10. 5. 3)

— if it is a union, at least one of its non-static data members is of non-volatile literal type, and

(10. 5. 4)

— if it is not a union, all of its non-static data members and base classes are of non-volatile literal types. [ Note: A literal type is one for which it might be possible to create an obj ect within a constant expression. It is not a guarantee that it is possible to create such an obj ect, nor is it a guarantee that any obj ect of that type will usable in a constant expression. — end note ]

11

Two types cv1 T1 and cv2 T2 are layout-compatible types if T1 and T2 are the same type, layout-compatible enumerations (10.2) , or layout-compatible standard-layout class types (12.2) .

1

Objects declared as characters ( char ) shall be large enough to store any member of the implementation’s basic character set. If a character from this set is stored in a character obj ect, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a char obj ect can hold negative values. Characters can be explicitly declared unsigned or signed. Plain char , signed char , and unsigned char are three distinct types, collectively called narrow character types. A char , a signed char , and an unsigned char occupy the same amount of storage and have the same alignment requirements (6.11) ; that is, they have the same object representation. For narrow character types, all bits of the object representation participate in the value representation. [ Note: A bit-field of narrow character type whose length is larger than the number of bits in the obj ect representation of that type has padding bits; see 12.2.4. — end note ] For unsigned narrow character types, each possible bit pattern of the value representation represents a distinct number. These requirements do not hold for other types. In any particular implementation, a plain char obj ect can take on either the same values as a signed char or an unsigned char ; which one is implementation-defined. For each value i of type unsigned char in the range 0 to 255 inclusive, there exists a value j of type char such that the result of an integral conversion (7.8) from i to char is j, and the result of an integral conversion from j to unsigned char is i.

2

There are five standard signed integer types : “ signed char ”, “ short int ”, “ int ”, “ long int ”, and “ long long int ”. In this list, each type provides at least as much storage as those preceding it in the list. There may also be implementation-defined extended signed integer types . The standard and extended signed integer types are collectively called signed integer types . Plain int s have the natural size suggested by the architecture of the execution environment 47 ; the other signed integer types are provided to meet special needs.

3

For each of the standard signed integer types, there exists a corresponding (but different) standard unsigned integer type: “ unsigned char ”, “ unsigned short int ”, “ unsigned int ”, “ unsigned long int ”,

6. 9. 1

Fundamental types

[basic. fundamental]

and “ unsigned long long int ”, each of which occupies the same amount of storage and has the same alignment requirements (6.11) as the corresponding signed integer type 48 ; that is, each signed integer type has the same object representation as its corresponding unsigned integer type. Likewise, for each of the extended 47) 48)

int

must also b e large enough t o cont ain any value in t he range

[INT_MIN, INT_MAX] ,

S ee 1 0. 1 . 7. 2 regarding t he corresp ondence b etween typ es and t he sequences of

§ 6.9.1

as defined in t he header

type-specifiers

© ISO/IEC 2017 – All rights reserved

.

t hat designat e t hem.

79

ISO/IEC 14882: 2017(E)

4

signed integer types there exists a corresponding extended unsigned integer type with the same amount of storage and alignment requirements. The standard and extended unsigned integer types are collectively called unsigned integer types. The range of non-negative values of a signed integer type is a subrange of the corresponding unsigned integer type, the representation of the same value in each of the two types is the same, and the value representation of each corresponding signed/unsigned type shall be the same. The standard signed integer types and standard unsigned integer types are collectively called the standard integer types, and the extended signed integer types and extended unsigned integer types are collectively called the extended integer types. The signed and unsigned integer types shall satisfy the constraints given in the C standard, section 5.2.4.2.1. Unsigned integers shall obey the laws of arithmetic modulo 2 n where n is the number of bits in the value representation of that particular size of integer. 49

5

Type wchar_t is a distinct type whose values can represent distinct codes for all members of the largest extended character set specified among the supported locales (25.3.1) . Type wchar_t shall have the same size, signedness, and alignment requirements (6.11) as one of the other integral types, called its underlying type. Types char16_t and char32_t denote distinct types with the same size, signedness, and alignment as uint_least16_t and uint_least32_t , respectively, in , called the underlying types.

6

Values of type bool are either true or false . 50 [ Note: There are no signed, unsigned, short , or long bool types or values. — end note ] Values of type bool participate in integral promotions (7.6) .

7

Types bool , char , char16_t , char32_t , wchar_t , and the signed and unsigned integer types are collectively called integral types. 51 A synonym for integral type is integer type. The representations of integral types shall define values by use of a pure binary numeration system. 52 [ Example: This document permits two’s complement, ones’ complement and signed magnitude representations for integral types. — end example ]

8

There are three floating-point types: float , double , and long double . The type double provides at least as much precision as float , and the type long double provides at least as much precision as double . The set of values of the type float is a subset of the set of values of the type double ; the set of values of the type double is a subset of the set of values of the type long double . The value representation of floating-point types is implementation-defined. [ Note: This document imposes no requirements on the accuracy of floating-point operations; see also 21.3. — end note ] Integral and floating types are collectively called arithmetic types. Specializations of the standard library template std: : numeric_limits (21.3) shall specify the maximum and minimum values of each arithmetic type for an implementation.

9

A type cv void is an incomplete type that cannot be completed; such a type has an empty set of values. It is used as the return type for functions that do not return a value. Any expression can be explicitly converted to type cv void (8.4) . An expression of type cv void shall be used only as an expression statement (9.2) , as an operand of a comma expression (8.19) , as a second or third operand of ?: (8.16) , as the operand of typeid, noexcept , or decltype , as the expression in a return statement (9.6.3) for a function with the return type cv void, or as the operand of an explicit conversion to type cv void.

10

A value of type std: : nullptr_t is a null pointer constant (7.11) . Such values participate in the pointer and the pointer to member conversions (7.11, 7.12) . sizeof(std: : nullptr_t) shall be equal to sizeof(void*) .

11

[ Note: Even if the implementation defines two or more basic types to have the same value representation, they are nevertheless different types. — end note ] 49)

This implies t hat unsigned arit hmet ic do es not overflow b ecause a result t hat cannot b e represent ed by t he result ing

unsigned int eger typ e is reduced mo dulo t he numb er t hat is one great er t han t he largest value t hat can b e represent ed by t he result ing unsigned int eger typ e.

50)

Using a

bool

value in ways describ ed by t his do cument as “ undefined” , such as by examining t he value of an uninit ialized

aut omat ic ob j ect , might cause it t o b ehave as if it is neit her

51 ) 52)

true

nor

false .

Therefore, enumerat ions ( 1 0. 2 ) are not int egral; however, enumerat ions can b e promot ed t o int egral typ es as sp ecified in 7. 6. A p osit ional represent at ion for int egers t hat uses t he binary digit s 0 and 1 , in which t he values represent ed by successive

bit s are addit ive, b egin wit h 1 , and are mult iplied by successive int egral p ower of 2 , except p erhaps for t he bit wit h t he highest p osit ion.

§ 6.9.1

( Adapt ed from t he

American National Dictionary for Information Processing Systems. )

© ISO/IEC 2017 – All rights reserved

80

ISO/IEC 14882: 2017(E)

6. 9. 2 1 (1. 1) (1. 2)

Compound types

Compound types can be constructed in the following ways:

arrays of obj ects of a given type, 11.3.4; — functions , which have parameters of given types and return

—

type, 11.3.5;

(1. 3) (1. 4) (1. 4. 1 ) (1. 4. 2) (1. 5)

[basic. compound]

void or references or obj ects of a given

pointers to cv void or objects or functions (including static members of classes) of a given type, 11.3.1; — references to obj ects or functions of a given type, 11.3.2. There are two types of references: — lvalue reference — rvalue reference — classes containing a sequence of obj ects of various types (Clause 12) , a set of types, enumerations and —

functions for manipulating these objects (12.2.1) , and a set of restrictions on the access to these entities (Clause 14) ;

(1. 6) (1. 7)

(1. 8)

unions, which are classes capable of containing obj ects of different types at different times, 12.3; — enumerations , which comprise a set of named constant values. Each distinct enumeration constitutes a different enumerated type , 10.2; — pointers to non-static class members , 53 which identify members of a given type within obj ects of a

—

given class, 11.3.3.

2

These methods of constructing types can be applied recursively; restrictions are mentioned in 11.3.1, 11.3.4, 11.3.5, and 11.3.2. Constructing a type such that the number of bytes in its object representation exceeds the maximum value representable in the type std: : size_t (21.2) is ill-formed.

3

The type of a pointer to cv void or a pointer to an obj ect type is called an object pointer type. [ Note: A pointer to void does not have a pointer-to-obj ect type, however, because void is not an obj ect type. — end note ] The type of a pointer that can designate a function is called a function pointer type. A pointer to objects of type T is referred to as a “pointer to T ”. [ Example: A pointer to an object of type int is referred to as “pointer to int ” and a pointer to an obj ect of class X is called a “pointer to X ”. — end example ] Except for pointers to static members, text referring to “pointers” does not apply to pointers to members. Pointers to incomplete types are allowed although there are restrictions on what can be done with them (6.11) . Every value of pointer type is one of the following:

(3. 1 )

pointer to an obj ect or function (the pointer is said to point to the obj ect or function) , or — a pointer past the end of an obj ect (8.7) , or — the null pointer value (7.11) for that type, or — an invalid pointer value. A value of a pointer type that is a pointer to or past the end of an obj ect represents the address of the

(3. 2) (3. 3) (3. 4)

— a

first byte in memory (4.4) occupied by the obj ect 54 or the first byte in memory after the end of the storage occupied by the obj ect, respectively. [ Note: A pointer past the end of an obj ect (8.7) is not considered to point to an unrelated obj ect of the obj ect’s type that might be located at that address. A pointer value becomes invalid when the storage it denotes reaches the end of its storage duration; see 6.7. — end note ] For purposes of pointer arithmetic (8.7) and comparison (8.9, 8.10) , a pointer past the end of the last element of an array x of n elements is considered to be equivalent to a pointer to a hypothetical element x[ n ] . The value representation of pointer types is implementation-defined. Pointers to layout-compatible types shall have the same value representation and alignment requirements (6.11) . [ Note: Pointers to over-aligned types (6.11) have no special representation, but their range of valid values is restricted by the extended 53) 54)

S t at ic class memb ers are ob j ect s or funct ions, and p oint ers t o t hem are ordinary p oint ers t o ob j ect s or funct ions. For an ob j ect t hat is not wit hin it s lifet ime, t his is t he first byt e in memory t hat it will o ccupy or used t o o ccupy.

§ 6.9.2

© ISO/IEC 2017 – All rights reserved

81

ISO/IEC 14882: 2017(E)

alignment requirement. — end 4

note ] Two obj ects a and b are pointer-interconvertible if:

(4. 1)

— they are the same obj ect, or

(4. 2)

— one is a standard-layout union obj ect and the other is a non-static data member of that obj ect (12.3) , or

(4. 3)

— one is a standard-layout class obj ect and the other is the first non-static data member of that obj ect, or, if the object has no non-static data members, the first base class subobject of that object (12.2) , or

(4. 4)

— there exists an obj ect interconvertible.

c such that a and c are pointer-interconvertible, and c and b are pointer-

If two obj ects are pointer-interconvertible, then they have the same address, and it is possible to obtain a pointer to one from a pointer to the other via a reinterpret_cast (8.2.10) . [ Note: An array object and its first element are not pointer-interconvertible, even though they have the same address. — end note ] 5

A pointer to cv -qualified (6.9.3) or cv-unqualified void can be used to point to obj ects of unknown type. Such a pointer shall be able to hold any obj ect pointer. An obj ect of type cv void* shall have the same representation and alignment requirements as cv char* . 6. 9. 3

1

(1. 1 ) (1. 2)

CV-qualifiers

[basic. type. qualifier]

A type mentioned in 6.9.1 and 6.9.2 is a cv-unqualified type. Each type which is a cv-unqualified complete or incomplete obj ect type or is void (6.9) has three corresponding cv-qualified versions of its type: a constqualified version, a volatile-qualified version, and a const-volatile-qualified version. The type of an object (4.5) includes the cv-qualifier s specified in the decl-specifier-seq (10.1) , declarator (Clause 11) , type-id (11.1) , or new-type-id (8.3.4) when the obj ect is created.

const object is an obj ect of type const T or a non-mutable subobj ect of such an obj ect. — A volatile object is an object of type volatile T , a subobject of such an object, or a mutable subobj ect

— A

of a const volatile obj ect.

(1. 3)

— A const volatile object is an obj ect of type const volatile T , a non-mutable subobj ect of such an obj ect, a const subobj ect of a volatile obj ect, or a non-mutable volatile subobj ect of a const obj ect. The cv-qualified or cv-unqualified versions of a type are distinct types; however, they shall have the same representation and alignment requirements (6.11) . 55

2

A compound type (6.9.2) is not cv-qualified by the cv-qualifiers (if any) of the types from which it is compounded. Any cv-qualifiers applied to an array type affect the array element type (11.3.4) .

3

See 11.3.5 and 12.2.2.1 regarding function types that have

4

There is a partial ordering on cv-qualifiers, so that a type can be said to be Table 10 shows the relations that constitute this ordering.

cv-qualifiers. more cv-qualified than another.

Table 10 — Relations on const and volatile

no cv-qualifier no cv-qualifier no cv-qualifier const volatile

55)

< < < <
0, where each cvi is a set of cv-qualifiers (6.9.3) , and each Pi is “pointer to” (11.3.1) , “pointer to member of class Ci of type” (11.3.3) , “array of Ni ”, or “array of unknown bound of” (11.3.4) . If Pi designates an array, the cv-qualifiers cvi +1 on the element type are also taken as the cv-qualifiers cvi of the array. [ Example: The type denoted by the type-id const int ** has two cv-decompositions, taking U as “ int ” and as “pointer to const int ”. — end example ] The n-tuple of cv-qualifiers after the first one in the longest cv-decomposition of T, that is, cv1 , cv2 , . . . , cvn , is called the cv-qualification signature of T . Two types T 1 and T2 are similar if they have cv-decompositions with the same n such that corresponding Pi “

2

Pi such that T is

[conv. qual]

· · ·

components are the same and the types denoted by U are the same.

3 A prvalue expression of type T1 can be converted to type T2 if the following conditions are satisfied, where cvij denotes the cv-qualifiers in the cv-qualification signature of Tj : 60

(3.1) (3.2) (3.3)

— T 1 and T2 are similar.

i > 0, if const is in cv1i then const is in cv2i , and similarly for volatile . — If the cv1i and cv 2i are different, then const is in every cv2k for 0 < k < i .

— For every

[ Note: If a program could assign a pointer of type T** to a pointer of type const T** (that is, if line #1 below were allowed) , a program could inadvertently modify a const obj ect (as it is done on line #2) . For example, int main() { const char c = ’ c’ ; char* pc; const char** pcc = &pc; *pcc = &c; *pc = ’ C’ ; }

// #1: not allowed // #2: modifies a const object

— end note ] 59)

This conversion never applies t o non- st at ic memb er funct ions b ecause an lvalue t hat refers t o a non- st at ic memb er funct ion

cannot b e obt ained.

60)

These rules ensure t hat const - safety is preserved by t he conversion.

§ 7.5

© ISO/IEC 2017 – All rights reserved

88

ISO/IEC 14882: 2017(E)

4

[ Note: A prvalue of type “pointer to cv1 T ” can be converted to a prvalue of type “pointer to cv2 T ” if “ cv2 T ” is more cv-qualified than “ cv1 T”. A prvalue of type “pointer to member of X of type cv1 T ” can be converted to a prvalue of type “pointer to member of X of type cv2 T ” if “ cv2 T ” is more cv-qualified than “ cv1 T”. — end note ]

5

[ Note: Function types (including those used in pointer to member function types) are never cv-qualified (11.3.5) . — end note ] 7. 6

Integral promotions

[conv. prom]

1

A prvalue of an integer type other than bool , char16_t , char32_t , or wchar_t whose integer conversion rank (7.15) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int .

2

A prvalue of type char16_t , char32_t , or wchar_t (6.9.1) can be converted to a prvalue of the first of the following types that can represent all the values of its underlying type: int , unsigned int , long int , unsigned long int , long long int , or unsigned long long int . If none of the types in that list can represent all the values of its underlying type, a prvalue of type char16_t , char32_t , or wchar_t can be converted to a prvalue of its underlying type.

3

A prvalue of an unscoped enumeration type whose underlying type is not fixed (10.2) can be converted to a prvalue of the first of the following types that can represent all the values of the enumeration (i.e., the values in the range bmin to bmax as described in 10.2) : int , unsigned int , long int , unsigned long int , long long int , or unsigned long long int . If none of the types in that list can represent all the values of the enumeration, a prvalue of an unscoped enumeration type can be converted to a prvalue of the extended integer type with lowest integer conversion rank (7.15) greater than the rank of long long in which all the values of the enumeration can be represented. If there are two such extended types, the signed one is chosen.

4

A prvalue of an unscoped enumeration type whose underlying type is fixed (10.2) can be converted to a prvalue of its underlying type. Moreover, if integral promotion can be applied to its underlying type, a prvalue of an unscoped enumeration type whose underlying type is fixed can also be converted to a prvalue of the promoted underlying type.

5

A prvalue for an integral bit-field (12.2.4) can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field. If the bit-field is larger yet, no integral promotion applies to it. If the bit-field has an enumerated type, it is treated as any other value of that type for promotion purposes.

6

A prvalue of type bool can be converted to a prvalue of type int , with false becoming zero and true becoming one.

7

These conversions are called 7. 7

integral promotions.

Floating-point promotion

[conv. fpprom]

1

A prvalue of type float can be converted to a prvalue of type double . The value is unchanged.

2

This conversion is called 7. 8

floating-point promotion.

Integral conversions

[conv. integral]

1

A prvalue of an integer type can be converted to a prvalue of another integer type. A prvalue of an unscoped enumeration type can be converted to a prvalue of an integer type.

2

If the destination type is unsigned, the resulting value is the least unsigned integer congruent to the source integer (modulo 2 n where n is the number of bits used to represent the unsigned type) . [ Note: In a two’s complement representation, this conversion is conceptual and there is no change in the bit pattern (if there is no truncation) . — end note ]

§ 7.8

© ISO/IEC 2017 – All rights reserved

89

ISO/IEC 14882: 2017(E)

3

If the destination type is signed, the value is unchanged if it can be represented in the destination type; otherwise, the value is implementation-defined.

4

If the destination type is bool , see 7.14. If the source type is bool , the value false is converted to zero and the value true is converted to one.

5

The conversions allowed as integral promotions are excluded from the set of integral conversions. 7. 9

Floating-point conversions

[conv. double]

1

A prvalue of floating-point type can be converted to a prvalue of another floating-point type. If the source value can be exactly represented in the destination type, the result of the conversion is that exact representation. If the source value is between two adj acent destination values, the result of the conversion is an implementation-defined choice of either of those values. Otherwise, the behavior is undefined.

2

The conversions allowed as floating-point promotions are excluded from the set of floating-point conversions. 7. 1 0

Floating-integral conversions

[conv. fpint]

1

A prvalue of a floating-point type can be converted to a prvalue of an integer type. The conversion truncates; that is, the fractional part is discarded. The behavior is undefined if the truncated value cannot be represented in the destination type. [ Note: If the destination type is bool , see 7.14. — end note ]

2

A prvalue of an integer type or of an unscoped enumeration type can be converted to a prvalue of a floatingpoint type. The result is exact if possible. If the value being converted is in the range of values that can be represented but the value cannot be represented exactly, it is an implementation-defined choice of either the next lower or higher representable value. [ Note: Loss of precision occurs if the integral value cannot be represented exactly as a value of the floating type. — end note ] If the value being converted is outside the range of values that can be represented, the behavior is undefined. If the source type is bool , the value false is converted to zero and the value true is converted to one. 7. 1 1

Pointer conversions

[conv. ptr]

1

A null pointer constant is an integer literal (5.13.2) with value zero or a prvalue of type std: : nullptr_t . A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type and is distinguishable from every other value of obj ect pointer or function pointer type. Such a conversion is called a null pointer conversion. Two null pointer values of the same type shall compare equal. The conversion of a null pointer constant to a pointer to cv-qualified type is a single conversion, and not the sequence of a pointer conversion followed by a qualification conversion (7.5) . A null pointer constant of integral type can be converted to a prvalue of type std: : nullptr_t . [ Note: The resulting prvalue is not a null pointer value. — end note ]

2

A prvalue of type “pointer to cv T ”, where T is an object type, can be converted to a prvalue of type “pointer to cv void”. The pointer value (6.9.2) is unchanged by this conversion.

3

A prvalue of type “pointer to cv D ”, where D is a class type, can be converted to a prvalue of type “pointer to cv B ”, where B is a base class (Clause 13) of D . If B is an inaccessible (Clause 14) or ambiguous (13.2) base

class of D , a program that necessitates this conversion is ill-formed. The result of the conversion is a pointer to the base class subobject of the derived class object. The null pointer value is converted to the null pointer value of the destination type.

7. 1 2 1

Pointer to member conversions

[conv. mem]

A null pointer constant (7.11) can be converted to a pointer to member type; the result is the null member pointer value of that type and is distinguishable from any pointer to member not created from a null pointer constant. Such a conversion is called a null member pointer conversion. Two null member pointer values of the same type shall compare equal. The conversion of a null pointer constant to a pointer to member of

§ 7.12

© ISO/IEC 2017 – All rights reserved

90

ISO/IEC 14882: 2017(E)

cv-qualified type is a single conversion, and not the sequence of a pointer to member conversion followed by a qualification conversion (7.5) . 2

A prvalue of type “pointer to member of B of type cv T”, where B is a class type, can be converted to a prvalue of type “pointer to member of D of type cv T ”, where D is a derived class (Clause 13) of B . If B is an inaccessible (Clause 14) , ambiguous (13.2) , or virtual (13.1) base class of D , or a base class of a virtual base class of D , a program that necessitates this conversion is ill-formed. The result of the conversion refers to the same member as the pointer to member before the conversion took place, but it refers to the base class member as if it were a member of the derived class. The result refers to the member in D ’s instance of B . Since the result has type “pointer to member of D of type cv T ”, indirection through it with a D obj ect is valid. The result is the same as if indirecting through the pointer to member of B with the B subobj ect of D . The null member pointer value is converted to the null member pointer value of the destination type. 61 7. 1 3

1

Function pointer conversions

[conv. fctptr]

A prvalue of type “pointer to noexcept function” can be converted to a prvalue of type “pointer to function”. The result is a pointer to the function. A prvalue of type “pointer to member of type noexcept function” can be converted to a prvalue of type “pointer to member of type function”. The result points to the member function. [ Example: void (*p) () ; void (**pp) () noexcept = &p;

// error: cannot convert to pointer to noexcept function

struct S { typedef void (*p) () ; operator p() ; }; void (*q) () noexcept = S() ; // error: cannot convert to pointer to noexcept function

— end example ] 7. 1 4 1

[conv. bool]

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool . A zero value, null pointer value, or null member pointer value is converted to false ; any other value is converted to true . For direct-initialization (11.6) , a prvalue of type std: : nullptr_t can be converted to a prvalue of type bool ; the resulting value is false . 7. 1 5

1

Boolean conversions

Integer conversion rank

Every integer type has an

[conv. rank]

integer conversion rank defined as follows:

(1. 1 )

— No two signed integer types other than char and signed char (if char is signed) shall have the same rank, even if they have the same representation.

(1. 2)

— The rank of a signed integer type shall be greater than the rank of any signed integer type with a smaller size.

(1. 3)

— The rank of long long int shall be greater than the rank of long int , which shall be greater than the rank of int , which shall be greater than the rank of short int , which shall be greater than the rank of signed char .

(1. 4)

— The rank of any unsigned integer type shall equal the rank of the corresponding signed integer type.

(1. 5)

— The rank of any standard integer type shall be greater than the rank of any extended integer type with the same size. 61 )

The rule for conversion of p oint ers t o memb ers ( from p oint er t o memb er of base t o p oint er t o memb er of derived) app ears

invert ed compared t o t he rule for p oint ers t o ob j ect s ( from p oint er t o derived t o p oint er t o base) ( 7. 1 1 , C lause 1 3) . is necessary t o ensure typ e safety.

This inversion

Not e t hat a p oint er t o memb er is not an ob j ect p oint er or a funct ion p oint er and t he rules for

conversions of such p oint ers do not apply t o p oint ers t o memb ers.

In part icular, a p oint er t o memb er cannot b e convert ed t o a

void* .

§ 7.15

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char

signed char

unsigned char

(1. 6)

—

The rank of

(1. 7)

—

The rank of

(1. 8)

—

The ranks of

(1. 9)

—

The rank of any ext ended signed int eger typ e relat ive t o anot her ext ended signed int eger typ e wit h t he same

siz e

is

bool

s hall equal t he rank of

and

.

s hall b e less t han t he rank of all ot her st andard int eger typ es.

char16_t char32_t ,

,

and

implement at ion- defined,

wchar_t

but

st ill

shall equal t he ranks of t heir underlying typ es (6 . 9 . 1 ) .

sub j ect

to

t he

ot her

rules

for

det ermining t he

int eger

conversion rank.

(1. 10)

—

For all int eger typ es t hen

[

Note:

T1

T1 T2 ,

,

T3

,

T1 T3

if

has great er rank t han

T2

and

T2

has great er rank t han

T3

,

.

The int eger conversion rank is used in t he definit ion of t he int egral promot ions (7. 6 )

arit hmet ic conversions ( C laus e 8 ) .

§ 7. 1 5

and

shall have great er rank t han

— end note

© IS O / IEC

and t he usual

]

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8 1

[

Expressions

Note:

C lause 8 defines t he synt ax,

[expr] order of evaluat ion,

s equence of op erat ors and op erands t hat

— end note

can cause side effect s.

2

[

Note:

1 2)

in 1 6 . 5 .

sp ecifies a comput at ion.

62

An exp ression is a

An exp ression can result

t hat is,

given meaning when applied t o exp ressions of class typ e ( C lause

Us es of overloaded op erat ors are t ransformed int o funct ion calls as describ ed

O verloaded op erat ors ob ey t he rules for synt ax and evaluat ion order sp ecified in C lause 8 ,

requirement s of op erand typ e and value cat egory are replaced by t he rules for funct ion call. op erat ors,

3

C laus e

in a value and

]

O p erat ors can b e overloaded,

or enumerat ion typ e ( 1 0 . 2 ) .

and meaning of expressions .

8

such as

defines

++a

t he

a+=1

meaning

effect s

,

are not

of op erat ors

O p erat or overloading shall not

when

applied

typ es for which t hey are defined by t his S t andard. resolut ion,

and as part

of t hat

— end note

guarant eed for overloaded op erat ors (1 6 . 5 ) .

mo dify t he rules for t he

to

typ es

for

which

built-in operators

However,

,

t hey t hat

have is,

but

t he

Relat ions b etween

not

b een

]

overloaded.

for op erat ors applied t o

t hese built - in op erat ors part icipat e in overload

pro cess user- defined conversions will b e considered where necessary t o convert

t he op erands t o typ es ap propriat e for t he built - in op erat or.

If a b uilt - in op erat or is select ed,

such conversions

will b e applied t o t he op erands b efore t he op erat ion is considered furt her according t o t he rules in C lause 8 ; s ee 1 6 . 3 . 1 . 2 ,

4

1 6. 6.

If during t he

evaluat ion

of an

expression,

represent able values for it s typ e, remainder

using a z ero

divis or,

and

adj ust able by a library funct ion.

5

If an any

expression

furt her

init ially

analysis.

has

The

6

t he

typ e

exp ression

has ended,

7

[

(7. 1 )

Note: —

T

not

mat hemat ically [

Note:

except ions

defined

Treat ment

“ reference

designat es

cv T

”,

or

not

in

t he

of division by z ero,

vary among machines,

and

is

range

of

forming a

somet imes

]

T

to t he

”

( 1 1 . 3. 2,

ob j ect

or

1 1 . 6. 3) ,

t he

typ e

funct ion denot ed

dep ending on t he exp ression.

[

Note: — end note

where

T

is

adj ust ed

by t he

to

T

reference,

prior and

to

t he

B efore t he lifet ime of t he reference

is a cv- unqualified non- class,

]

non- array typ e,

t he typ e of

prior t o any furt her analysis .

An express ion is an xvalue if it

t he res ult of calling a funct ion, t o ob j ect

is

t he b ehavior is undefined ( see 6 . 8 ) .

If a prvalue init ially has t he typ e “ t he expres sion is adj ust ed t o

result

all float ing- p oint

— end note

express ion is an lvalue or an xvalue, has st art ed or aft er it

t he

t he b ehavior is undefined.

is:

whet her imp licit ly or explicit ly,

whose ret urn typ e is an rvalue reference

typ e,

(7. 2)

—

a cast

(7. 3)

—

a class memb er access express ion designat ing a non- st at ic dat a memb er of non- reference typ e in which

t o an rvalue reference t o ob j ect

t he ob j ect

(7. 4)

—

a

.*

expression is an xvalue,

typ e,

or

p oint er- t o- memb er express ion in which t he first

op erand is an xvalue and t he second op erand is a

p oint er t o dat a memb er.

In general, references

t he effect to

ob j ect s

named or not .

[

of t his rule is t hat are

t reat ed

— end note

as

named rvalue references are t reat ed as lvalues and unnamed rvalue

xvalues;

rvalue

references

to

funct ions

are

t reat ed

as

lvalues

whet her

]

Example: struct A { int m; 62)

The precedence of op erat ors is not direct ly sp ecified, but it can b e derived from t he synt ax.

Expressions

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}; A&& operator+(A, A) ; A&& f() ; A a; A&& ar = static_cast(a) ;

The expressions f() , f() . m, static_cast(a) , and a + a are xvalues. The expression ar is an lvalue.

— end example ] 8

In some contexts, unevaluated operands appear (8.2.8, 8.3.3, 8.3.7, 10.1.7.2) . An unevaluated operand is not evaluated. [ Note: In an unevaluated operand, a non-static class member may be named (8.1) and naming of obj ects or functions does not, by itself, require that a definition be provided (6.2) . An unevaluated operand is considered a full-expression (4.6) . — end note ]

9

Whenever a glvalue expression appears as an operand of an operator that expects a prvalue for that operand, the lvalue-to-rvalue (7.1) , array-to-pointer (7.2) , or function-to-pointer (7.3) standard conversions are applied to convert the expression to a prvalue. [ Note: Because cv-qualifiers are removed from the type of an expression of non-class type when the expression is converted to a prvalue, an lvalue expression of type const int can, for example, be used where a prvalue expression of type int is required. — end note ]

10

Whenever a prvalue expression appears as an operand of an operator that expects a glvalue for that operand, the temporary materialization conversion (7.4) is applied to convert the expression to an xvalue.

11

Many binary operators that expect operands of arithmetic or enumeration type cause conversions and yield result types in a similar way. The purpose is to yield a common type, which is also the type of the result. This pattern is called the usual arithmetic conversions , which are defined as follows:

(11. 1)

— If either operand is of scoped enumeration type (10.2) , no conversions are performed; if the other operand does not have the same type, the expression is ill-formed.

(11. 2)

— If either operand is of type long double , the other shall be converted to long double .

(11. 3)

— Otherwise, if either operand is double , the other shall be converted to double .

(11. 4)

— Otherwise, if either operand is float , the other shall be converted to float .

(11. 5)

— Otherwise, the integral promotions (7.6) shall be performed on both operands. 63 Then the following rules shall be applied to the promoted operands:

(11. 5. 1)

— If both operands have the same type, no further conversion is needed.

(11. 5. 2)

— Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank shall be converted to the type of the operand with greater rank.

(11. 5. 3)

— Otherwise, if the operand that has unsigned integer type has rank greater than or equal to the rank of the type of the other operand, the operand with signed integer type shall be converted to the type of the operand with unsigned integer type.

(11. 5. 4)

— Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, the operand with unsigned integer type shall be converted to the type of the operand with signed integer type.

(11. 5. 5)

— Otherwise, both operands shall be converted to the unsigned integer type corresponding to the type of the operand with signed integer type.

12

In some contexts, an expression only appears for its side effects. Such an expression is called a discarded-value expression. The array-to-pointer (7.2) and function-to-pointer (7.3) standard conversions are not applied. 63)

As a consequence, op erands of typ e

bool , char16_t , char32_t , wchar_t ,

or an enumerat ed typ e are convert ed t o some

int egral typ e.

Expressions

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The lvalue-to-rvalue conversion (7.1) is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following: (12. 1) (12. 2)

expression ) , where expression is one of these expressions, — id-expression (8.1.4) ,

— (

(12. 3)

— subscripting (8.2.1) ,

(12. 4)

— class member access (8.2.5) ,

(12. 5)

— indirection (8.3.1) ,

(12. 6)

— pointer-to-member operation (8.5) ,

(12. 7)

— conditional expression (8.16) where both the second and the third operands are one of these expressions, or

(12. 8)

— comma expression (8.19) where the right operand is one of these expressions. [ Note: Using an overloaded operator causes a function call; the above covers only operators with builtin meaning. — end note ] If the expression is a prvalue after this optional conversion, the temporary materialization conversion (7.4) is applied. [ Note: If the expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary that is the result object of the lvalue-to-rvalue conversion. — end note ] The glvalue expression is evaluated and its value is discarded.

13

The values of the floating operands and the results of floating expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby. 64

14

The is:

(14. 1) (14. 2)

15

cv-combined type of two types

T1 and T2 is a type T3 similar to T1 whose cv-qualification signature (7.5)

— for every i > 0, cvi3 is the union of cvi1 and cvi2 ; — if the resulting cvi3 is different from cvi1 or cvi2 , then const is added to every cvk3 for 0 < k < i . [ Note: Given similar types T1 and T2 , this construction ensures that both can be converted to T3 . — end note ] The composite pointer type of two operands p1 and p2 having types T1 and T2 , respectively, where at least one is a pointer or pointer to member type or std: : nullptr_t , is:

(15. 1)

— if both p1 and p2 are null pointer constants, std: : nullptr_t ;

(15. 2)

— if either p1 or p2 is a null pointer constant, T2 or T1 , respectively;

(15. 3)

— if T1 or T2 is “pointer to cv1 void” and the other type is “pointer to cv2 T”, where T is an object type or void, “pointer to cv12 void”, where cv12 is the union of cv1 and cv2 ;

(15. 4)

— if T1 or T2 is “pointer to noexcept function” and the other type is “pointer to function”, where the function types are otherwise the same, “pointer to function”;

(15. 5)

— if T1 is “pointer to cv1 C1 ” and T2 is “pointer to cv2 C2 ”, where C1 is reference-related to C2 or C2 is reference-related to C1 (11.6.3) , the cv-combined type of T1 and T2 or the cv-combined type of T2 and T1 , respectively;

(15. 6)

— if T1 is “pointer to member of C1 of type cv1 U1 ” and T2 is “pointer to member of C2 of type cv2 U2 ” where C1 is reference-related to C2 or C2 is reference-related to C1 (11.6.3) , the cv-combined type of T2 and T1 or the cv-combined type of T1 and T2 , respectively;

(15. 7)

— if T1 and T2 are similar types (7.5) , the cv-combined type of T1 and T2 ;

(15. 8)

— otherwise, a program that necessitates the determination of a composite pointer type is ill-formed. 64)

The cast and assignment op erat ors must st ill p erform t heir sp ecific conversions as describ ed in 8. 4, 8. 2. 9 and 8. 1 8.

Expressions

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[

Example: typedef typedef typedef typedef

void *p; const int *q; int **pi; const int **pci;

The comp osit e p oint er typ e of

pci

const

is “ p oint er t o

8. 1

p

q is “ p oint er t o const void” ; const int ”. — end example ]

and

p oint er t o

t he comp osit e p oint er typ e of

Primary expressions

pi

and

[expr. prim]

primary-expression : literal this ( expression ) id-expression lambda-expression fold-expression

8. 1 . 1 1

A

Literals

literal

[expr. prim. literal]

is a primary expression.

It s typ e dep ends on it s form (5 . 1 3 ) .

A st ring lit eral is an lvalue;

all ot her

lit erals are prvalues.

8. 1 . 2 1

This

The keyword

[expr. prim. this]

this

names a p oint er t o t he ob j ect for which a non- st at ic memb er funct ion (1 2 . 2 . 2 . 1 )

or a non- st at ic dat a memb er’ s init ializ er (1 2 . 2 )

2

If a declarat ion declares a memb er funct ion or memb er funct ion t emplat e of a class

cv-qualifier-seq X function-definition member-declarator declarator is a prvalue of typ e “ p oint er t o

”

,

and

it

shall

not

,

app ear

wit hin

t he

is invoked

is evaluat ed.

b etween t he opt ional

or

declarat ion

.

of a

It

shall not

st at ic

X,

t he expression

cv-qualifier-seq

app ear b efore t he op t ional

memb er

funct ion

( alt hough

it s

cv-qualifier-seq

typ e

and

Unlike t he ob j ect

expression in ot her cont ext s,

class memb er access ( 8 . 2 . 5 ) t he declarat ion are visible.

o ccur unt il t he complet e declarat or is known.

*this

is not

]

[

[

]

required t o b e of complet e typ e for purp oses of

out side t he memb er funct ion b o dy.

— end note

value

Note: — end note

cat egory are defined wit hin a st at ic memb er funct ion as t hey are wit hin a non- st at ic memb er funct ion) . This is b ecause declarat ion mat ching do es not

this

and t he end of t he

Example:

[

Note:

O nly class memb ers declared prior t o

struct A { char g() ; template auto f(T t) -> decltype(t + g() ) { return t + g() ; } }; template auto A: : f(int t) -> decltype(t + g() ) ;

— end example member-declarator ]

3

O t herwise,

if a

is a prvalue of typ e “ p oint er t o elsewhere in t he

4

The express ion

X”

declares a non- st at ic dat a memb er (1 2 . 2 ) wit hin t he opt ional default

member-declarator

this

shall not

of a class

X,

memb er init ializ er (1 2 . 2 ) .

t he expression It

shall not

this

app ear

.

app ear in any ot her cont ext .

class Outer { int a[sizeof(*this) ] ; unsigned int sz = sizeof(*this) ;

[

Example:

// error: not inside a member function // OK: in default member initializer

void f() {

§ 8. 1 . 2

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int b[sizeof(*this) ] ;

// OK

struct Inner { int c[sizeof(*this) ] ; };

// error: not inside a member function of Inner

} };

— end example ] 8. 1 . 3 1

Parentheses

[expr. prim. paren]

A parenthesized expression (E) is a primary expression whose type, value, and value category are identical to those of E. The parenthesized expression can be used in exactly the same contexts as those where E can be used, and with the same meaning, except as otherwise indicated. 8. 1 . 4

Names

[expr. prim. id]

id-expression : unqualified-id qualified-id 1

An id-expression is a restricted form of a and -> operators (8.2.5) . — end note ]

2

An id-expression that denotes a non-static data member or non-static member function of a class can only be used:

(2. 1 )

— as part of a class member access (8.2.5) in which the object expression refers to the member’s class 65 or a class derived from that class, or

(2. 2)

— to form a pointer to member (8.3.1) , or

(2. 3)

— if that id-expression denotes a non-static data member and it appears in an unevaluated operand. [ Example: struct S { int m; }; int i = sizeof(S: : m) ; int j = sizeof(S: : m + 42) ;

primary-expression. [ Note: An id-expression can appear after

.

// OK // OK

— end example ] 8.1 .4.1

Unqualified names

[expr.prim.id.unqual]

unqualified-id : identifier operator-function-id conversion-function-id literal-operator-id ~ class-name ~ decltype-specifier template-id 1

identifier is an id-expression provided it has been suitably declared (Clause 10) . [ Note: For operatorfunction-ids, see 16.5; for conversion-function-ids, see 15.3.2; for literal-operator-ids, see 16.5.8; for templateids, see 17.2. A class-name or decltype-specifier prefixed by ~ denotes a destructor; see 15.4. Within the definition of a non-static member function, an identifier that names a non-static member is transformed to a An

65)

This also applies when t he ob j ect expression is an implicit

§ 8.1.4.1

(*this)

( 1 2 . 2. 2 ) .

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class member access expression (12.2.2) . — end note ] The type of the expression is the type of the identifier . The result is the entity denoted by the identifier. The expression is an lvalue if the entity is a function, variable, or data member and a prvalue otherwise; it is a bit-field if the identifier designates a bit-field (11.5) . 8. 1. 4. 2

Qualified names

[expr. prim. id.qual]

qualified-id : nested-name-specifier template opt unqualified-id nested-name-specifier: :: type-name : : namespace-name : : decltype-specifier : : nested-name-specifier identifier : : nested-name-specifier template opt simple-template-id : :

decltype-specifier in a nested-name-specifier shall be a class or enumeration type.

1

The type denoted by a

2

A nested-name-specifier that denotes a class, optionally followed by the keyword template (17.2) , and then followed by the name of a member of either that class (12.2) or one of its base classes (Clause 13) , is a qualified-id; 6.4.3.1 describes name lookup for class members that appear in qualified-ids. The result is the member. The type of the result is the type of the member. The result is an lvalue if the member is a static member function or a data member and a prvalue otherwise. [ Note: A class member can be referred to using a qualified-id at any point in its potential scope (6.3.7) . — end note ] Where class-name : : ~ class-name is used, the two class-name s shall refer to the same class; this notation names the destructor (15.4) . The form ~ decltype-specifier also denotes the destructor, but it shall not be used as the unqualified-id in a qualified-id . [ Note: A typedef-name that names a class is a class-name (12.1) . — end note ]

3

The nested-name-specifier : : names the global namespace. A nested-name-specifier that names a namespace (10.3) , optionally followed by the keyword template (17.2) , and then followed by the name of a member of that namespace (or the name of a member of a namespace made visible by a using-directive ) , is a qualified-id; 6.4.3.2 describes name lookup for namespace members that appear in qualified-ids. The result is the member. The type of the result is the type of the member. The result is an lvalue if the member is a function or a variable and a prvalue otherwise.

4

A nested-name-specifier that denotes an enumeration (10.2) , followed by the name of an enumerator of that enumeration, is a qualified-id that refers to the enumerator. The result is the enumerator. The type of the result is the type of the enumeration. The result is a prvalue.

5

In a qualified-id , if the unqualified-id is a conversion-function-id , its conversion-type-id shall denote the same type in both the context in which the entire qualified-id occurs and in the context of the class denoted by the nested-name-specifier. 8. 1 . 5

Lambda expressions

[expr. prim. lambda]

lambda-expression : lambda-introducer lambda-declaratoropt compound-statement lambda-introducer: [ lambda-captureopt ] lambda-declarator: ( parameter-declaration-clause ) decl-specifier-seqopt noexcept-specifieropt attribute-specifier-seqopt trailing-return-typeopt 1

Lambda expressions provide a concise way to create simple function obj ects. [ Example: #include #include void abssort(float* x, unsigned N) {

§ 8.1.5

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std: : sort(x, x + N, [] (float a, float b) { return std: : abs(a) < std: : abs(b) ; }) ; }

2

— end example ] A lambda-expression is a prvalue whose result obj ect is called the closure object . A lambda-expression shall not appear in an unevaluated operand (Clause 8) , in a template-argument , in an alias-declaration , in a typedef

declaration, or in the declaration of a function or function template outside its function body and default arguments. [ Note: The intention is to prevent lambdas from appearing in a signature. — end note ] [ Note: A closure obj ect behaves like a function obj ect (23.14) . — end note ] 3 4

decl-specifier-seq of the lambda-declarator, each decl-specifier shall either be mutable or constexpr . If a lambda-expression does not include a lambda-declarator, it is as if the lambda-declarator were () . The lambda return type is auto , which is replaced by the type specified by the trailing-return-type if provided and/or deduced from return statements as described in 10.1.7.4. [ Example: In the

auto x1 = [] (int i) { return i; }; // OK: return type is int auto x2 = [] { return { 1, 2 }; }; // error: deducing return type from braced-init-list int j ; auto x3 = [] () ->auto&& { return j ; }; // OK: return type is int&

— end example ] 8. 1. 5. 1

Closure types

[expr.prim.lambda.closure]

1

The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union class type, called the closure type, whose properties are described below.

2

The closure type is declared in the smallest block scope, class scope, or namespace scope that contains the corresponding lambda-expression . [ Note: This determines the set of namespaces and classes associated with the closure type (6.4.2) . The parameter types of a lambda-declarator do not affect these associated namespaces and classes. — end note ] The closure type is not an aggregate type (11.6.1) . An implementation may define the closure type differently from what is described below provided this does not alter the observable behavior of the program other than by changing:

(2. 1 )

— the size and/or alignment of the closure type,

(2. 2)

— whether the closure type is trivially copyable (Clause 12) ,

(2. 3)

— whether the closure type is a standard-layout class (Clause 12) , or

(2. 4)

— whether the closure type is a POD class (Clause 12) . An implementation shall not add members of rvalue reference type to the closure type.

3

The closure type for a non-generic lambda-expression has a public inline function call operator (16.5.4) whose parameters and return type are described by the lambda-expression ’s parameter-declaration-clause and trailing-return-type respectively. For a generic lambda, the closure type has a public inline function call operator member template (17.5.2) whose template-parameter-list consists of one invented type templateparameter for each occurrence of auto in the lambda’s parameter-declaration-clause, in order of appearance. The invented type template-parameter is a parameter pack if the corresponding parameter-declaration declares a function parameter pack (11.3.5) . The return type and function parameters of the function call operator template are derived from the lambda-expression ’s trailing-return-type and parameter-declaration-clause by replacing each occurrence of auto in the decl-specifier s of the parameter-declaration-clause with the name of the corresponding invented template-parameter . [ Example: auto glambda = [] (auto a, auto&& b) { return a < b; }; bool b = glambda(3, 3. 14) ; // OK auto vglambda = [] (auto printer) {

§ 8.1.5.1

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// OK: ts is a function parameter pack

return [=] (auto&& . . . ts) { printer(std: : forward(ts) . . . ) ; return [=] () { printer(ts . . . ) ; };

}; }; auto p = vglambda( [] (auto v1, auto v2, auto v3) { std: : cout x + (*this) . y) // this has type S1* }; } };

— end example ] Further, a variable __func__ is implicitly defined at the beginning of the compound-statement lambda-expression, with semantics as described in 11.4.1. The closure type associated with a lambda-expression has no default constructor and a deleted copy assignment operator. It has a defaulted copy constructor and a defaulted move constructor (15.8) . [ Note: These special member functions are implicitly defined as usual, and might therefore be defined as deleted. — end note ] The closure type associated with a lambda-expression has an implicitly-declared destructor (15.4) . of the

11

12 13

A member of a closure type shall not be explicitly instantiated (17.7.2) , explicitly specialized (17.7.3) , or named in a friend declaration (14.3) .

§ 8.1.5.1

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8.1 . 5. 2

Captures

[expr.prim.lambda.capture]

lambda-capture : capture-default capture-list capture-default , capture-list capture-default : & = capture-list : capture . . . opt capture-list , capture . . . opt capture : simple-capture init-capture simple-capture : identifier & identifier this * this init-capture : identifier initializer & identifier initializer

1 The body of a lambda-expression may refer to variables with automatic storage duration and the *this obj ect (if any) of enclosing block scopes by capturing those entities, as described below.

2 If a lambda-capture includes a capture-default that is & , no identifier in a simple-capture of that lambda-capture shall be preceded by & . If a lambda-capture includes a capture-default that is = , each simple-capture of that lambda-capture shall be of the form “ & identifier” or “ * this ”. [ Note: The form [&, this] is redundant but accepted for compatibility with ISO C++ 2014. — end note ] Ignoring appearances in initializer s of init-captures, an identifier or this shall not appear more than once in a lambda-capture. [ Example: struct S2 { void f(int i) ; }; void S2: : f(int i) { [&, i] { }; // OK [&, &i] { }; // error: i preceded by & when & is the default [=, *this] { }; // OK [=, this] { }; // error: this when = is the default [i, i] { }; // error: i repeated [this, *this] { }; // error: this appears twice }

— end example ] 3 A lambda-expression whose smallest enclosing scope is a block scope (6.3.3) is a local lambda expression; any other lambda-expression shall not have a capture-default or simple-capture in its lambda-introducer . The reaching scope of a local lambda expression is the set of enclosing scopes up to and including the innermost enclosing function and its parameters. [ Note: This reaching scope includes any intervening lambda-expression s. — end note ] 4 The identifier in a simple-capture is looked up using the usual rules for unqualified name lookup (6.4.1) ; each such lookup shall find an entity. An entity that is designated by a simple-capture is said to be explicitly captured, and shall be *this (when the simple-capture is “ this ” or “ * this ”) or a variable with automatic storage duration declared in the reaching scope of the local lambda expression.

§ 8.1.5.2

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5

identifier in a simple-capture appears as the declarator-id of a parameter of the lambda-declarator’s parameter-declaration-clause, the program is ill-formed. [ Example: If an

void f() { int x = 0; auto g = [x] (int x) { return 0; } }

6

(6. 1 )

(6. 2)

// error: parameter and simple-capture have the same name

— end example ] An init-capture behaves as if it declares and explicitly captures a variable of the form “ auto init-capture ; ” whose declarative region is the lambda-expression ’s compound-statement , except that: — if the capture is by copy (see below) , the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same obj ect, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and — if the capture is by reference, the variable’s lifetime ends when the closure obj ect’s lifetime ends. [ Note: This enables an init-capture like “ x = std: : move(x) ”; the second “ x ” must bind to a declaration in the surrounding context. — end note ] [ Example: int x = 4; auto y = [&r = x, x = x+1] () ->int { r += 2; return x+2; }() ; // Updates : : x to 6, and initializes y to 7. auto z = [a = 42] (int a) { return 1; } // error: parameter and local variable have the same name

7

— end example ] A lambda-expression with an associated capture-default that does not explicitly capture *this or a variable with automatic storage duration (this excludes any id-expression that has been found to refer to an initcapture’s associated non-static data member) , is said to implicitly capture the entity (i.e., *this or a variable) if the compound-statement :

(7. 1 )

— odr-uses (6.2) the entity (in the case of a variable) ,

(7. 2)

— odr-uses (6.2) this (in the case of the obj ect designated by *this ) , or

(7. 3)

— names the entity in a potentially-evaluated expression (6.2) where the enclosing full-expression depends on a generic lambda parameter declared within the reaching scope of the lambda-expression . [ Example: void f(int, const int (&) [2] = {}) { } // #1 void f(const int&, const int (&) [1] ) { } // #2 void test() { const int x = 17; auto g = [] (auto a) { f(x) ; // OK: calls #1, does not capture x }; auto g2 = [=] (auto a) { int selector[sizeof(a) == 1 ? 1 : 2] {}; f(x, selector) ; // OK: is a dependent expression, so captures x }; }

— end example ] All such implicitly captured entities shall be declared within the reaching scope of the lambda expression. [ Note: The implicit capture of an entity by a nested lambda-expression can cause its implicit § 8.1.5.2

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capt ure by t he cont aining capt ure.

8

— end note captured

An ent ity is is

o dr- used

express ion,

lambda-expression

( see b elow) .

Implicit

o dr- uses

of

this

(6. 2)

in

if it

t he

it s nearest

is

capt ured

scop e

explicit ly or implicit ly.

cont aining

t he

An ent ity capt ured

lambda-expression

.

If

*this

is

enclosing funct ion shall b e a non- st at ic memb er funct ion.

aut omat ic st orage durat ion from it s reaching scop e,

lambda-expression

cap t ures

an

ent ity

enclosing lamb da expression or funct ion,

void f1(int i) { int const N = 20; auto m1 = [=] { int const M = 30; auto m2 = [i] { int x[N] [M] ; x[0] [0] = i; }; }; struct s1 { int f; void work(int n) { int m = n*n; int j = 40; auto m3 = [this, m] { auto m4 = [&, j ] { int x = n; x += m; x += i; x += f; }; }; } }; } struct s2 { double ohseven = . 007; auto f() { return [this] { return [*this] { return ohseven; } }() ; } auto g() { return [] { return [*this] { }; }() ; } };

— end example

§ 8. 1 . 5. 2

and

t hat

t hat

by a

capt ured If a

inst ant iat ion of t he funct ion call op erat or t emplat e of a generic lamb da o dr- uses (6 . 2 )

If a

can result

in implicit

]

is

not

t he program is ill- formed.

defined [

or

this

capt ured

Example:

a

lo cal

lamb da

lambda-expression

ent ity shall b e capt ured by t he

ent ity

lambda-expression by

or an

or a variable wit h

lambda-expression

in

t he

.

immediat ely

// OK: N and M are not odr-used // OK: i is explicitly captured by m2 and implicitly captured by m1

// error: j not captured by m3 // error: n implicitly captured by m4 but not captured by m3 // OK: m implicitly captured by m4 and explicitly captured by m3 // error: i is outside of the reaching scope // OK: this captured implicitly by m4 and explicitly by m3

// OK

// error: *this not captured by outer lambda-expression

]

© IS O / IEC

2 0 1 7 – All right s reserved

1 05

ISO/IEC 1 4882:201 7(E)

9

A lambda-expression appearing in a default argument shall not implicitly or explicitly capture any entity. [ Example: void f2() { int i = 1; void g1(int void g2(int void g3(int void g4(int void g5(int }

10

= = = = =

([i] { return i; }) () ) ; ([i] { return 0; }) () ) ; ([=] { return i; }) () ) ; ([=] { return 0; }) () ) ; ([] { return sizeof i; }) () ) ;

// ill-formed // ill-formed // ill-formed // OK // OK

— end example ] An entity is captured by copy if capture-default is = , and the captured entity is not *this , or identifier, or & identifier

(10. 1)

— it is implicitly captured, the

(10. 2)

— it is explicitly captured with a capture that is not of the form this , & initializer.

For each entity captured by copy, an unnamed non-static data member is declared in the closure type. The declaration order of these members is unspecified. The type of such a data member is the referenced type if the entity is a reference to an obj ect, an lvalue reference to the referenced function type if the entity is a reference to a function, or the type of the corresponding captured entity otherwise. A member of an anonymous union shall not be captured by copy. 11

Every id-expression within the compound-statement of a lambda-expression that is an odr-use (6.2) of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type. [ Note: An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type. Furthermore, such an id-expression does not cause the implicit capture of the entity. — end note ] If *this is captured by copy, each odr-use of this is transformed into a pointer to the corresponding unnamed data member of the closure type, cast (8.4) to the type of this . [ Note: The cast ensures that the transformed expression is a prvalue. — end note ] An id-expression within the compound-statement of a lambda-expression that is an odr-use of a reference captured by reference refers to the entity to which the captured reference is bound and not to the captured reference. [ Note: The validity of such captures is determined by the lifetime of the obj ect to which the reference refers, not by the lifetime of the reference itself. — end note ] [ Example: void f(const int*) ; void g() { const int N = 10; [=] { int arr[N] ; // OK: not an odr-use, refers to automatic variable f(&N) ; // OK: causes N to be captured; &N points to // the corresponding member of the closure type }; } auto h(int &r) { return [&] { ++r; // Valid after h returns if the lifetime of the // object to which r is bound has not ended }; }

— end example ]

§ 8.1.5.2

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ISO/IEC 1 4882:201 7(E)

12

An ent ity is

captured by reference

if it

is

implicit ly or explicit ly capt ured but

not

capt ured by copy.

It

is

unsp ecified whet her addit ional unnamed non- st at ic dat a memb ers are declared in t he clos ure typ e for ent it ies capt ured by reference.

If declared,

such non- st at ic dat a memb ers shall b e of lit eral typ e.

[

Example:

// The inner closure type must be a literal type regardless of how reference captures are represented. static_assert([] (int n) { return [&n] { return ++n; }() ; }(3) == 4) ;

— end example lambda-expression m2 lambda-expression m1 m2 ]

13

If

A bit - field or a memb er of an anonymous union shall not

a

capt ures

,

(13. 1)

—

if

m1

t hen

an

ent ity

and

t hat

ent ity

is

b e capt ured by reference.

capt ured

by

an

immediat ely

enclosing

’ s capt ure is t ransformed as follows:

m2

capt ures t he ent ity by copy,

capt ures t he corresp onding non- st at ic dat a memb er of

m1

’ s closure

typ e;

(13. 2)

—

[

if

m1

Example:

m2

capt ures t he ent ity by reference,

m1

capt ures t he same ent ity capt ured by

The nest ed lamb da expres sions and invo cat ions b elow will out put

123234

.

.

int a = 1, b = 1, c = 1; auto m1 = [a, &b, &c] () mutable { auto m2 = [a, b, &c] () mutable { std: : cout ( expression ) static_cast < type-id > ( expression ) reinterpret_cast < type-id > ( expression ) const_cast < type-id > ( expression ) typeid ( expression ) typeid ( type-id ) expression-list : initializer-list pseudo-destructor-name : nested-name-specifieropt type-name : : ~ type-name nested-name-specifier template simple-template-id : : ~ type-name ~ decltype-specifier

2

~

type-name

[ Note: The > token following the type-id in a dynamic_cast , static_cast , reinterpret_cast , or const_cast may be the product of replacing a >> token by two consecutive > tokens (17.2) . — end note ] 8. 2. 1

Subscripting

[expr. sub]

1

A postfix expression followed by an expression in square brackets is a postfix expression. One of the expressions shall be a glvalue of type “array of T ” or a prvalue of type “pointer to T” and the other shall be a prvalue of unscoped enumeration or integral type. The result is of type “ T ”. The type “ T” shall be a completely-defined object type. 66 The expression E1[E2] is identical (by definition) to *((E1) +(E2) ) [ Note: see 8.3 and 8.7 for details of * and + and 11.3.4 for details of arrays. — end note ] , except that in the case of an array operand, the result is an lvalue if that operand is an lvalue and an xvalue otherwise. The expression E1 is sequenced before the expression E2 .

2

A

braced-init-list shall not be used with the built-in subscript operator.

8. 2. 2 1

Function call

[expr. call]

A function call is a postfix expression followed by parentheses containing a possibly empty, comma-separated list of initializer-clause s which constitute the arguments to the function. The postfix expression shall have function type or function pointer type. For a call to a non-member function or to a static member function, the postfix expression shall be either an lvalue that refers to a function (in which case the function-to-pointer standard conversion (7.3) is suppressed on the postfix expression) , or it shall have function pointer type. Calling a function through an expression whose function type is different from the function type of the 66)

This is t rue even if t he subscript op erat or is used in t he following common idiom:

§ 8.2.2

© ISO/IEC 2017 – All rights reserved

&x[0] .

109

ISO/IEC 14882: 2017(E)

called function’s definition results in undefined behavior (10.5) . For a call to a non-static member function, the postfix expression shall be an implicit (12.2.2, 12.2.3) or explicit class member access (8.2.5) whose id-expression is a function member name, or a pointer-to-member expression (8.5) selecting a function member; the call is as a member of the class object referred to by the object expression. In the case of an implicit class member access, the implied object is the one pointed to by this . [ Note: A member function call of the form f() is interpreted as (*this) . f() (see 12.2.2) . — end note ] If a function or member function name is used, the name can be overloaded (Clause 16) , in which case the appropriate function shall be selected according to the rules in 16.3. If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id , that function is called. Otherwise, its final overrider (13.3) in the dynamic type of the obj ect expression is called; such a call is referred to as a virtual function call. [ Note: The dynamic type is the type of the object referred to by the current value of the object expression. 15.7 describes the behavior of virtual function calls when the obj ect expression refers to an obj ect under construction or destruction. — end note ] 2

[ Note: If a function or member function name is used, and name lookup (6.4) does not find a declaration of that name, the program is ill-formed. No function is implicitly declared by such a call. — end note ]

3

If the postfix-expression designates a destructor (15.4) , the type of the function call expression is void; otherwise, the type of the function call expression is the return type of the statically chosen function (i.e., ignoring the virtual keyword) , even if the type of the function actually called is different. This return type shall be an obj ect type, a reference type or cv void.

4

When a function is called, each parameter (11.3.5) shall be initialized (11.6, 15.8, 15.1) with its corresponding argument. If the function is a non-static member function, the this parameter of the function (12.2.2.1) shall be initialized with a pointer to the object of the call, converted as if by an explicit type conversion (8.4) . [ Note: There is no access or ambiguity checking on this conversion; the access checking and disambiguation are done as part of the (possibly implicit) class member access operator. See 13.2, 14.2, and 8.2.5. — end note ] When a function is called, the parameters that have obj ect type shall have completely-defined obj ect type. [ Note: this still allows a parameter to be a pointer or reference to an incomplete class type. However, it prevents a passed-by-value parameter to have an incomplete class type. — end note ] It is implementation-defined whether the lifetime of a parameter ends when the function in which it is defined returns or at the end of the enclosing full-expression. The initialization and destruction of each parameter occurs within the context of the calling function. [ Example: The access of the constructor, conversion functions or destructor is checked at the point of call in the calling function. If a constructor or destructor for a function parameter throws an exception, the search for a handler starts in the scope of the calling function; in particular, if the function called has a function-try-block (Clause 18) with a handler that could handle the exception, this handler is not considered. — end example ]

5

The postfix-expression is sequenced before each expression in the expression-list and any default argument. The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter. [ Note: All side effects of argument evaluations are sequenced before the function is entered (see 4.6) . — end note ] [ Example: void f() { std: : string s = " but I have heard it works even if you don’ t believe in it" ; s. replace(0, 4, " " ) . replace(s. find(" even" ) , 4, " only" ) . replace(s. find(" don’ t" ) , 6, " " ) ; assert(s == " I have heard it works only if you believe in it" ) ; // OK }

— end example ] [ Note: If an operator function is invoked using operator notation, argument evaluation is sequenced as specified for the built-in operator; see 16.3.1.2. — end note ] [ Example: struct S { S(int) ; }; int operator(static_cast< cv void*>(v) ) . [ Note: Converting a prvalue of type “pointer to T1 ” to the type “pointer to T2 ” (where T1 and T2 are obj ect types and where the alignment requirements of T2 are no stricter than those of T1 ) and back to its original type yields the original pointer value. — end note ]

8

Converting a function pointer to an object pointer type or vice versa is conditionally-supported. The meaning of such a conversion is implementation-defined, except that if an implementation supports conversions in both directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification, shall yield the original pointer value.

9

The null pointer value (7.11) is converted to the null pointer value of the destination type. [ Note: A null pointer constant of type std: : nullptr_t cannot be converted to a pointer type, and a null pointer constant of integral type is not necessarily converted to a null pointer value. — end note ]

10

A prvalue of type “pointer to member of X of type T1 ” can be explicitly converted to a prvalue of a different type “pointer to member of Y of type T2 ” if T1 and T2 are both function types or both obj ect types. 74 The null member pointer value (7.12) is converted to the null member pointer value of the destination type. The result of this conversion is unspecified, except in the following cases:

(10. 1)

— converting a prvalue of type “pointer to member function” to a different pointer to member function type and back to its original type yields the original pointer to member value.

(10. 2)

— converting a prvalue of type “pointer to data member of X of type T1 ” to the type “pointer to data member of Y of type T2 ” (where the alignment requirements of T2 are no stricter than those of T1 ) and back to its original type yields the original pointer to member value.

11

A glvalue expression of type T1 can be cast to the type “reference to T2 ” if an expression of type “pointer to T1 ” can be explicitly converted to the type “pointer to T2 ” using a reinterpret_cast . The result refers to

the same obj ect as the source glvalue, but with the specified type. [ Note: That is, for lvalues, a reference cast reinterpret_cast(x) has the same effect as the conversion *reinterpret_cast(&x) with the built-in & and * operators (and similarly for reinterpret_cast(x) ) . — end note ] No temporary is created, no copy is made, and constructors (15.1) or conversion functions (15.3) are not called. 75 73)

The typ es may have different

cv- qualifiers,

sub j ect t o t he overall rest rict ion t hat a

reinterpret_cast

cannot cast away

cv- qualifiers,

sub j ect t o t he overall rest rict ion t hat a

reinterpret_cast

cannot cast away

const ness.

74) T1

and

T2

may have different

const ness.

75)

This is somet imes referred t o as a

§ 8.2.10

type pun.

© ISO/IEC 2017 – All rights reserved

118

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8. 2. 1 1

Const cast

[expr. const. cast]

1

The result of the expression const_cast(v) is of type T . If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (7.1) , array-to-pointer (7.2) , and function-to-pointer (7.3) standard conversions are performed on the expression v. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast .

2

[ Note: Subject to the restrictions in this section, an expression may be cast to its own type using a const_cast operator. — end note ]

3

For two similar types T1 and T2 (7.5) , a prvalue of type T1 may be explicitly converted to the type T2 using a const_cast . The result of a const_cast refers to the original entity. [ Example: typedef int *A[3] ; typedef const int *const CA[3] ;

// array of 3 pointer to int // array of 3 const pointer to const int

CA &&r = A{}; // OK, reference binds to temporary array object after qualification conversion to type CA A &&r1 = const_cast(CA{}) ; // error: temporary array decayed to pointer A &&r2 = const_cast(CA{}) ; // OK

— end example ] 4

For two obj ect types T1 and T2 , if a pointer to T1 can be explicitly converted to the type “pointer to T2 ” using a const_cast , then the following conversions can also be made:

(4. 1 )

— an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast ;

(4. 2)

— a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast ; and

(4. 3)

— if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast . The result of a reference const_cast refers to the original object if the operand is a glvalue and to the result of applying the temporary materialization conversion (7.4) otherwise.

5

A null pointer value (7.11) is converted to the null pointer value of the destination type. The null member pointer value (7.12) is converted to the null member pointer value of the destination type.

6

[ Note: Depending on the type of the obj ect, a write operation through the pointer, lvalue or pointer to data member resulting from a const_cast that casts away a const-qualifier76 may produce undefined behavior (10.1.7.1) . — end note ]

7

A conversion from a type T1 to a type T2 casts away constness if T1 and T2 are different, there is a cv-decomposition (7.5) of T1 yielding n such that T2 has a cv-decomposition of the form cv02 P02 cv12 P12

· · ·

2 2 2 cv n − 1 Pn− 1 cvn

U2 ,

and there is no qualification conversion that converts T1 to cv02 P01 cv12 P11

· · ·

2 1 2 cv n − 1 Pn− 1 cvn

U1 .

8

Casting from an lvalue of type T1 to an lvalue of type T2 using an lvalue reference cast or casting from an expression of type T1 to an xvalue of type T2 using an rvalue reference cast casts away constness if a cast from a prvalue of type “pointer to T1 ” to the type “pointer to T2 ” casts away constness.

9

[ Note: Some conversions which involve only changes in cv-qualification cannot be done using const_cast. For instance, conversions between pointers to functions are not covered because such conversions lead to values whose use causes undefined behavior. For the same reasons, conversions between pointers to member functions, and in particular, the conversion from a pointer to a const member function to a pointer to a 76) const_cast

§ 8.2.11

is not limit ed t o conversions t hat cast away a const - qualifier.

© ISO/IEC 2017 – All rights reserved

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non-const member function, are not covered. 8. 3 1

— end note ]

Unary expressions

[expr. unary]

Expressions with unary operators group right-to-left. unary-expression : postfix-expression ++ cast-expression -- cast-expression unary-operator cast-expression sizeof unary-expression sizeof ( type-id ) sizeof . . . ( identifier ) alignof ( type-id ) noexcept-expression new-expression delete-expression unary-operator: one * & + - ! ~

8. 3. 1

of

Unary operators

[expr. unary. op]

1

The unary * operator performs indirection : the expression to which it is applied shall be a pointer to an obj ect type, or a pointer to a function type and the result is an lvalue referring to the obj ect or function to which the expression points. If the type of the expression is “pointer to T ”, the type of the result is “ T”. [ Note: Indirection through a pointer to an incomplete type (other than cv void) is valid. The lvalue thus obtained can be used in limited ways (to initialize a reference, for example) ; this lvalue must not be converted to a prvalue, see 7.1. — end note ]

2

The result of each of the following unary operators is a prvalue.

3

The result of the unary & operator is a pointer to its operand. The operand shall be an lvalue or a qualified-id . If the operand is a qualified-id naming a non-static or variant member m of some class C with type T , the result has type “pointer to member of class C of type T ” and is a prvalue designating C: : m. Otherwise, if the type of the expression is T , the result has type “pointer to T ” and is a prvalue that is the address of the designated object (4.4) or a pointer to the designated function. [ Note: In particular, the address of an object of type “ cv T ” is “pointer to cv T ”, with the same cv-qualification. — end note ] For purposes of pointer arithmetic (8.7) and comparison (8.9, 8.10) , an object that is not an array element whose address is taken in this way is considered to belong to an array with one element of type T. [ Example: struct A { int i; }; struct B : A { }; . . . &B: : i . . . // has type int A: : * int a; int* p1 = &a; int* p2 = p1 + 1; // defined behavior bool b = p2 > p1; // defined behavior, with value true

— end example ] [ Note: A pointer to member formed from a mutable non-static data member (10.1.1) does not reflect the mutable specifier associated with the non-static data member. — end note ] 4

A pointer to member is only formed when an explicit & is used and its operand is a qualified-id not enclosed in parentheses. [ Note: That is, the expression &(qualified-id) , where the qualified-id is enclosed in parentheses, does not form an expression of type “pointer to member”. Neither does qualified-id, because there is no implicit conversion from a qualified-id for a non-static member function to the type “pointer to member function” as there is from an lvalue of function type to the type “pointer to function” (7.3) . Nor is &unqualified-id a pointer to member, even within the scope of the unqualified-id’s class. — end note ] § 8.3.1

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5

If & is applied to an lvalue of incomplete class type and the complete type declares operator&() , it is unspecified whether the operator has the built-in meaning or the operator function is called. The operand of & shall not be a bit-field.

6

The address of an overloaded function (Clause 16) can be taken only in a context that uniquely determines which version of the overloaded function is referred to (see 16.4) . [ Note: Since the context might determine whether the operand is a static or non-static member function, the context can also affect whether the expression has type “pointer to function” or “pointer to member function”. — end note ]

7

The operand of the unary + operator shall have arithmetic, unscoped enumeration, or pointer type and the result is the value of the argument. Integral promotion is performed on integral or enumeration operands. The type of the result is the type of the promoted operand.

8

The operand of the unary - operator shall have arithmetic or unscoped enumeration type and the result is the negation of its operand. Integral promotion is performed on integral or enumeration operands. The negative of an unsigned quantity is computed by subtracting its value from 2 n , where n is the number of bits in the promoted operand. The type of the result is the type of the promoted operand.

9

The operand of the logical negation operator ! is contextually converted to bool (Clause 7) ; its value is true if the converted operand is false and false otherwise. The type of the result is bool .

10

The operand of ~ shall have integral or unscoped enumeration type; the result is the ones’ complement of its operand. Integral promotions are performed. The type of the result is the type of the promoted operand. There is an ambiguity in the grammar when ~ is followed by a class-name or decltype-specifier . The ambiguity is resolved by treating ~ as the unary complement operator rather than as the start of an unqualified-id naming a destructor. [ Note: Because the grammar does not permit an operator to follow the . , -> , or : : tokens, a ~ followed by a class-name or decltype-specifier in a member access expression or qualified-id is unambiguously parsed as a destructor name. — end note ] 8. 3. 2

Increment and decrement

[expr. pre. incr]

1

The operand of prefix ++ is modified by adding 1 . The operand shall be a modifiable lvalue. The type of the operand shall be an arithmetic type other than cv bool , or a pointer to a completely-defined object type. The result is the updated operand; it is an lvalue, and it is a bit-field if the operand is a bit-field. The expression ++x is equivalent to x+=1 . [ Note: See the discussions of addition (8.7) and assignment operators (8.18) for information on conversions. — end note ]

2

The operand of prefix -- is modified by subtracting 1 . The requirements on the operand of prefix -- and the properties of its result are otherwise the same as those of prefix ++ . [ Note: For postfix increment and decrement, see 8.2.6. — end note ] 8. 3. 3

Sizeof

[expr. sizeof]

1

The sizeof operator yields the number of bytes in the obj ect representation of its operand. The operand is either an expression, which is an unevaluated operand (Clause 8) , or a parenthesized type-id . The sizeof operator shall not be applied to an expression that has function or incomplete type, to the parenthesized name of such types, or to a glvalue that designates a bit-field. sizeof(char) , sizeof(signed char) and sizeof(unsigned char) are 1 . The result of sizeof applied to any other fundamental type (6.9.1) is implementation-defined. [ Note: In particular, sizeof(bool) , sizeof(char16_t) , sizeof(char32_t) , and sizeof(wchar_t) are implementation-defined. 77 — end note ] [ Note: See 4.4 for the definition of byte and 6.9 for the definition of object representation . — end note ]

2

When applied to a reference or a reference type, the result is the size of the referenced type. When applied to a class, the result is the number of bytes in an obj ect of that class including any padding required for placing obj ects of that type in an array. The size of a most derived class shall be greater than zero (4.5) . 77) sizeof( bool)

§ 8.3.3

is not required t o b e

1.

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The result of applying sizeof to a base class subobj ect is the size of the base class type. 78 When applied to an array, the result is the total number of bytes in the array. This implies that the size of an array of n elements is n times the size of an element. 3

The sizeof operator can be applied to a pointer to a function, but shall not be applied directly to a function.

4

The lvalue-to-rvalue (7.1) , array-to-pointer (7.2) , and function-to-pointer (7.3) standard conversions are not applied to the operand of sizeof . If the operand is a prvalue, the temporary materialization conversion (7.4) is applied.

5

The identifier in a sizeof. . . expression shall name a parameter pack. The sizeof. . . operator yields the number of arguments provided for the parameter pack identifier . A sizeof. . . expression is a pack expansion (17.5.3) . [ Example: template struct count { static const std: : size_t value = sizeof. . . (Types) ; };

— end example ] 6

The result of sizeof and sizeof. . . is a constant of type std: : size_t . [ Note: std: : size_t is defined in the standard header (21.2.1, 21.2.4) . — end note ]

1

The new-expression attempts to create an obj ect of the type-id (11.1) or new-type-id to which it is applied. The type of that obj ect is the allocated type. This type shall be a complete obj ect type, but not an abstract class type or array thereof (4.5, 6.9, 13.4) . [ Note: Because references are not obj ects, references cannot be created by new-expression s. — end note ] [ Note: The type-id may be a cv-qualified type, in which case the obj ect created by the new-expression has a cv-qualified type. — end note ]

8. 3. 4

New

[expr. new]

new-expression : : : opt new new-placementopt new-type-id new-initializeropt : : opt new new-placementopt ( type-id ) new-initializeropt new-placement : ( expression-list ) new-type-id : type-specifier-seq new-declaratoropt new-declarator: ptr-operator new-declaratoropt noptr-new-declarator noptr-new-declarator: [ expression ] attribute-specifier-seqopt noptr-new-declarator [ constant-expression ] attribute-specifier-seqopt new-initializer: ( expression-listopt ) braced-init-list

Entities created by a new-expression have dynamic storage duration (6.7.4) . [ Note: The lifetime of such an entity is not necessarily restricted to the scope in which it is created. — end note ] If the entity is a non-array object, the new-expression returns a pointer to the object created. If it is an array, the new-expression returns a pointer to the initial element of the array. 78)

The act ual size of a base class sub ob j ect may b e less t han t he result of applying

sizeof

t o t he sub ob j ect , due t o virt ual

base classes and less st rict padding requirement s on base class sub ob j ect s.

§ 8.3.4

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2

If a placeholder type (10.1.7.4) appears in the type-specifier-seq of a new-type-id or type-id of a new-expression , the allocated type is deduced as follows: Let init be the new-initializer , if any, and T be the new-type-id or type-id of the new-expression, then the allocated type is the type deduced for the variable x in the invented declaration (10.1.7.4) : T x init ;

[ Example: new auto(1) ; auto x = new auto(’ a’ ) ;

// allocated type is int // allocated type is char , x is of type char*

template struct A { A(T, T) ; }; auto y = new A{1, 2}; // allocated type is A

3

— end example ] The new-type-id in a new-expression is the longest possible sequence of new-declarator s. [ Note: This prevents ambiguities between the declarator operators & , && , * , and [] and their expression counterparts. — end note ] [ Example: // syntax error: parsed as (new int*) i , not as (new int) *i

new int * i;

— end example ] new-type-id of a new-expression can have surprising effects. [ Example:

The * is the pointer declarator and not the multiplication operator. 4

[ Note: Parentheses in a new int(*[10] ) () ;

// error

is ill-formed because the binding is (new int) (*[10] ) () ;

// error

Instead, the explicitly parenthesized version of the new operator can be used to create obj ects of compound types (6.9.2) : new (int (*[10] ) () ) ;

allocates an array of 10 pointers to functions (taking no argument and returning int ) .

— end note ] 5

6

7

— end example ]

When the allocated object is an array (that is, the noptr-new-declarator syntax is used or the new-type-id or type-id denotes an array type) , the new-expression yields a pointer to the initial element (if any) of the array. [ Note: Both new int and new int[10] have type int* and the type of new int[i] [10] is int (*) [10] — end note ] The attribute-specifier-seq in a noptr-new-declarator appertains to the associated array type. Every constant-expression in a noptr-new-declarator shall be a converted constant expression (8.20) of type std: : size_t and shall evaluate to a strictly positive value. The expression in a noptr-new-declarator is implicitly converted to std: : size_t . [ Example: Given the definition int n = 42 , new float[n] [5] is well-formed (because n is the expression of a noptr-new-declarator ) , but new float[5] [n] is ill-formed (because n is not a constant expression) . — end example ] The expression in a noptr-new-declarator is erroneous if:

(7. 1 )

— the expression is of non-class type and its value before converting to std: : size_t is less than zero;

(7. 2)

— the expression is of class type and its value before application of the second standard conversion (16.3.3.1.2) 79 is less than zero;

(7. 3)

— its value is such that the size of the allocated obj ect would exceed the implementation-defined limit (Annex B) ; or

(7. 4)

— the 79)

new-initializer is a braced-init-list and the number of array elements for which initializers are

If t he conversion funct ion ret urns a signed int eger typ e, t he second st andard conversion convert s t o t he unsigned typ e

std: : size_t

§ 8.3.4

and t hus t hwart s any at t empt t o det ect a negat ive value aft erwards.

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provided (including the terminating ’ \0’ in a string literal (5.13.5) ) exceeds the number of elements to initialize.

expression is erroneous after converting to std: : size_t : — if the expression is a core constant expression, the program is ill-formed;

If the

(7.5) (7.6) (7.6.1)

— otherwise, an allocation function is not called; instead — if the allocation function that would have been called has a non-throwing exception specification (18.4) , the value of the new-expression is the null pointer value of the required result type;

(7.6.2)

— otherwise, the new-expression terminates by throwing an exception of a type that would match a handler (18.3) of type std: : bad_array_new_length (21.6.3.2) . When the value of the elements.

expression is zero, the allocation function is called to allocate an array with no

8 A new-expression may obtain storage for the obj ect by calling an allocation function (6.7.4.1) . If the new-expression terminates by throwing an exception, it may release storage by calling a deallocation function (6.7.4.2) . If the allocated type is a non-array type, the allocation function’s name is operator new and the deallocation function’s name is operator delete . If the allocated type is an array type, the allocation function’s name is operator new[] and the deallocation function’s name is operator delete[] . [ Note: An implementation shall provide default definitions for the global allocation functions (6.7.4, 21.6.2.1, 21.6.2.2) . A C++ program can provide alternative definitions of these functions (20.5.4.6) and/or class-specific versions (15.5) . The set of allocation and deallocation functions that may be called by a new-expression may include functions that do not perform allocation or deallocation; for example, see 21.6.2.3. — end note ]

9 If the new-expression begins with a unary : : operator, the allocation function’s name is looked up in the

global scope. Otherwise, if the allocated type is a class type T or array thereof, the allocation function’s name is looked up in the scope of T. If this lookup fails to find the name, or if the allocated type is not a class type, the allocation function’s name is looked up in the global scope.

10 An implementation is allowed to omit a call to a replaceable global allocation function (21.6.2.1, 21.6.2.2) .

(10.1) (10.2) (10.3) (10.4) (10.5) (10.6)

When it does so, the storage is instead provided by the implementation or provided by extending the allocation of another new-expression . The implementation may extend the allocation of a new-expression e1 to provide storage for a new-expression e2 if the following would be true were the allocation not extended: — the evaluation of e1 is sequenced before the evaluation of e2 , and — e2 is evaluated whenever e1 obtains storage, and — both e1 and e2 invoke the same replaceable global allocation function, and

— if the allocation function invoked by e1 and e2 is throwing, any exceptions thrown in the evaluation of either e1 or e2 would be first caught in the same handler, and

delete-expressions, and — the evaluation of e2 is sequenced before the evaluation of the delete-expression whose operand is the

— the pointer values produced by e1 and e2 are operands to evaluated pointer value produced by e1 .

[ Example: void mergeable(int x) { // These allocations are safe for merging: std: : unique_ptr a{new (std: : nothrow) char[8] }; std: : unique_ptr b{new (std: : nothrow) char[8] }; std: : unique_ptr c{new (std: : nothrow) char[x] }; g(a. get() , b. get() , c. get() ) ; }

§ 8.3.4

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void unmergeable(int x) { std: : unique_ptr a{new char[8] }; try { // Merging this allocation would change its catch handler. std: : unique_ptr b{new char[x] }; } catch (const std: : bad_alloc& e) { std: : cerr *

Abb reviat ing typ e

of

E1

expression

5

E1

not

U E1->*E2

The expression

cont ain

t o pro duce t he

cv-

cv-

T

”

t o it s first

is an unambiguous and accessible base

binds it s second op erand, which shall b e of typ e “ p oint er t o memb er of ”

t he

memb er

to

where

as

qualificat ion,

U

is eit her

E1. *E2 E1 E2 E2 ,

or a class of which

is called t he

refers,

t he

form

is

T

” t o it s firs t

is an unambiguous

(*(E1) ) . *E2

object expression

b ehavior

cv-

undefined.

.

.

If t he dynamic O t herwise,

t he

qualifiers of t he op erands are combined

are t he same as t he rules for

p oss ible t o use a p oint er t o memb er t hat

T

.

and t he manner in which t he

qualifiers of t he result ,

T

is convert ed int o t he equivalent

which

is sequenced b efore t he expression

The rest rict ions on

T

or a funct ion of t he typ e sp ecified by t he second op erand.

pm-expression. * cast-expression

do es

which shall b e of typ e “ p oint er t o memb er of

or of a class of which

which shall b e of typ e “ p oint er t o

and accessible base class.

4

T

which shall b e a glvalue of class

refers t o a

mutable

E1. E2

given in 8 . 2 . 5 .

memb er t o mo dify a

const

[

Note:

It

is not

class ob j ect .

For

examp le,

§ 8. 5

© IS O / IEC

2 0 1 7 – All right s reserved

1 30

ISO/IEC 14882: 2017(E)

struct S { S() : i(0) { } mutable int i; }; void f() { const S cs; int S: : * pm = &S: : i; cs. *pm = 88; }

— end note 6

]

If t he result op erat or

()

// pm refers to mutable member S: : i // ill-formed: cs is a const object

of .

[

.*

or

->*

is a funct ion,

Example:

t hen t hat

result

can b e used only as t he op erand for t he funct ion call

(ptr_to_obj ->*ptr_to_mfct) (10) ; calls t he memb er funct ion denot ed by In a

.*

expression whose ob j ect

ptr_to_mfct

for t he ob j ect p oint ed t o by

expression is an rvalue,

p oint er t o memb er funct ion wit h

ref-qualifier &

.

In a

.*

ptr_to_obj — end example .

expression whose ob j ect

expression is an lvalue,

p rogram is ill- formed if t he second op erand is a p oint er t o memb er funct ion wit h of a an

.*

]

t he program is ill- formed if t he second op erand is a

ref-qualifier &&

t he

.

The result

expres sion whose second op erand is a p oint er t o a dat a memb er is an lvalue if t he first

op erand is

lvalue

and

an xvalue

ot herwise.

memb er funct ion is a prvalue.

The

result

of a

.*

expression whose

second

op erand

is

If t he second op erand is t he null memb er p oint er value (7. 1 2 ) ,

a p oint er

to

a

t he b ehavior is

undefined.

8. 6 1

Multiplicative operators

The mult iplicat ive op erat ors

* / ,

,

[expr. mul]

and

%

group left - t o- right .

multiplicative-expression : pm-expression multiplicative-expression * pm-expression multiplicative-expression / pm-expression multiplicative-expression % pm-expression 2

The op erands of

*

and

/

shall have arit hmet ic or unscop ed enumerat ion typ e;

int egral or unscop ed enumerat ion typ e.

t he op erands of

%

shall have

The us ual arit hmet ic conversions are p erformed on t he op erands and

det ermine t he typ e of t he result .

3

The b inary

4

The binary of t he first

* /

op erat or indicat es mult iplicat ion.

op erat or yields t he quot ient ,

expression by t he s econd.

int egral op erands t he quot ient of b ot h

8. 7 1

a/b a/b

/

and t he binary

a%b

op erat or yields t he remainder from t he division

op erat or yields t he algebraic quot ient

is represent able in t he typ e of t he result ,

and

%

If t he second op erand of

/

or

%

is z ero t he b ehavior is undefined.

wit h any fract ional part

(a/b) *b + a%b

is equal t o

a

;

discarded;

ot herwise,

addit ive

For

if t he

t he b ehavior

is undefined.

Additive operators

The

85

op erat ors

+

and

[expr. add]

-

group

left - t o- right .

The

usual

arit hmet ic

conversions

are

p erformed

for

op erands of arit hmet ic or enumerat ion typ e.

additive-expression : multiplicative-expression additive-expression + multiplicative-expression additive-expression - multiplicative-expression 85)

This is oft en called t runcat ion t owards zero.

§ 8. 7

© IS O / IEC

2 0 1 7 – All right s reserved

1 31

ISO/IEC 14882: 2017(E)

For addit ion,

eit her b ot h op erands shall have arit hmet ic or unscop ed enumerat ion typ e,

b e a p oint er t o a complet ely- defined ob j ect

or one op erand shall

typ e and t he ot her shall have int egral or unscop ed enumerat ion

typ e.

2

For subt ract ion,

one of t he following shall hold:

(2. 1 )

—

b ot h op erands have arit hmet ic or unscop ed enumerat ion typ e;

(2. 2)

—

b ot h op erands are p oint ers t o cv- qualified or cv- unqualified versions of t he same complet ely- defined ob j ect

(2. 3)

—

typ e;

t he left

or

or

op erand is a p oint er t o a complet ely- defined ob j ect

typ e and t he right

op erand has int egral or

unscop ed enumerat ion typ e.

3

The result

+

of t he binary

op erat or is t he sum of t he op erands.

The result

of t he binary

-

op erat or is t he

difference result ing from t he subt ract ion of t he s econd op erand from t he first .

4

When an expression t hat of t he t he

p oint er

op erand.

P + J

expressions

x[i +

j

]

if 0

≤i j≤n +

( p ossibly- hyp ot het ical)

5

When

two

p oint ers

has int egral typ e is added t o or subt ract ed from a p oint er, If t he

and ;

expression

J + P

( where

ot herwise,

element

to

p oint s has

to

t he

x[i ]

element

value

j

)

of

j

]

if 0

t he

≤ i−j≤ n

same

array

;

ob j ect

to

t he

L ikewise,

ot herwise,

are

t he result

of an array ob j ect

p oint

t he b ehavior is undefined.

x[i −

element s

P J

x

has t he typ e

n

wit h

element s,

( p ossib ly- hyp ot het ical)

t he expression

P - J

86

element

p oint s t o t he

t he b ehavior is undefined.

subt ract ed,

t he

typ e

of

t he

result

is

an

std: : ptrdiff_element s x [ i ] and x [ j ]

implement at ion- defined signed int egral typ e; t his typ e shall b e t he same typ e t hat is defined as

t

in t he

header ( 2 1 . 2 ) .

x,

of t he same array ob j ect [

Note:

undefined.

6

For

cannot

is not

P - Q

P

Q

and

p oint

has t he value

i−j

t o, ;

resp ect ively,

ot herwise,

in t he range of repres ent able values of typ e

t he b ehavior is undefined.

std: : ptrdiff_t ,

or

subt ract ion,

typ e are not

if t he

similar (7. 5 ) ,

exp ressions

P

or

Q

have

typ e

t he b ehavior is undefined.

[

“ p oint er

Note:

to

cv

T” ,

In part icular,

and

t he result is a null p oint er value.

op erat ors

>

and

group left - t o- right .

shift-expression : additive-expression shift-expression > additive-expression The op erands shall b e of int egral or unscop ed enumerat ion typ e and int egral promot ions are p erformed. typ e of t he result negat ive,

2

t hat

of t he p romot ed left

E1 > E2

is

right

The

op erand is

op erand.

vacat ed bit s are z ero- filled.

has a signed typ e and non- negat ive value,

If

E1

has an unsigned

typ e,

t hen t hat

value,

and E1

×

2

E2

is rep resent ab le

convert ed t o t he result

typ e,

is t he

t he b ehavior is undefined.

E1

right - s hift ed

E2

bit

p os it ions.

If

E1

has an unsigned typ e or if

E1

has a signed

An ob j ect t hat is not an array element is considered t o b elong t o a single- element array for t his purp ose; see 8. 3. 1 .

p oint er past t he last element of an array

x [n ]

b ehavior is

reduced mo dulo one more t han t he maximum value repres ent able in

in t he corresp onding unsigned typ e of t he result

3

The

or great er t han or equal t o t he lengt h in bit s of t he promot ed left

The value of typ e,

is

x

of

n

A

element s is considered t o b e equivalent t o a p oint er t o a hyp ot het ical element

for t his purp ose; see 6. 9. 2 .

§ 8. 8

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ISO/IEC 14882: 2017(E)

type and a non-negative value, the value of the result is the integral part of the quotient of E1 / 2 E2 . If E1 has a signed type and a negative value, the resulting value is implementation-defined. 4

The expression E1 is sequenced before the expression E2 .

1

The relational operators group left-to-right. [ Example: am = 7; T(a) ++; T(a, 5) x. i = 99; p->x. j = 99;

— end example ] 5 The semantics of an access through a volatile glvalue are implementation-defined. If an attempt is made to

access an obj ect defined with a volatile-qualified type through the use of a non-volatile glvalue, the behavior is undefined.

6 [ Note: volatile is a hint to the implementation to avoid aggressive optimization involving the object because

the value of the obj ect might be changed by means undetectable by an implementation. Furthermore, for some implementations, volatile might indicate that special hardware instructions are required to access the obj ect. See 4.6 for detailed semantics. In general, the semantics of volatile are intended to be the same in C++ as they are in C. — end note ]

1 0. 1.7. 2

Simple type specifiers

1 The simple type specifiers are

[dcl.type.simple]

simple-type-specifier: nested-name-specifieropt type-name nested-name-specifier template simple-template-id nested-name-specifieropt template-name char char16_t char32_t wchar_t bool short int long signed unsigned float double void auto decltype-specifier type-name : class-name enum-name typedef-name simple-template-id decltype-specifier: decltype ( expression ) decltype ( auto )

2 The simple-type-specifier auto is a placeholder for a type to be deduced (10.1.7.4) . A type-specifier of the form typename opt nested-name-specifier opt template-name is a placeholder for a deduced class type (10.1.7.5) . The template-name shall name a class template that is not an injected-class-name. The other simple-type-specifiers

specify either a previously-declared type, a type determined from an expression, or one of the fundamental types (6.9.1) . Table 11 summarizes the valid combinations of simple-type-specifier s and the types they specify.

3 When multiple simple-type-specifiers are allowed, they can be freely intermixed with other decl-specifiers in any order. [ Note: It is implementation-defined whether obj ects of char type are represented as signed or

unsigned quantities. The signed specifier forces char obj ects to be signed; it is redundant in other contexts. § 10.1.7.2

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Table 1 1

—

simple-type-specifier

Specifier(s)

Type

type-name simple-template-id template-name

t he typ e named t he typ e as defined in 1 7. 2 placeholder for a typ e t o b e deduced

char unsigned char signed char char16_t char32_t bool unsigned unsigned int signed signed int int unsigned short int unsigned short unsigned long int unsigned long unsigned long long int unsigned long long signed long int signed long signed long long int signed long long long long int long long long int long signed short int signed short short int short wchar_t float double long double void auto decltype(auto) decltype( expression)

— end note 4 (4. 1)

if

e

“ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “ “

char unsigned char signed char char16_t char32_t bool unsigned int unsigned int int int int unsigned short int unsigned short int unsigned long int unsigned long int unsigned long long int unsigned long long int long int long int long long int long long int long long int long long int long int long int short int short int short int short int wchar_t float double long double void ”

”

”

” ”

”

” ”

”

”

”

”

”

”

”

” ”

” ”

” ” ” ”

”

”

” ” ”

”

”

”

”

”

”

placeholder for a typ e t o b e deduced placeholder for a typ e t o b e deduced t he typ e as defined b elow

]

For an express ion

—

s and t he typ es t hey sp ecify

is

an

e

,

t he typ e denot ed by

unparent hes iz ed

decltype(e)

id-expression

is defined as follows:

naming

a

st ruct ured

binding

(1 1 . 5 ) ,

decltype(e)

is

t he

referenced typ e as given in t he sp ecificat ion of t he s t ruct ured binding declarat ion;

§ 1 0 . 1 . 7. 2

© IS O / IEC

2 0 1 7 – All right s reserved

1 68

ISO/IEC 1 4882:201 7(E)

(4. 2)

— otherwise, if e is an unparenthesized id-expression or an unparenthesized class member access (8.2.5) , decltype(e) is the type of the entity named by e . If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;

(4. 3)

— otherwise, if e is an xvalue, decltype(e) is T&& , where T is the type of e ;

(4. 4)

— otherwise, if e is an lvalue, decltype(e) is T& , where T is the type of e ;

(4. 5)

— otherwise, decltype(e) is the type of e . The operand of the decltype specifier is an unevaluated operand (Clause 8) . [ Example: const int&& foo() ; int i; struct A { double x; }; const A* a = new A() ; decltype(foo() ) x1 = 17; decltype(i) x2; decltype(a->x) x3; decltype((a->x) ) x4 = x3;

5

// type // type // type // type

is is is is

const int&& int double const double&

— end example ] [ Note: The rules for determining types involving decltype(auto) are specified in 10.1.7.4. — end note ] If the operand of a decltype-specifier is a prvalue, the temporary materialization conversion is not applied (7.4) and no result obj ect is provided for the prvalue. The type of the prvalue may be incomplete. [ Note: As a result, storage is not allocated for the prvalue and it is not destroyed. Thus, a class type is not instantiated as a result of being the type of a function call in this context. In this context, the common purpose of writing the expression is merely to refer to its type. In that sense, a decltype-specifier is analogous to a use of a typedef-name , so the usual reasons for requiring a complete type do not apply. In particular, it is not necessary to allocate storage for a temporary obj ect or to enforce the semantic constraints associated with invoking the type’s destructor. — end note ] [ Note: Unlike the preceding rule, parentheses have no special meaning in this context. — end note ] [ Example: template struct A { template auto h() -> A; template auto i(T) -> T; template auto f(T) -> decltype(i(h() ) ) ;

template auto f(T) -> void; auto g() -> void { f(42) ;

} template auto q(T) -> decltype((h() ) ) ; void r() { q(42) ;

§ 10.1.7.2

~ A()

= delete; };

// identity // #1 // forces completion of A and implicitly uses A: : ~ A() // for the temporary introduced by the use of h() . // (A temporary is not introduced as a result of the use of i() .) // #2

// OK: calls #2. (#1 is not a viable candidate: type deduction // fails (17.8.2) because A: : ~ A() is implicitly used in its // decltype-specifier) // does not force completion of A ; A: : ~ A() is not implicitly // used within the context of this decltype-specifier // error: deduction against q succeeds, so overload resolution selects // the specialization “q(T) -> decltype((h() ) ) [with T=int] ”; // the return type is A , so a temporary is introduced and its

© ISO/IEC 2017 – All rights reserved

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// destructor is used, so the program is ill-formed

}

— end example ] 10. 1. 7.3

Elaborated type specifiers

[dcl.type. elab]

elaborated-type-specifier: class-key attribute-specifier-seqopt nested-name-specifieropt identifier class-key simple-template-id class-key nested-name-specifier template opt simple-template-id enum nested-name-specifieropt identifier 1

An attribute-specifier-seq shall not appear in an elaborated-type-specifier unless the latter is the sole constituent of a declaration. If an elaborated-type-specifier is the sole constituent of a declaration, the declaration is ill-formed unless it is an explicit specialization (17.7.3) , an explicit instantiation (17.7.2) or it has one of the following forms: class-key attribute-specifier-seqopt identifier ; friend class-key : : opt identifier ; friend class-key : : opt simple-template-id ; friend class-key nested-name-specifier identifier ; friend class-key nested-name-specifier template opt simple-template-id ;

2

In the first case, the attribute-specifier-seq , if any, appertains to the class being declared; the attributes in the attribute-specifier-seq are thereafter considered attributes of the class whenever it is named. 6.4.4 describes how name lookup proceeds for the identifier in an elaborated-type-specifier . If the identifier resolves to a class-name or enum-name , the elaborated-type-specifier introduces it into the declaration the same way a simple-type-specifier introduces its type-name . If the identifier resolves to a typedef-name or the simple-template-id resolves to an alias template specialization, the elaborated-type-specifier is ill-formed. [ Note: This implies that, within a class template with a template type-parameter T, the declaration friend class T;

is ill-formed. However, the similar declaration friend T; is allowed (14.3) . 3

— end note ]

The class-key or enum keyword present in the elaborated-type-specifier shall agree in kind with the declaration to which the name in the elaborated-type-specifier refers. This rule also applies to the form of elaborated-typespecifier that declares a class-name or friend class since it can be construed as referring to the definition of the class. Thus, in any elaborated-type-specifier , the enum keyword shall be used to refer to an enumeration (10.2) , the union class-key shall be used to refer to a union (Clause 12) , and either the class or struct class-key shall be used to refer to a class (Clause 12) declared using the class or struct class-key . [ Example: enum class E { a, b }; enum E x = E: : a;

// OK

— end example ] auto specifier [dcl. spec. auto] The auto and decltype(auto) type-specifier s are used to designate a placeholder type that will be replaced later by deduction from an initializer. The auto type-specifier is also used to introduce a function type having a trailing-return-type or to signify that a lambda is a generic lambda (8.1.5.1) . The auto type-specifier is also

10. 1. 7.4 1

The

used to introduce a structured binding declaration (11.5) . 2

The placeholder type can appear with a function declarator in the decl-specifier-seq , type-specifier-seq , conversion-function-id, or trailing-return-type, in any context where such a declarator is valid. If the function declarator includes a trailing-return-type (11.3.5) , that trailing-return-type specifies the declared return type of the function. Otherwise, the function declarator shall declare a function. If the declared return type of the

§ 10.1.7.4

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function contains a placeholder type, the return type of the function is deduced from non-discarded return statements, if any, in the body of the function (9.4.1) . 3

If the auto type-specifier appears as one of the decl-specifier s in the decl-specifier-seq of a parameter-declaration of a lambda-expression , the lambda is a generic lambda (8.1.5.1) . [ Example: auto glambda = [] (int i, auto a) { return i; };

// OK: a generic lambda

— end example ] 4

The type of a variable declared using auto or decltype(auto) is deduced from its initializer. This use is allowed in an initializing declaration (11.6) of a variable. auto or decltype(auto) shall appear as one of the decl-specifiers in the decl-specifier-seq and the decl-specifier-seq shall be followed by one or more declarator s, each of which shall be followed by a non-empty initializer . In an initializer of the form ( expression-list )

the

expression-list shall be a single assignment-expression . [ Example:

auto x = 5; const auto *v = &x, u = 6; static auto y = 0. 0; auto int r; auto f() -> int; auto g() { return 0. 0; } auto h() ;

// OK: x has type int // OK: v has type const int* , u has type const int // OK: y has type double // error: auto is not a storage-class-specifier // OK: f returns int // OK: g returns double // OK: h’s return type will be deduced when it is defined

— end example ] 5

A placeholder type can also be used in the type-specifier-seq in the new-type-id or type-id of a newexpression (8.3.4) and as a decl-specifier of the parameter-declaration’s decl-specifier-seq in a templateparameter (17.1) .

6

A program that uses auto or decltype(auto) in a context not explicitly allowed in this section is ill-formed.

7

If the init-declarator-list contains more than one init-declarator , they shall all form declarations of variables. The type of each declared variable is determined by placeholder type deduction (10.1.7.4.1) , and if the type that replaces the placeholder type is not the same in each deduction, the program is ill-formed. [ Example: auto x = 5, *y = &x; auto a = 5, b = { 1, 2 };

// OK: auto is int // error: different types for auto

— end example ] 8

If a function with a declared return type that contains a placeholder type has multiple non-discarded return statements, the return type is deduced for each such return statement. If the type deduced is not the same in each deduction, the program is ill-formed.

9

If a function with a declared return type that uses a placeholder type has no non-discarded return statements, the return type is deduced as though from a return statement with no operand at the closing brace of the function body. [ Example: auto f() { } auto* g() { }

// OK, return type is void // error, cannot deduce auto* from void()

— end example ] 10

If the type of an entity with an undeduced placeholder type is needed to determine the type of an expression, the program is ill-formed. Once a non-discarded return statement has been seen in a function, however, the return type deduced from that statement can be used in the rest of the function, including in other return statements. [ Example:

§ 10.1.7.4

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ISO/IEC 1 4882:201 7(E)

// error, n’s type is unknown

auto auto void auto if

n = n; f() ; g() { &f; } sum(int i) { (i == 1) return i; else return sum(i-1) +i;

// error, f ’s return type is unknown

// sum’s return type is int // OK, sum’s return type has been deduced

}

— end example ] 11

Return type deduction for a function template with a placeholder in its declared type occurs when the definition is instantiated even if the function body contains a return statement with a non-type-dependent operand. [ Note: Therefore, any use of a specialization of the function template will cause an implicit instantiation. Any errors that arise from this instantiation are not in the immediate context of the function type and can result in the program being ill-formed (17.8.2) . — end note ] [ Example: template auto f(T t) { return t; } typedef decltype(f(1) ) fint_t; template auto f(T* t) { return *t; } void g() { int (*p) (int*) = &f; }

// return type deduced at instantiation time // instantiates f to deduce return type // instantiates both f s to determine return types, // chooses second

— end example ] 12

Redeclarations or specializations of a function or function template with a declared return type that uses a placeholder type shall also use that placeholder, not a deduced type. [ Example: auto f() ; auto f() { return 42; } auto f() ; int f() ; decltype(auto) f() ;

// return type is int // OK // error, cannot be overloaded with auto f() // error, auto and decltype(auto) don’t match

template auto g(T t) { return t; } // #1 template auto g(int) ; // OK, return type is int template char g(char) ; // error, no matching template template auto g(double) ; // OK, forward declaration with unknown return type template T g(T t) { return t; } template char g(char) ; template auto g(float) ;

// OK, not functionally equivalent to #1 // OK, now there is a matching template // still matches #1

void h() { return g(42) ; }

// error, ambiguous

template struct A { friend T frf(T) ; }; auto frf(int i) { return i; }

// not a friend of A

— end example ] 13

A function declared with a return type that uses a placeholder type shall not be virtual (13.3) .

14

An explicit instantiation declaration (17.7.2) does not cause the instantiation of an entity declared using a placeholder type, but it also does not prevent that entity from being instantiated as needed to determine its type. [ Example: template auto f(T t) { return t; }

§ 10.1.7.4

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ISO/IEC 14882: 2017(E)

extern template auto f(int) ; int (*p) (int) = f;

// does not instantiate f // instantiates f to determine its return type, but an explicit // instantiation definition is still required somewhere in the program

— end example ] 10. 1. 7.4.1 1

Placeholder type deduction

[dcl. type. auto.deduct]

Placeholder type deduction is the process by which a type containing a placeholder type is replaced by a deduced type.

2

A type T containing a placeholder type, and a corresponding initializer e , are determined as follows:

(2. 1 )

— for a non-discarded return statement that occurs in a function declared with a return type that contains a placeholder type, T is the declared return type and e is the operand of the return statement. If the return statement has no operand, then e is void() ;

(2. 2)

— for a variable declared with a type that contains a placeholder type, T is the declared type of the variable and e is the initializer. If the initialization is direct-list-initialization, the initializer shall be a braced-init-list containing only a single assignment-expression and e is the assignment-expression ;

(2. 3)

— for a non-type template parameter declared with a type that contains a placeholder type, T is the declared type of the non-type template parameter and e is the corresponding template argument. In the case of a return statement with no operand or with an operand of type void, T shall be either decltype(auto) or cv auto .

braced-init-list (11.6.4) , the program is ill-formed.

3

If the deduction is for a return statement and e is a

4

If the placeholder is the auto type-specifier , the deduced type T 0 replacing T is determined using the rules for template argument deduction. Obtain P from T by replacing the occurrences of auto with either a new invented type template parameter U or, if the initialization is copy-list-initialization, with std: : initializer_list . Deduce a value for U using the rules of template argument deduction from a function call (17.8.2.1) , where P is a function template parameter type and the corresponding argument is e . If the deduction fails, the declaration is ill-formed. Otherwise, T 0 is obtained by substituting the deduced U into P . [ Example: auto auto auto auto auto

x1 = { 1, 2 }; x2 = { 1, 2. 0 }; x3{ 1, 2 }; x4 = { 3 }; x5{ 3 };

// decltype(x1) is std: : initializer_list // error: cannot deduce element type // error: not a single element // decltype(x4) is std: : initializer_list // decltype(x5) is int

— end example ] [ Example: const auto &i = expr;

The type of i is the deduced type of the parameter u in the call f(expr) of the following invented function template: template void f(const U& u) ;

— end example ] 5

If the placeholder is the decltype(auto) type-specifier , T shall be the placeholder alone. The type deduced for T is determined as described in 10.1.7.2, as though e had been the operand of the decltype . [ Example: int i; int&& f() ; auto decltype(auto) auto decltype(auto)

§ 10.1.7.4.1

x2a(i) ; x2d(i) ; x3a = i; x3d = i;

// decltype(x2a) // decltype(x2d) // decltype(x3a) // decltype(x3d)

is is is is

int int int int

© ISO/IEC 2017 – All rights reserved

173

ISO/IEC 14882: 2017(E)

auto x4a decltype(auto) x4d auto x5a decltype(auto) x5d auto x6a decltype(auto) x6d auto *x7a decltype(auto) *x7d

= = = = = = = =

(i) ; (i) ; f() ; f() ; { 1, 2 }; { 1, 2 }; &i; &i;

// decltype(x4a) is int // decltype(x4d) is int& // decltype(x5a) is int // decltype(x5d) is int&& // decltype(x6a) is std: : initializer_list // error, { 1, 2 } is not an expression // decltype(x7a) is int* // error, declared type is not plain decltype(auto)

— end example ] 10. 1. 7.5

Deduced class template specialization types

[dcl. type.class.deduct]

1

If a placeholder for a deduced class type appears as a decl-specifier in the decl-specifier-seq of an initializing declaration (11.6) of a variable, the placeholder is replaced by the return type of the function selected by overload resolution for class template deduction (16.3.1.8) . If the decl-specifier-seq is followed by an init-declarator-list or member-declarator-list containing more than one declarator , the type that replaces the placeholder shall be the same in each deduction.

2

A placeholder for a deduced class type can also be used in the type-specifier-seq in the new-type-id or type-id of a new-expression (8.3.4) , or as the simple-type-specifier in an explicit type conversion (functional notation) (8.2.3) . A placeholder for a deduced class type shall not appear in any other context.

3

[ Example: template struct container { container(T t) {} template container(Iter beg, Iter end) ; }; template container(Iter b, Iter e) -> container; std: : vector v = { /* . . . */ }; container c(7) ; // OK, deduces int for T auto d = container(v. begin() , v. end() ) ; // OK, deduces double for T container e{5, 6}; // error, int is not an iterator

— end example ] 1 0. 2 1

Enumeration declarations

An enumeration is a distinct type (6.9.2) with named constants. Its name becomes an scope.

[dcl. enum]

enum-name within its

enum-name : identifier enum-specifier: enum-head { enumerator-listopt } enum-head { enumerator-list , } enum-head : enum-key attribute-specifier-seqopt enum-head-nameopt enum-baseopt enum-head-name : nested-name-specifieropt identifier opaque-enum-declaration : enum-key attribute-specifier-seqopt nested-name-specifieropt identifier enum-baseopt ;

§ 10.2

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ISO/IEC 14882: 2017(E)

enum-key : enum enum class enum struct enum-base : : type-specifier-seq enumerator-list : enumerator-definition enumerator-list , enumerator-definition enumerator-definition : enumerator enumerator = constant-expression enumerator: identifier attribute-specifier-seqopt

attribute-specifier-seq enum-head opaque-enum-declaration attribute-specifier-seq : enum nested-name-specifier opt identifier member-declaration enum-base Note: enum-base Example:

The opt ional merat ion;

whenever it of a

in t he

and t he

t he at t ribut es in t hat is named.

A

following “

is

p arsed

as

”

part

of an

.

[

b etween t he declarat ion of an enumerat ion wit h an enumerat ion typ e.

This

decl-specifier-seq

wit hin t he

resolves

a

p ot ent ial

ambiguity

and t he declarat ion of an unnamed bit - field of

[

struct S { enum E : int {}; enum E : int {}; };

// error: redeclaration of enumeration

— end example — end note ]

s hall b e an explicit

2

ap p ert ains t o t he enu-

are t hereaft er considered at t ribut es of t he enumerat ion

]

If an

opaque-enum-declaration

cont ains a

nested-name-specifier

, t he declarat ion

sp ecializ at ion (1 7. 7. 3 ) .

enum-key enum unscoped enumeration enumerator enum-key enum class enum struct scoped enumeration enumerator scoped enumerators identifier type-specifier-seq enum-base opaque-enum-declaration enum-base enumerator-list enumeratordefinition = enumerator constant-expression enumerator initializer enumerator-definition initializer enumerator enumerator Example: The enumerat ion typ e declared wit h an are

unscoped enumerators

enumerat ion

typ e

.

.

declared

The

of only

The

wit h

one

of

opt ional

The

is an

s

t hese

shall

of an

, and it s

and

is

not

shall

a

,

be

name

omit t ed

an

in

int egral

t he

and

it s

declarat ion

typ e;

declaring an unscop ed enumerat ion shall not

an

any

omit

s

of

a

scop ed

cv- qualificat ion

t he

.

gives t he as so ciat ed

has

wit hout

no

,

an

gives

by one.

t he

is

of t he

t he

corresp onding const ant

t he

value

obt ained

by

is

are

enumerat ion. ignored.

.

z ero.

An

increasing

t he

An

An

t he value indicat ed by t he

value

an

The ident ifiers in

are declared as const ant s, and can app ear wherever cons t ant s are required.

wit h

s

are semant ically equivalent ;

value

of

If t he first

t he

previous

[

enum { a, b, c=0 }; enum { d, e, f=e+2 }; defines

a c ,

,

d

and

to

be

in an

3

An

and

is

a

enum-specifier a

different

[

complet e .

typ e.

— end note

underlying

typ e.

redeclarat ion shall include an

4

and

e

to

be

1

,

app ert ains t o t hat

f

and

to

3

be

— end example

.

]

The

If t he

§ 1 0. 2

enum-key

]

A

An

list

of enumerat ors

scop ed

enumerat ion

unscop ed

attribute-

can shall

enumerat ion

enum-base nested-name-specifier

scop e or a declarat ion

opaque-enum-declaration

An enumerat ion declared by an The

opt ional

enumerat or.

is eit her a redeclarat ion of an enumerat ion in t he current

of a new enumerat ion. typ e

b

z ero,

specifier-seq enumerator opaque-enum-declaration Note:

be

provided

not

shall

be

not

in

lat er

be

a

lat er

redeclared

lat er

has fixed underlying

redeclarat ion as

redeclared

unscop ed

as

scop ed

wit h

an

or

wit h

and

each

sp ecifying t he same underlying typ e as in t he original declarat ion.

is followed by a

© IS O / IEC

,

t he

enum-specifier

2 0 1 7 – All right s reserved

s hall refer t o an enumerat ion t hat

1 75

ISO/IEC 14882: 2017(E)

was

previously

declared

direct ly in

t he

neit her inherit ed nor int ro duced by a

class

or

namespace

using-declaration

),

to

nested-name-specifier enum-specifier

which t he

and t he

refers

( i. e. ,

shall app ear in a namespace

enclosing t he previous declarat ion.

5

Each enumerat ion defines a typ e t hat is different from all ot her typ es.

type

.

underlying typ e is

fixed

.

int

if it is not exp licit ly sp ecified.

Following t he closing brace of an

Each enumerat ion also has an

enum-base

The underlying typ e can b e explicit ly sp ecified using an

.

In b ot h of t hese cases,

enum-specifier

,

underlying

For a scop ed enumerat ion typ e,

t he

t he underlying typ e is said t o b e

each enumerat or has t he typ e of it s enumerat ion.

If

t he underlying typ e is fixed,

t he typ e of each enumerat or prior t o t he closing brace is t he underlying typ e

and

in

t he

constant-expression

underlying typ e ( 8 . 2 0 ) .

enumerator-definition

t he

If t he underlying typ e is not

shall

fixed,

be

a convert ed

const ant

expression

of t he

t he typ e of each enumerat or prior t o t he closing

brace is det ermined as follows:

(5. 1 )

If an init ializ er

—

is

expression ( 8 . 2 0 ) . typ e of t hat

sp ecified for an enumerat or,

enumerat ion typ e,

ot herwise it

(5. 2)

—

If no init ializ er is sp ecified for t he first

(5. 3)

—

O t herwise

t he

typ e

of t he

increment ed value is not typ e sufficient

6

enumerat or,

enumerat or

is

shall b e

t he

it s typ e is an uns p ecified signed int egral typ e.

same

represent able in t hat

as

typ e,

t hat

of t he

preceding

enumerat or

whose underlying typ e is not t he closing

For

an

}

of it s

If no s uch typ e exist s,

all

t he

whose

at

values

which p oint

which p oint

typ e

defined

is

fit

in an

t hat

int

or

it

not

in

it

b ecomes

t he

t he program is ill- formed.

of declarat ion (6 . 3 . 2 )

a complet e typ e.

An enumerat ion

of declarat ion t o immediat ely aft er

underlying

enumerat ion.

It

If no

typ e

is

int egral

an

int egral

typ e

can

typ e

t hat

represent

can

all

t he

is implement at ion- defined which int egral typ e is used

t he underlying typ e shall not

unsigned int .

t he

b ecomes a comp let e typ e.

fixed,

t he

t he enumerat ion is ill- formed.

as t he underlying typ e except enumerat or cannot

,

underlying

enumerat or

enumerat or values,

at

fixed is an incomplet e typ e from it s p oint

enum-specifier

enumerat ion

represent

( if any) ,

unless

in which case t he typ e is an unsp ecified int egral

An enumerat ion whose underlying typ e is fixed is an incomplet e typ e from it s p oint

enum-base

an int egral const ant

t he enumerat or has t he underlying

has t he same typ e as t he exp ression.

t o cont ain t he increment ed value.

t o immediat ely aft er it s

7

constant-expression

t he

If t he expression has unscop ed enumerat ion typ e,

If t he

b e larger t han

enumerator-list

int

is empty,

unless t he value of an

t he underlying typ e is as

if t he enumerat ion had a single enumerat or wit h value 0 .

8

For

an

enumerat ion

underlying typ e.

whose

underlying

typ e

is

fixed,

t he

O t herwise, for an enumerat ion where e

min

values

t he values of t he enumerat ion are t he values in t he range b a two’ s is

t he

complement

smallest

value

non- negat ive int eger.

represent at ion and 0 great er

b

min

t han or

is z ero if e

for a ones’

|

equal t o max ( e

min

of t he

min

to b

complement

min

enumerat ion

max ,

t he

It

| − | max | − max K,

is non- negat ive and

enumerator-list

wit h value 0 .

9 10

is p ossible t o define an enumerat ion t hat is empty,

(b

e

)

and

+ K)

equal t o

ot herwise.

value

has values not

2

M−

values

max

of t he

is t he largest ,

L et

K be 1

for

b

max

represent at ion. 1,

where

M is a

The siz e of t he smallest

if b

min

is z ero and M + 1

defined by any of it s enumerat ors.

If

t he values of t he enumerat ion are as if t he enumerat ion had a single enumerat or

96

Two enumerat ion typ es are

The

t he

defined as follows:

or sign- magnit ude

bit - field large enough t o hold all t he values of t he enumerat ion typ e is max ( M, 1 ) ot herwise.

are

is t he smallest enumerat or and e

of an

layout-compatible enumerations

enumerat or

int egral promot ion ( 7. 6 ) .

[

or

an

ob j ect

Example:

of an

unscop ed

if t hey have t he same underlying typ e.

enumerat ion

typ e

is

convert ed

to

an

int eger

by

enum color { red, yellow, green=20, blue }; color col = red; color* cp = &col; if (*cp == blue) // ... 96)

This set of values is used t o define promot ion and conversion semant ics for t he enumerat ion typ e.

It do es not preclude an

expression of enumerat ion typ e from having a value t hat falls out side t his range.

§ 1 0. 2

© IS O / IEC

2 0 1 7 – All right s reserved

1 76

ISO/IEC 14882: 2017(E)

makes color a type describing various colors, and then declares col as an obj ect of that type, and cp as a pointer to an obj ect of that type. The possible values of an obj ect of type color are red, yellow, green, blue ; these values can be converted to the integral values 0 , 1 , 20 , and 21 . Since enumerations are distinct types, obj ects of type color can be assigned only values of type color . color c = 1; int i = yellow;

// error: type mismatch, no conversion from int to color // OK: yellow converted to integral value 1 , integral promotion

Note that this implicit enum to int conversion is not provided for a scoped enumeration: enum class Col { red, yellow, green }; int x = Col: : red; // error: no Col to int conversion Col y = Col: : red; if (y) { } // error: no Col to bool conversion

11

— end example ] Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier . Each scoped enumerator is declared in the scope of the enumeration. These names obey the scope rules defined for all names in 6.3 and 6.4. [ Example: enum direction { left=’ l’ , right=’ r’ }; void g() { direction d; d = left; d = direction: : right; }

// OK // OK // OK

enum class altitude { high=’ h’ , low=’ l’ }; void h() { altitude a; a = high; a = altitude: : low; }

// OK // error: high not in scope // OK

— end example ] An enumerator declared in class scope can be referred to using the class member access operators ( : : , . (dot) and -> (arrow) ) , see 8.2.5. [ Example: struct X { enum direction { left=’ l’ , right=’ r’ }; int f(int i) { return i==left ? 0 : i==right ? 1 : 2; } }; void g(X* p) { direction d; int i; i = p->f(left) ; i = p->f(X: : right) ; i = p->f(p->left) ; // ... }

12

// error: direction not in scope // error: left not in scope // OK // OK

— end example ] If an enum-head contains a nested-name-specifier , the enum-specifier shall refer to an enumeration that was previously declared directly in the class or namespace to which the nested-name-specifier refers, or in an element of the inline namespace set (10.3.1) of that namespace (i.e., not merely inherited or introduced by a

using-declaration) , and the enum-specifier shall appear in a namespace enclosing the previous declaration. In § 10.2

© ISO/IEC 2017 – All rights reserved

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such cases, the nested-name-specifier of the enum-head of the definition shall not begin with a decltype-specifier . 1 0. 3

Namespaces

[basic. namespace]

1

A namespace is an optionally-named declarative region. The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace. Unlike other declarative regions, the definition of a namespace can be split over several parts of one or more translation units.

2

The outermost declarative region of a translation unit is a namespace; see 6.3.6. 1 0. 3. 1

Namespace definition

[namespace. def]

namespace-name : identifier namespace-alias namespace-definition : named-namespace-definition unnamed-namespace-definition nested-namespace-definition named-namespace-definition : inline opt namespace attribute-specifier-seqopt identifier { namespace-body } unnamed-namespace-definition : inline opt namespace attribute-specifier-seqopt { namespace-body } nested-namespace-definition : namespace enclosing-namespace-specifier : : identifier { namespace-body } enclosing-namespace-specifier: identifier enclosing-namespace-specifier : : identifier namespace-body : declaration-seqopt 1 2

3

namespace-definition shall appear in the global scope or in a namespace scope (6.3.6) . In a named-namespace-definition , the identifier is the name of the namespace. If the identifier , when looked up (6.4.1) , refers to a namespace-name (but not a namespace-alias ) that was introduced in the namespace in which the named-namespace-definition appears or that was introduced in a member of the inline namespace set of that namespace, the namespace-definition extends the previously-declared namespace. Otherwise, the identifier is introduced as a namespace-name into the declarative region in which the named-namespace-definition appears. Because a namespace-definition contains declaration s in its namespace-body and a namespace-definition is itself a declaration, it follows that namespace-definition s can be nested. [ Example: Every

namespace Outer { int i; namespace Inner { void f() { i++; } int i; void g() { i++; } } }

4

// Outer: : i // Inner: : i

— end example ] The enclosing namespaces of a declaration are those namespaces in which the declaration lexically appears, except for a redeclaration of a namespace member outside its original namespace (e.g., a definition as

§ 10.3.1

© ISO/IEC 2017 – All rights reserved

178

ISO/IEC 14882: 2017(E)

specified in 10.3.1.2) . Such a redeclaration has the same enclosing namespaces as the original declaration. [ Example: namespace Q { namespace V { void f() ; class C { void m() ; }; } void V: : f() { extern void h() ; } void V: : C: : m() { } }

// enclosing namespaces are the global namespace, Q , and Q: : V

// enclosing namespaces are the global namespace, Q , and Q: : V // ... so this declares Q: : V: : h // enclosing namespaces are the global namespace, Q , and Q: : V

— end example ] 5

If the optional initial inline keyword appears in a namespace-definition for a particular namespace, that namespace is declared to be an inline namespace . The inline keyword may be used on a namespace-definition that extends a namespace only if it was previously used on the namespace-definition that initially declared the namespace-name for that namespace.

6

The optional attribute-specifier-seq in a defined or extended.

7

Members of an inline namespace can be used in most respects as though they were members of the enclosing namespace. Specifically, the inline namespace and its enclosing namespace are both added to the set of associated namespaces used in argument-dependent lookup (6.4.2) whenever one of them is, and a usingdirective (10.3.4) that names the inline namespace is implicitly inserted into the enclosing namespace as for an unnamed namespace (10.3.1.1) . Furthermore, each member of the inline namespace can subsequently be partially specialized (17.5.5) , explicitly instantiated (17.7.2) , or explicitly specialized (17.7.3) as though it were a member of the enclosing namespace. Finally, looking up a name in the enclosing namespace via explicit qualification (6.4.3.2) will include members of the inline namespace brought in by the using-directive even if there are declarations of that name in the enclosing namespace.

8

These properties are transitive: if a namespace N contains an inline namespace M, which in turn contains an inline namespace O , then the members of O can be used as though they were members of M or N. The inline namespace set of N is the transitive closure of all inline namespaces in N. The enclosing namespace set of O is the set of namespaces consisting of the innermost non-inline namespace enclosing an inline namespace O , together with any intervening inline namespaces.

9

A nested-namespace-definition with an equivalent to

named-namespace-definition appertains to the namespace being

enclosing-namespace-specifier E, identifier

I and

namespace-body

B is

namespace E { namespace I { B } }

[ Example: namespace A: : B: : C { int i; }

The above has the same effect as: namespace A { namespace B { namespace C { int i; } }

§ 10.3.1

© ISO/IEC 2017 – All rights reserved

179

ISO/IEC 14882: 2017(E)

}

— end example ] 10. 3. 1 .1 1

An

Unnamed namespaces

[namespace.unnamed]

unnamed-namespace-definition behaves as if it were replaced by inline opt namespace unique { /* empty body */ } using namespace unique ; namespace unique { namespace-body }

where inline appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the translation unit. The optional attribute-specifier-seq in the unnamed-namespace-definition appertains to unique. [ Example: namespace { int i; } void f() { i++; } namespace A { namespace { int i; int j ; } void g() { i++; } } using namespace A; void h() { i++; A: : i++; j ++; }

// unique: : i // unique: : i++

// A: : unique: : i // A: : unique: : j // A: : unique: : i++

// error: unique: : i or A: : unique: : i // A: : unique: : i // A: : unique: : j

— end example ] 10. 3. 1 .2 1

Namespace member definitions

[namespace. memdef]

A declaration in a namespace N (excluding declarations in nested scopes) whose declarator-id is an unqualifiedid (11.3) , whose class-head-name (Clause 12) or enum-head-name (10.2) is an identifier, or whose elaboratedtype-specifier is of the form class-key attribute-specifier-seqopt identifier (10.1.7.3) , or that is an opaque-enumdeclaration, declares (or redeclares) its unqualified-id or identifier as a member of N. [ Note: An explicit instantiation (17.7.2) or explicit specialization (17.7.3) of a template does not introduce a name and thus may be declared using an unqualified-id in a member of the enclosing namespace set, if the primary template is declared in an inline namespace. — end note ] [ Example: namespace X { void f() { /* . . . */ } namespace M { void g() ; } using M: : g; void g() ; }

// OK: introduces X: : f()

// OK: introduces X: : M: : g()

// error: conflicts with X: : M: : g()

— end example ] 2

Members of a named namespace can also be defined outside that namespace by explicit qualification (6.4.3.2) § 10.3.1.2

© ISO/IEC 2017 – All rights reserved

180

ISO/IEC 14882: 2017(E)

of the name being defined, provided that the entity being defined was already declared in the namespace and the definition appears after the point of declaration in a namespace that encloses the declaration’s namespace. [ Example: namespace Q { namespace V void f() ; } void V: : f() void V: : g() namespace V void g() ; } }

{

{ /* . . . */ } { /* . . . */ } {

namespace R { void Q: : V: : g() { /* . . . */ } }

// OK // error: g() is not yet a member of V

// error: R doesn’t enclose Q

— end example ] 3

If a friend declaration in a non-local class first declares a class, function, class template or function template97 the friend is a member of the innermost enclosing namespace. The friend declaration does not by itself make the name visible to unqualified lookup (6.4.1) or qualified lookup (6.4.3) . [ Note: The name of the friend will be visible in its namespace if a matching declaration is provided at namespace scope (either before or after the class definition granting friendship) . — end note ] If a friend function or function template is called, its name may be found by the name lookup that considers functions from namespaces and classes associated with the types of the function arguments (6.4.2) . If the name in a friend declaration is neither qualified nor a template-id and the declaration is a function or an elaborated-type-specifier , the lookup to determine whether the entity has been previously declared shall not consider any scopes outside the innermost enclosing namespace. [ Note: The other forms of friend declarations cannot declare a new member of the innermost enclosing namespace and thus follow the usual lookup rules. — end note ] [ Example: // Assume f and g have not yet been declared. void h(int) ; template void f2(T) ; namespace A { class X { friend void f(X) ; // A: : f(X) is a friend class Y { friend void g() ; // A: : g is a friend friend void h(int) ; // A: : h is a friend // : : h not considered friend void f2(int) ; // : : f2(int) is a friend }; };

}

// A: : f , A: : g and A: : h are not visible here X x; void g() { f(x) ; } // definition of A: : g void f(X) { /* . . . */ } // definition of A: : f void h(int) { /* . . . */ } // definition of A: : h // A: : f , A: : g and A: : h are visible here and known to be friends

97) t his implies t hat t he name of t he class or funct ion is unqualified.

§ 10.3.1.2

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ISO/IEC 14882: 2017(E)

using A: : x; void h() { A: : f(x) ; A: : X: : f(x) ; A: : X: : Y: : g() ; }

// error: f is not a member of A: : X // error: g is not a member of A: : X: : Y

— end example ] 1 0. 3. 2 1

A

Namespace alias

[namespace. alias]

namespace-alias-definition declares an alternate name for a namespace according to the following grammar: namespace-alias : identifier namespace-alias-definition : namespace identifier = qualified-namespace-specifier ; qualified-namespace-specifier: nested-name-specifieropt namespace-name

2

3

The identifier in a namespace-alias-definition is a synonym for the name of the namespace denoted by the qualified-namespace-specifier and becomes a namespace-alias. [ Note: When looking up a namespace-name in a namespace-alias-definition , only namespace names are considered, see 6.4.6. — end note ] In a declarative region, a namespace-alias-definition can be used to redefine a namespace-alias declared in that declarative region to refer only to the namespace to which it already refers. [ Example: The following declarations are well-formed: namespace namespace namespace namespace

Company_with_very_long_name { /* . . . */ } CWVLN = Company_with_very_long_name; CWVLN = Company_with_very_long_name; // OK: duplicate CWVLN = CWVLN;

— end example ] 1 0. 3. 3

The using declaration

[namespace. udecl]

using-declaration : using using-declarator-list ; using-declarator-list : using-declarator . . . opt using-declarator-list , using-declarator . . . opt using-declarator: typename opt nested-name-specifier unqualified-id 1

Each using-declarator in a using-declaration 98 introduces a set of declarations into the declarative region in which the using-declaration appears. The set of declarations introduced by the using-declarator is found by performing qualified name lookup (6.4.3, 13.2) for the name in the using-declarator , excluding functions that are hidden as described below. If the using-declarator does not name a constructor, the unqualified-id is declared in the declarative region in which the using-declaration appears as a synonym for each declaration introduced by the using-declarator . [ Note: Only the specified name is so declared; specifying an enumeration name in a using-declaration does not declare its enumerators in the using-declaration ’s declarative region. — end note ] If the using-declarator names a constructor, it declares that the class inherits the set of constructor declarations introduced by the using-declarator from the nominated base class. 98)

A

using-declaration wit h more using-declarator each.

t han one

using-declarator

is equivalent t o a corresp onding sequence of

using-declarations

wit h one

§ 10.3.3

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ISO/IEC 14882: 2017(E)

2

Every

using-declaration Example:

definit ion.

is

declaration

a

and

a

member-declaration

and

can

t herefore

be

used

in

a

class

[

struct B { void f(char) ; void g(char) ; enum E { e }; union { int x; }; }; struct D : B { using B: : f; void f(int) { f(’ c’ ) ; } void g(int) { g(’ c’ ) ; } };

— end example using-declaration

// calls B: : f(char) // recursively calls D: : g(int)

]

3

In a

used as a

member-declaration using-declarator nested-name-specifier using-declarator nested-name-specifier Example: ,

a base class of t he clas s b eing defined. s hall name a direct

each

’s

If a

shall name

names a const ruct or,

base class of t he class b eing defined.

it s

[

template struct X : bases. . . { using bases: : g. . . ; };

// OK: B: : g and D: : g introduced

X x;

— end example

]

[

Example:

class C { int g() ; }; class D2 : public B { using B: : f; using B: : e; using B: : x; using C: : g; }; 4

— end example Note:

[

S ince

class.

S ince

t hey are not

// OK: B is a base of D2 // OK: e is an enumerator of base B // OK: x is a union member of base B // error: C isn’t a base of D2

]

dest ruct ors

do

sp ecializ at ions

not

have

of memb er

considered when a

a const ruct or or assignment

names,

a

using-declaration

t emplat es

using-declaration

op erat or brought

copy/ move const ruct or or assignment

for

conversion

cannot

refer

funct ions

are

to

not

a

dest ruct or

found

sp ecifies a conversion funct ion (1 7. 5 . 2 ) .

for

by name

a

base

lo okup,

— end note

]

If

from a base class int o a derived clas s has t he signat ure of a

op erat or for t he derived class (1 5 . 8 ) ,

it s elf sup press t he implicit declarat ion of t he derived class memb er;

t he

using-declaration

do es not

by

t he memb er from t he base class is hidden

or overridden by t he implicit ly- declared copy/ move const ruct or or assignment

op erat or of t he derived class,

as describ ed b elow.

5

A

using-declaration

shall not

name a

template-id

.

[

Example:

struct A { template void f(T) ; template struct X { }; }; struct B : A { § 1 0. 3. 3

© IS O / IEC

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1 83

ISO/IEC 14882: 2017(E)

using A: : f; using A: : X; };

6 7 8

// ill-formed // ill-formed

— end example ] A using-declaration shall not name a namespace. A using-declaration shall not name a scoped enumerator. A using-declaration that names a class member shall be a member-declaration . [ Example: struct X { int i; static int s; }; void f() { using X: : i; using X: : s; }

// error: X: : i is a class member and this is not a member declaration. // error: X: : s is a class member and this is not a member declaration.

— end example ] 9

Members declared by a using-declaration can be referred to by explicit qualification j ust like other member names (6.4.3.2) . [ Example: void f() ; namespace A { void g() ; } namespace X { using : : f; using A: : g; } void h() { X: : f() ; X: : g() ; }

10

// global f // A ’s g

// calls : : f // calls A: : g

— end example ] A using-declaration is a declaration and can therefore be used repeatedly where (and only where) multiple declarations are allowed. [ Example: namespace A { int i; } namespace A1 { using A: : i, A: : i; }

// OK: double declaration

struct B { int i; };

§ 10.3.3

© ISO/IEC 2017 – All rights reserved

184

ISO/IEC 14882: 2017(E)

struct X : B { using B: : i, B: : i; };

// error: double member declaration

— end example Note: using-declaration using-declaration ]

11

[

For

a

whose

namespace aft er t he

when a use of t he name is made.

nested-name-specifier

names

a

names pace,

are not in t he set of int ro duced declarat ions,

Thus, addit ional overloads added aft er t he

default

funct ion argument s ( 1 1 . 3 . 6) ,

1 7. 7. 3 )

are considered.

— end note

default ]

[

t emplat e argument s (1 7. 1 ) ,

Example:

memb ers

added

to

t he

so t hey are not considered

using-declaration

are ignored, but

and t emplat e sp ecializ at ions (1 7. 5 . 5 ,

namespace A { void f(int) ; } using A: : f; namespace A { void f(char) ; }

12

// f is a synonym for A: : f ; that is, for A: : f(int) .

void foo() { f(’ a’ ) ; }

// calls f(int) , even though f(char) exists.

void bar() { using A: : f; f(’ a’ ) ; }

// f is a synonym for A: : f ; that is, for A: : f(int) and A: : f(char) . // calls f(char)

— end example Note:

[

]

Part ial sp ecializ at ions of clas s t emplat es are found by lo oking up t he primary class t emplat e and t hen

considering all part ial sp ecializ at ions of t hat s p ecializ at ions int ro duced aft er t he

— end note using-declaration

vis ible ( 1 7. 5 . 5 ) .

13

S ince

a

declarat ive region ( 6 . 3)

is

t emplat e.

using-declaration

If a

using-declaration

names a class t emplat e,

part ial

are effect ively visible b ecause t he primary t emplat e is

]

a

declarat ion,

also apply t o

t he

rest rict ions

using-declaration

s.

[

on

declarat ions

Example:

of t he

s ame

name

in

t he

same

namespace A { int x; } namespace B { int i; struct g { }; struct x { }; void f(int) ; void f(double) ; void g(char) ; } void func() { int i; using B: : i; void f(char) ; using B: : f;

§ 1 0. 3. 3

// OK: hides struct g

// error: i declared twice // OK: each f is a function

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ISO/IEC 14882: 2017(E)

// calls B: : f(double)

f(3. 5) ; using B: : g; g(’ a’ ) ; struct g g1; using B: : x; using A: : x; x = 99; struct x x1;

// calls B: : g(char) // g1 has class type B: : g // OK: hides struct B: : x // assigns to A: : x // x1 has class type B: : x

}

— end example 14

]

If a funct ion declarat ion in names pace scop e or b lo ck s cop e has

t he same name and t he same paramet er-

typ e- list

,

(1 1 . 3. 5)

s ame funct ion, name,

as a funct ion int ro duced by a

t he program is ill- formed.

paramet er- typ e- list ,

using-declaration

,

ret urn typ e,

using-declaration

and t emplat e paramet er list

t he program is ill- formed.

[

Note:

t he same name and t he same paramet er- typ e- list .

Two

declare t he

If,

as a funct ion t emplat e int ro duced by a

using-declaration

s may int ro duce funct ions wit h

for a call t o an unqualified funct ion name,

overload resolut ion select s t he funct ions int ro duced by such [

and t he declarat ions do not

If a funct ion t emplat e declarat ion in namespace scop e has t he same

Example:

using-declaration

s,

funct ion

t he funct ion call is ill- formed.

namespace B { void f(int) ; void f(double) ; } namespace C { void f(int) ; void f(double) ; void f(char) ; } void h() { using B: : f; using C: : f; f(’ h’ ) ; f(1) ; void f(int) ; }

// B: : f(int) and B: : f(double) // C: : f(int) , C: : f(double) , and C: : f(char) // calls C: : f(char) // error: ambiguous: B: : f(int) or C: : f(int) ? // error: f(int) conflicts with C: : f(int) and B: : f(int)

— end example — end note using-declarator ]

15

When a

]

b rings declarat ions from a base class int o a derived class,

memb er funct ions and

memb er funct ion t emplat es in t he derived class override and/ or hide memb er funct ions and memb er funct ion t emp lat es

wit h

t he

same

name,

paramet er- typ e- list

a base class ( rat her t han conflict ing) . declarat ions int ro duced by t he

(1 1 . 3 . 5 ) ,

cv- qualificat ion,

and

ref-qualifier

( if any)

S uch hidden or overridden declarat ions are excluded from t he s et

using-declarator

.

[

Example:

in of

struct B { virtual void f(int) ; virtual void f(char) ; void g(int) ; void h(int) ; }; struct D : B { using B: : f; void f(int) ;

§ 1 0. 3. 3

// OK: D: : f(int) overrides B: : f(int) ;

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1 86

ISO/IEC 14882: 2017(E)

using B: : g; void g(char) ;

// OK

using B: : h; void h(int) ;

// OK: D: : h(int) hides B: : h(int)

}; void k(D* p) { p->f(1) ; p->f(’ a’ ) ; p->g(1) ; p->g(’ a’ ) ; }

// calls // calls // calls // calls

D: : f(int) B: : f(char) B: : g(int) D: : g(char)

struct B1 { B1(int) ; }; struct B2 { B2(int) ; }; struct D1 : B1, B2 { using B1: : B1; using B2: : B2; }; D1 d1(0) ; // ill-formed: ambiguous struct D2 : B1, B2 { using B1: : B1; using B2: : B2; D2(int) ; // OK: D2: : D2(int) hides B1: : B1(int) and B2: : B2(int) }; D2 d2(0) ; // calls D2: : D2(int)

— end example ] 16

For the purpose of overload resolution, the functions that are introduced by a using-declaration into a derived class are treated as though they were members of the derived class. In particular, the implicit this parameter shall be treated as if it were a pointer to the derived class rather than to the base class. This has no effect on the type of the function, and in all other respects the function remains a member of the base class. Likewise, constructors that are introduced by a using-declaration are treated as though they were constructors of the derived class when looking up the constructors of the derived class (6.4.3.1) or forming a set of overload candidates (16.3.1.3, 16.3.1.4, 16.3.1.7) . If such a constructor is selected to perform the initialization of an obj ect of class type, all subobj ects other than the base class from which the constructor originated are implicitly initialized (15.6.3) .

17

In a using-declarator that does not name a constructor, all members of the set of introduced declarations shall be accessible. In a using-declarator that names a constructor, no access check is performed. In particular, if a derived class uses a using-declarator to access a member of a base class, the member name shall be accessible. If the name is that of an overloaded member function, then all functions named shall be accessible. The base class members mentioned by a using-declarator shall be visible in the scope of at least one of the direct base classes of the class where the using-declarator is specified. § 10.3.3

© ISO/IEC 2017 – All rights reserved

187

ISO/IEC 14882: 2017(E)

18

[ Note: Because a using-declarator designates a base class member (and not a member subobject or a member function of a base class subobject) , a using-declarator cannot be used to resolve inherited member ambiguities. [ Example: struct A { int x() ; }; struct B : A { }; struct C : A { using A: : x; int x(int) ; }; struct D : B, C { using C: : x; int x(double) ; }; int f(D* d) { return d->x() ; }

// error: overload resolution selects A: : x , but A is an ambiguous base class

— end example ] — end note ] 19

A synonym created by a using-declaration has the usual accessibility for a member-declaration . A usingdeclarator that names a constructor does not create a synonym; instead, the additional constructors are accessible if they would be accessible when used to construct an obj ect of the corresponding base class, and the accessibility of the using-declaration is ignored. [ Example: class A { private: void f(char) ; public: void f(int) ; protected: void g() ; }; class B : public A { using A: : f; // error: A: : f(char) is inaccessible public: using A: : g; // B: : g is a public synonym for A: : g };

20

— end example ] If a using-declarator uses the keyword typename and specifies a dependent name (17.6.2) , the name introduced by the using-declaration is treated as a typedef-name (10.1.3) . 1 0. 3. 4

Using directive

[namespace. udir]

using-directive : attribute-specifier-seqopt using namespace nested-name-specifieropt namespace-name ; 1

A using-directive shall not appear in class scope, but may appear in namespace scope or in block scope. [ Note: When looking up a namespace-name in a using-directive , only namespace names are considered, see 6.4.6. — end note ] The optional attribute-specifier-seq appertains to the using-directive .

2

A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive. During unqualified name lookup (6.4.1) , the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace. [ Note: In this context, “contains” means “contains directly or indirectly”. — end § 10.3.4

© ISO/IEC 2017 – All rights reserved

188

ISO/IEC 14882: 2017(E)

3

note ] A using-directive does not add any members to the declarative region in which it appears. [ Example: namespace A { int i; namespace B { namespace C { int i; } using namespace void f1() { i = 5; } } namespace D { using namespace using namespace void f2() { i = 5; } } void f3() { i = 5; } } void f4() { i = 5; }

A: : B: : C;

// OK, C: : i visible in B and hides A: : i

B; C;

// ambiguous, B: : C: : i or A: : i ?

// uses A: : i

// ill-formed; neither i is visible

— end example ] 4

For unqualified lookup (6.4.1) , the using-directive is transitive: if a scope contains a using-directive that nominates a second namespace that itself contains using-directives, the effect is as if the using-directive s from the second namespace also appeared in the first. [ Note: For qualified lookup, see 6.4.3.2. — end note ] [ Example: namespace M { int i; } namespace N { int i; using namespace M; } void f() { using namespace N; i = 7; // error: both M: : i and N: : i are visible }

For another example, namespace A { int i; } namespace B { int i; int j ;

§ 10.3.4

© ISO/IEC 2017 – All rights reserved

189

ISO/IEC 14882: 2017(E)

namespace C { namespace D { using namespace A; int j ; int k; int a = i; // B: : i hides A: : i } using namespace D; int k = 89; // no problem yet int l = k; // ambiguous: C: : k or D: : k int m = i; // B: : i hides A: : i int n = j ; // D: : j hides B: : j } }

— end example ] 5

6

If a namespace is extended (10.3.1) after a using-directive for that namespace is given, the additional members of the extended namespace and the members of namespaces nominated by using-directive s in the extending namespace-definition can be used after the extending namespace-definition . If name lookup finds a declaration for a name in two different namespaces, and the declarations do not declare the same entity and do not declare functions, the use of the name is ill-formed. [ Note: In particular, the name of a variable, function or enumerator does not hide the name of a class or enumeration declared in a different namespace. For example, namespace A { class X { }; extern " C" int extern " C++" int } namespace B { void X(int) ; extern " C" int extern " C++" int } using namespace A; using namespace B; void f() { X(1) ; g() ; h() ; }

g() ; h() ;

g() ; h(int) ;

// error: name X found in two namespaces // OK: name g refers to the same entity // OK: overload resolution selects A: : h

— end note ] 7

During overload resolution, all functions from the transitive search are considered for argument matching. The set of declarations found by the transitive search is unordered. [ Note: In particular, the order in which namespaces were considered and the relationships among the namespaces implied by the using-directive s do not cause preference to be given to any of the declarations found by the search. — end note ] An ambiguity exists if the best match finds two functions with the same signature, even if one is in a namespace reachable through using-directive s in the namespace of the other. 99 [ Example: namespace D { 99) D uring name lo okup in a class hierarchy, some ambiguit ies may b e resolved by considering whet her one memb er hides t he ot her along some pat hs ( 1 3. 2) .

There is no such disambiguat ion when considering t he set of names found as a result of following

using-directive s.

§ 10.3.4

© ISO/IEC 2017 – All rights reserved

190

ISO/IEC 14882: 2017(E)

int d1; void f(char) ; } using namespace D;

// OK: no conflict with D: : d1

int d1; namespace E { int e; void f(int) ; }

namespace D { // namespace extension int d2; using namespace E; void f(int) ; } void f() { d1++; : : d1++; D: : d1++; d2++; e++; f(1) ; f(’ a’ ) ; }

// error: ambiguous : : d1 or D: : d1 ? // OK // OK // OK: D: : d2 // OK: E: : e // error: ambiguous: D: : f(int) or E: : f(int) ? // OK: D: : f(char)

— end example 1

1 0. 4

asm

The

asm

An

]

declaration

[dcl. asm]

declarat ion has t he form

asm-definition : attribute-specifier-seqopt asm ( string-literal ) ; The

asm

declarat ion is condit ionally- supp ort ed;

specifier-seq

in

asm-definition

an

ap p ert ains

it s meaning is implement at ion- defined. to

t he

asm

declarat ion.

— end note

informat ion t hrough t he implement at ion t o an assembler.

1

1 0. 5

Note:

The op t ional

Typically

it

All funct ion typ es ,

each

.

[

implement at ion

attribute-

used

to

pass

[dcl. link]

funct ion names wit h ext ernal linkage,

Note:

is

]

Linkage specifications

language linkage to

[

and variable names wit h ext ernal linkage have a

S ome of t he prop ert ies asso ciat ed wit h an ent ity wit h language linkage are sp ecific and

are

not

describ ed

here.

For

example,

a

part icular

language

linkage

may

be

as so ciat ed wit h a part icular form of represent ing names of ob j ect s and funct ions wit h ext ernal linkage, wit h a part icular calling convent ion, funct ion names,

L inkage ( 6 . 5 )

— end note ++

and variable names is C

linkages are dis t inct

2

et c.

]

The default

language linkage.

or

language linkage of all funct ion typ es,

Two funct ion typ es wit h different

language

typ es even if t hey are ot herwise ident ical.

b etween C

++

and non- C

++

co de fragment s can b e achieved using a

linkage-specification

:

linkage-specification : extern string-literal { declaration-seqopt } extern string-literal declaration

string-literal string-literal " C" The

s

§ 1 0. 5

indicat es and

t he

" C++"

.

required Use

of a

language

linkage.

string-literal

© IS O / IEC

ot her

This t han

do cument

" C"

or

2 0 1 7 – All right s reserved

sp ecifies

" C++"

is

t he

semant ics

for

t he

condit ionally- supp ort ed,

1 91

ISO/IEC 14882: 2017(E)

wit h imp lement at ion- defined semant ics.

[

Note:

Therefore,

is unknown t o t he imp lement at ion requires a diagnost ic. s p elling of t he and

3

Fortran

string-literal FORTRAN

or

,

b e t aken from t he do cument

dep ending on t he vint age.

defining t hat

— end note

]

[

Note:

++

funct ions,

" C++"

A

t hat

For example,

Ada

( not

t he

ADA

)

]

Example:

programming language,

" C"

,

// C linkage

]

L inkage sp ecificat ions nest . linkage.

[

string-literal

is recommended t hat

// C++ linkage by default

complex sqrt(complex) ; extern " C" { double sqrt(double) ; }

— end example

.

It

language.

Every implement at ion s hall provide for linkage t o funct ions writ t en in t he C and linkage t o C

4

a linkage- sp ecificat ion wit h a

— end note

linkage

namespace scop e ( 6 . 3 ) .

In a

of all funct ion declarat ors, declared wit hin t he

When linkage sp ecificat ions nest ,

s p ecificat ion

do es

not

es t ablish

linkage-specification

,

a

scop e.

t he innermost A

extern " C" void f1(void(*pf) (int) ) ;

.

[

shall

o ccur

only

in

t he sp ecified language linkage applies t o t he funct ion typ es

funct ion names wit h ext ernal linkage,

linkage-specification

one det ermines t he language

linkage-specification

Example:

and variable names wit h ext ernal linkage

// the name f1 and its function type have C language linkage; // pf is a pointer to a C function

extern " C" typedef void FUNC() ; FUNC f2; // the name f2 has C++ language linkage and the // function’s type has C language linkage extern " C" FUNC f3;

// the name of function f3 and the function’s type have C language linkage

void (*pf2) (FUNC*) ;

// the name of the variable pf2 has C++ linkage and the type // of pf2 is “pointer to C++ function that takes one parameter of type // pointer to C function”

extern " C" { static void f4() ;

// the name of the function f4 has internal linkage (not C language linkage) // and the function’s type has C language linkage.

} extern " C" void f5() { extern void f4() ; }

// OK: Name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration.

extern void f4() ;

void f6() { extern void f4() ;

// OK: Name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration.

}

— end example

]

A C

// OK: Name linkage (internal) and function type linkage (C language linkage) // obtained from previous declaration.

language linkage is ignored in det ermining t he language linkage of t he names of class

memb ers and t he funct ion typ e of class memb er funct ions.

[

Example:

extern " C" typedef void FUNC_c() ; class C { void mf1(FUNC_c*) ;

§ 1 0. 5

// the name of the function mf1 and the member function’s type have

© IS O / IEC

2 0 1 7 – All right s reserved

1 92

ISO/IEC 14882: 2017(E)

// C++ language linkage; the parameter has type “pointer to C function” FUNC_c mf2;

// the name of the function mf2 and the member function’s type have // C++ language linkage

static FUNC_c* q;

// the name of the data member q has C++ language linkage and // the data member’s type is “pointer to C function”

}; extern " C" { class X { void mf() ;

// the name of the function mf and the member function’s type have // C++ language linkage // the name of the function mf2 has C++ language linkage; // the parameter has type “pointer to C function”

void mf2(void(*) () ) ; }; }

5

— end example

]

If two declarat ions declare funct ions wit h t he same name and paramet er- typ e- list

(1 1 . 3 . 5 )

t o b e memb ers of

t he same namespace or declare ob j ect s wit h t he same name t o b e memb ers of t he same namespace and t he declarat ions give t he names different

language linkages,

if t he declarat ions app ear in different declarat ion wit hout

a linkage sp ecificat ion shall not

A funct ion can b e declared wit hout s een;

6

At

t he program is ill- formed;

t ranslat ion unit s.

Except

precede t he first

one funct ion wit h a part icular name can have C

++

linkage,

affect ed by such a funct ion declarat ion.

language linkage.

Two declarat ions for a funct ion

language linkage wit h t he same funct ion name ( ignoring t he namespace names t hat

app ear

in

namespace

scop es

refer

to

t he

same

funct ion.

Two

declarat ions

language linkage wit h t he same name ( ignoring t he names pace names t hat qualify it ) namespace s cop es refer t o t he same variable. t he same name as a variable in global scop e,

An ent ity wit h C

b e declared wit h t he same name as a funct ion wit h C qualify t he resp ect ive names) ; unit s. t he

[

Note:

— end note

6. 2) ; ]

int x; namespace A { extern " C" int extern " C" int extern " C" int extern " C" int }

[

t his

no diagnost ic

implies

Example:

t hat

f() ; g() { return 1; } h() ; x() ;

a

is

such

t hat

wit h

C

b e declared wit h no diagnost ic is

A variable wit h C language linkage s hall not

language linkage ( ignoring t he namespace names t hat

required

an

qualify it )

variable

t hat app ear in different

language linkage shall not

if t he declarat ions

O nly one definit ion for an ent ity wit h a given name wit h C

program ( see

s cop e.

for

unless b ot h declarat ions denot e t he same ent ity;

required if t he declarat ions app ear in different t rans lat ion unit s.

funct ion.

linkage sp ecificat ion has b een

wit h C

different

a funct ion

linkage sp ecificat ion for t hat

a linkage sp ecificat ion aft er an exp licit

t he linkage explicit ly sp ecified in t he earlier declarat ion is not

mos t

no diagnost ic is required

for funct ions wit h C

ent ity

must

not

be

app ear

in different

t ranslat ion

language linkage may app ear in

defined

in more

t han

one

namespace

// ill-formed: same name as global-space object x

namespace B { extern " C" int f() ; extern " C" int g() { return 1; } }

// A: : f and B: : f refer to the same function // ill-formed, the function g with C language linkage has two definitions

int A: : f() { return 98; }

// definition for the function f with C language linkage

§ 1 0. 5

© IS O / IEC

2 0 1 7 – All right s reserved

1 93

ISO/IEC 14882: 2017(E)

extern " C" int h() { return 97; }

// definition for the function h with C language linkage // A: : h and : : h refer to the same function

— end example ] 7 A declaration directly contained in a linkage-specification is treated as if it contains the extern specifier (10.1.1) for the purpose of determining the linkage of the declared name and whether it is a definition. Such a declaration shall not specify a storage class. [ Example: extern " C" double f() ; static double f() ; extern " C" int i; extern " C" { int i; } extern " C" static void g() ;

// error // declaration // definition // error

— end example ] 8 [ Note: Because the language linkage is part of a function type, when indirecting through a pointer to C function, the function to which the resulting lvalue refers is considered a C function. — end note ] 9 Linkage from C++ to obj ects defined in other languages and to obj ects defined in C++ from other languages

is implementation-defined and language-dependent. Only where the obj ect layout strategies of two language implementations are similar enough can such linkage be achieved.

1 0. 6

Attributes

1 0. 6. 1

[dcl. attr]

Attribute syntax and semantics

[dcl. attr. grammar]

1 Attributes specify additional information for various source constructs such as types, variables, names, blocks, or translation units.

attribute-specifier-seq : attribute-specifier-seqopt attribute-specifier attribute-specifier: [ [ attribute-using-prefixopt attribute-list ] ] alignment-specifier alignment-specifier: alignas ( type-id . . . opt ) alignas ( constant-expression . . . opt ) attribute-using-prefix : using attribute-namespace : attribute-list : attributeopt attribute-list , attributeopt attribute . . . attribute-list , attribute . . . attribute : attribute-token attribute-argument-clauseopt attribute-token : identifier attribute-scoped-token attribute-scoped-token : attribute-namespace : : identifier

§ 10.6.1

© ISO/IEC 2017 – All rights reserved

194

ISO/IEC 14882: 2017(E)

attribute-namespace : identifier attribute-argument-clause : ( balanced-token-seqopt ) balanced-token-seq : balanced-token balanced-token-seq balanced-token balanced-token : ( balanced-token-seqopt ) [ balanced-token-seqopt ] { balanced-token-seqopt } any token ot her t han a parent hesis, 2

4

5

6

7

or a brace

If an attribute-specifier contains an attribute-using-prefix , the attribute-list following that attribute-using-prefix shall not contain an attribute-scoped-token and every attribute-token in that attribute-list is treated as if its identifier were prefixed with N: : , where N is the attribute-namespace specified in the attribute-usingprefix. [ Note: This rule imposes no constraints on how an attribute-using-prefix affects the tokens in an attribute-argument-clause. — end note ] [ Example: [[using CC: opt(1) , debug] ] void f() {} [[using CC: opt(1) ] ] [[CC: : debug] ] void g() {} [[using CC: CC: : opt(1) ] ] void h() {}

3

a bracket ,

// same as [[CC: : opt(1) , CC: : debug] ] // same as [[CC: : opt(1) ] ] [[CC: : debug] ] // error: cannot combine using and scoped attribute token

— end example ] [ Note: For each individual attribute, the form of the balanced-token-seq will be specified. — end note ] In an attribute-list , an ellipsis may appear only if that attribute ’s specification permits it. An attribute followed by an ellipsis is a pack expansion (17.5.3) . An attribute-specifier that contains no attribute s has no effect. The order in which the attribute-token s appear in an attribute-list is not significant. If a keyword (5.11) or an alternative token (5.5) that satisfies the syntactic requirements of an identifier (5.10) is contained in an attribute-token , it is considered an identifier. No name lookup (6.4) is performed on any of the identifiers contained in an attribute-token . The attribute-token determines additional requirements on the attribute-argument-clause (if any) . Each attribute-specifier-seq is said to appertain to some entity or statement, identified by the syntactic context where it appears (Clause 9, Clause 10, Clause 11) . If an attribute-specifier-seq that appertains to some entity or statement contains an attribute or alignment-specifier that is not allowed to apply to that entity or statement, the program is ill-formed. If an attribute-specifier-seq appertains to a friend declaration (14.3) , that declaration shall be a definition. No attribute-specifier-seq shall appertain to an explicit instantiation (17.7.2) . For an attribute-token (including an attribute-scoped-token ) not specified in this document, the behavior is implementation-defined. Any attribute-token that is not recognized by the implementation is ignored. [ Note: Each implementation should choose a distinctive name for the attribute-namespace in an attribute-scoped-token . — end note ] Two consecutive left square bracket tokens shall appear only when introducing an attribute-specifier or within the balanced-token-seq of an attribute-argument-clause . [ Note: If two consecutive left square brackets appear where an attribute-specifier is not allowed, the program is ill-formed even if the brackets match an alternative grammar production. — end note ] [ Example: int p[10] ; void f() { int x = 42, y[5] ;

§ 10.6.1

© ISO/IEC 2017 – All rights reserved

195

ISO/IEC 14882: 2017(E)

// error: invalid attribute on a nested declarator-id and // not a function-style cast of an element of p . y[[] { return 2; }() ] = 2; // error even though attributes are not allowed in this context. int i [[vendor: : attr([[] ] ) ] ] ; // well-formed implementation-defined attribute.

int(p[[x] { return x; }() ] ) ;

}

— end example 1 0. 6. 2 1

An

]

Alignment specifier

alignment-specifier

b it - field,

[dcl. align]

may b e applied t o a variable or t o a class dat a memb er, but it shall not b e app lied t o a

a funct ion paramet er,

or an

exception-declaration alignment-specifier elaborated-type-specifier class-head opaque-enum-declaration alignment-specifier alignas( constant-expression ) (1 8. 3) .

An

may als o b e applied

t o t he declarat ion or definit ion of a class ( in an resp ect ively)

(2. 1 )

to

t he

declarat ion

enum-head alignment-specifier constant-expression ,

2

and

resp ect ively ( 1 0 . 2 ) ) .

When t he

(2. 2)

—

t he

—

if t he const ant alignment

An

or

( 1 0 . 1 . 7. 3 )

definit ion

of an

enumerat ion

( in

or

( C lause 1 2 ) ,

an

or

wit h an ellipsis is a pack expansion (1 7. 5 . 3 ) .

is of t he form

:

shall b e an int egral cons t ant

expression do es not

expres sion

evaluat e t o an alignment

and t he implement at ion do es not

supp ort

t hat

value (6 . 1 1 ) ,

alignment

or evaluat es t o an ext ended

in t he cont ext

of t he declarat ion,

t he program is ill- formed.

3

alignment-specifier id ) ) An

of

t he

alignas( type-id )

form

has

t he

same

effect

as

alignas(alignof( type-

( 8. 3. 6) .

4

The alignment requirement of an ent ity is t he st rict est nonz ero alignment sp ecified by it s if any;

5

ot herwis e,

t he

The combined effect st rict

t han

t he

alignment-specifier alignment-specifier

of all

alignment

app ert aining t o t hat

alignment-specifier

s,

s have no effect .

t hat

would

be

ent ity were omit t ed.

s in a declarat ion shall not

required [

for

Example:

t he

ent ity

b eing

s p ecify an alignment

declared

if all

t hat

is less

alignment-specifier

s

struct alignas(8) S {}; struct alignas(1) U { S s; }; // error: U specifies an alignment that is less strict than if the alignas(1) were omitted.

— end example 6

]

If t he defining declarat ion of an ent ity has an s hall eit her an

ent ity

alignment .

sp ecify equivalent

has

an

alignment

alignment-specifier

,

alignment-specifier alignment-specifier ,

any non- defining declarat ion of t hat

or have no

every

defining

declarat ion

.

of t hat

C onversely, ent ity

No diagnost ic is required if declarat ions of an ent ity have different

t ranslat ion unit s.

[

Example:

shall

ent ity

if any declarat ion of s p ecify

an

alignment-specifier

equivalent

s in different

// Translation unit #1: struct S { int x; } s, *p = &s; // Translation unit #2: struct alignas(16) S; extern S* p; 7

— end example Example:

[

// error: definition of S lacks alignment, no diagnostic required

]

An aligned buffer wit h an alignment

requirement

of

A

and holding

N

element s of typ e

T

can b e

declared as :

alignas(T) alignas(A) T buffer[N] ; S p ecifying

§ 1 0. 6. 2

alignas(T)

ensures t hat

t he final request ed alignment

© IS O / IEC

will not

2 0 1 7 – All right s reserved

b e weaker t han

alignof(T)

,

and

1 96

ISO/IEC 14882: 2017(E)

therefore the program will not be ill-formed. 8

— end example ]

[ Example: alignas(double) void f() ; alignas(double) unsigned char c[sizeof(double) ] ; extern unsigned char c[sizeof(double) ] ; alignas(float) extern unsigned char c[sizeof(double) ] ;

// error: alignment applied to function // array of characters, suitably aligned for a double // no alignas necessary // error: different alignment in declaration

— end example ] 1 0. 6. 3

Carries dependency attribute

[dcl. attr. depend]

1

The attribute-token carries_dependency specifies dependency propagation into and out of functions. It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present. The attribute may be applied to the declarator-id of a parameter-declaration in a function declaration or lambda, in which case it specifies that the initialization of the parameter carries a dependency to (4.7) each lvalue-to-rvalue conversion (7.1) of that object. The attribute may also be applied to the declarator-id of a function declaration, in which case it specifies that the return value, if any, carries a dependency to the evaluation of the function call expression.

2

The first declaration of a function shall specify the carries_dependency attribute for its declarator-id if any declaration of the function specifies the carries_dependency attribute. Furthermore, the first declaration of a function shall specify the carries_dependency attribute for a parameter if any declaration of that function specifies the carries_dependency attribute for that parameter. If a function or one of its parameters is declared with the carries_dependency attribute in its first declaration in one translation unit and the same function or one of its parameters is declared without the carries_dependency attribute in its first declaration in another translation unit, the program is ill-formed, no diagnostic required.

3

[ Note: The carries_dependency attribute does not change the meaning of the program, but may result in generation of more efficient code. — end note ]

4

[ Example: /∗ Translation unit A. ∗/ struct foo { int* a; int* b; }; std: : atomic foo_head[10] ; int foo_array[10] [10] ; [[carries_dependency] ] struct foo* f(int i) { return foo_head[i] . load(memory_order_consume) ; } int g(int* x, int* y [[carries_dependency] ] ) { return kill_dependency(foo_array[*x] [*y] ) ; }

/∗ Translation unit B. ∗/ [[carries_dependency] ] struct foo* f(int i) ; int g(int* x, int* y [[carries_dependency] ] ) ; int c = 3; void h(int i) { struct foo* p;

§ 10.6.3

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ISO/IEC 14882: 2017(E)

p = f(i) ; do_something_with(g(&c, p->a) ) ; do_something_with(g(p->a, &c) ) ; }

The carries_dependency attribute on function f means that the return value carries a dependency out of f , so that the implementation need not constrain ordering upon return from f . Implementations of f and its caller may choose to preserve dependencies instead of emitting hardware memory ordering instructions (a.k.a. fences) . Function g’s second parameter has a carries_dependency attribute, but its first parameter does not. Therefore, function h’s first call to g carries a dependency into g, but its second call does not. The implementation might need to insert a fence prior to the second call to g. — end example ] 1 0. 6. 4 1

Deprecated attribute

[dcl. attr. deprecated]

The attribute-token deprecated can be used to mark names and entities whose use is still allowed, but is discouraged for some reason. [ Note: In particular, deprecated is appropriate for names and entities that are deemed obsolescent or unsafe. — end note ] It shall appear at most once in each attribute-list . An attribute-argument-clause may be present and, if present, it shall have the form: ( string-literal )

[ Note: The string-literal in the attribute-argument-clause could be used to explain the rationale for deprecation and/or to suggest a replacing entity. — end note ] 2

The attribute may be applied to the declaration of a class, a typedef-name , a variable, a non-static data member, a function, a namespace, an enumeration, an enumerator, or a template specialization.

3

A name or entity declared without the deprecated attribute can later be redeclared with the attribute and vice-versa. [ Note: Thus, an entity initially declared without the attribute can be marked as deprecated by a subsequent redeclaration. However, after an entity is marked as deprecated, later redeclarations do not un-deprecate the entity. — end note ] Redeclarations using different forms of the attribute (with or without the attribute-argument-clause or with different attribute-argument-clause s) are allowed.

4

[ Note: Implementations may use the deprecated attribute to produce a diagnostic message in case the program refers to a name or entity other than to declare it, after a declaration that specifies the attribute. The diagnostic message may include the text provided within the attribute-argument-clause of any deprecated attribute applied to the name or entity. — end note ] 1 0. 6. 5

Fallthrough attribute

[dcl. attr. fallthrough]

1

The attribute-token fallthrough may be applied to a null statement (9.2) ; such a statement is a fallthrough statement. The attribute-token fallthrough shall appear at most once in each attribute-list and no attributeargument-clause shall be present. A fallthrough statement may only appear within an enclosing switch statement (9.4.2) . The next statement that would be executed after a fallthrough statement shall be a labeled statement whose label is a case label or default label for the same switch statement. The program is ill-formed if there is no such statement.

2

[ Note: The use of a fallthrough statement is intended to suppress a warning that an implementation might otherwise issue for a case or default label that is reachable from another case or default label along some path of execution. Implementations should issue a warning if a fallthrough statement is not dynamically reachable. — end note ]

3

[ Example: void f(int n) { void g() , h() , i() ; switch (n) { case 1:

§ 10.6.5

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ISO/IEC 14882: 2017(E)

case 2: g() ; [[fallthrough] ] ; case 3: h() ; case 4: i() ; [[fallthrough] ] ; }

// warning on fallthrough discouraged // implementation may warn on fallthrough // ill-formed

}

— end example 1 0. 6. 6 1

The

Maybe unused attribute

The

indicat es t hat

[

Note:

may

be

a funct ion,

or t hat

app lied

to

t he

an enumerat ion,

[

and no

declarat ion

a name or ent ity is p ossibly int ent ionally unused.

attribute-argument-clause typedef-name

It

shall

non- st at ic

dat a

shall b e present .

of a

class,

a

,

a

variable,

a

or an enumerat or.

maybe_unused

,

implement at ions should not

t he

maybe_unused

emit

a warning t hat

— end note

t he ent ity is used despit e t he presence of t he at t ribut e.

A name or ent ity declared wit hout and vice versa.

5

once in each

For an ent ity marked

unused,

4

most

at t ribut e

memb er,

3

[dcl. attr. unused]

attribute-token maybe_unused attribute-list

app ear at

2

]

t he ent ity is

]

at t ribut e can lat er b e redeclared wit h t he at t ribut e

An ent ity is considered marked aft er t he first

declarat ion t hat

marks it .

Example: [[maybe_unused] ] void f([[maybe_unused] ] bool thing1, [[maybe_unused] ] bool thing2) { [[maybe_unused] ] bool b = thing1 && thing2; assert(b) ; }

Implement at ions should not

1 0. 6. 7 1

The

warn t hat

attribute-token nodiscard

argument-clause Note: [

is unused,

whet her or not

NDEBUG

is defined.

Nodiscard attribute may

declarat ion of a class or enumerat ion.

2

b

be It

applied

to

t he

shall app ear at

declarator-id most

in

a

funct ion

once in each

.

Implement at ions should issue a warning in such cases.

ret urn value of a no discard call has s urprising consequences.

3

[

and no

calls a funct ion p reviously declared

no discard call as a p ot ent ially- evaluat ed discarded- value expres sion ( C lause 8 )

void

declarat ion

attribute-list

or

to

t he

attribute-

shall b e pres ent .

A no discard call is a funct ion call expression t hat

to

]

[dcl. attr. nodiscard]

whose ret urn typ e is a p ossibly cv- qualified class or enumerat ion typ e marked

cast

— end example

nodiscard

.

nodiscard

,

or

App earance of a

is discouraged unless explicit ly

This is typically b ecause discarding t he

— end note

]

Example: struct [[nodiscard] ] error_info { /* . . . */ }; error_info enable_missile_safety_mode() ; void launch_missiles() ; void test_missiles() { enable_missile_safety_mode() ; // warning encouraged launch_missiles() ; } error_info &foo() ; void f() { foo() ; } // warning not encouraged: not a nodiscard call, because neither // the (reference) return type nor the function is declared nodiscard

§ 1 0. 6. 7

© IS O / IEC

2 0 1 7 – All right s reserved

1 99

ISO/IEC 14882: 2017(E)

— end example 1 0. 6. 8 1

The

]

Noreturn attribute

attribute-token attribute-list declarator-id

each t he

noreturn

and

no

sp ecifies

in a funct ion declarat ion.

noreturn

at t ribut e

in

If a funct ion t he

f

b ehavior

one

Note: note Example: [

is called where is

undefined.

a

funct ion

[

f

The

t ranslat ion

unit

first

and

do es

shall

not

be

t he

noreturn same

t he program is ill- formed,

was previous ly declared wit h t he

Note:

The

funct ion

ret urn.

present .

It

The

shall

app ear

at t ribut e

at

may

most be

declarat ion of a funct ion shall sp ecify t he

funct ion sp ecifies t he

at t ribut e in anot her t ranslat ion unit ,

2

t hat

attribute-argument-clause

at t ribut e if any declarat ion of t hat t he

[dcl. attr. noreturn]

may

at t ribut e.

funct ion

is

once

in

applied

to

noreturn

If a funct ion is declared wit h

declared

wit hout

t he

noreturn

no diagnost ic required.

noreturn

t erminat e

by

at t ribut e and

t hrowing

Implement at ions should issue a warning if a funct ion marked

an

f

event ually ret urns ,

except ion.

[[noreturn] ]

might

— end note — end ]

ret urn.

]

3

[

[[ noreturn ] ] void f() { throw " error" ; } [[ noreturn ] ] void q(int i) { if (i > 0) throw " positive" ; }

— end example

§ 1 0. 6. 8

// OK

// behavior is undefined if called with an argument type-id ptr-operator: * attribute-specifier-seqopt cv-qualifier-seqopt & attribute-specifier-seqopt && attribute-specifier-seqopt nested-name-specifier * attribute-specifier-seqopt cv-qualifier-seqopt cv-qualifier-seq : cv-qualifier cv-qualifier-seqopt cv-qualifier: const volatile ref-qualifier: & && declarator-id : . . . opt id-expression

11.1 1

Type names

[dcl. name]

To s p ecify typ e conversions explicit ly, a typ e shall b e sp ecified. or funct ion of t hat

and as an argument

This can b e done wit h a

typ e t hat

of

type-id ,

sizeof alignof new ,

,

,

or

typeid

,

t he name of

which is s ynt act ically a declarat ion for a variable

omit s t he name of t he ent ity.

type-id : type-specifier-seq abstract-declaratoropt defining-type-id : defining-type-specifier-seq abstract-declaratoropt abstract-declarator: ptr-abstract-declarator noptr-abstract-declaratoropt parameters-and-qualifiers trailing-return-type abstract-pack-declarator ptr-abstract-declarator: noptr-abstract-declarator ptr-operator ptr-abstract-declaratoropt noptr-abstract-declarator: noptr-abstract-declaratoropt parameters-and-qualifiers noptr-abstract-declaratoropt [ constant-expressionopt ] attribute-specifier-seqopt ( ptr-abstract-declarator ) abstract-pack-declarator: noptr-abstract-pack-declarator ptr-operator abstract-pack-declarator

§ 11.1

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202

ISO/IEC 14882: 2017(E)

noptr-abstract-pack-declarator: noptr-abstract-pack-declarator parameters-and-qualifiers noptr-abstract-pack-declarator [ constant-expressionopt ] attribute-specifier-seqopt ... It

is p ossible t o ident ify uniquely t he lo cat ion in t he

t he cons t ruct ion were hyp ot het ical ident ifier.

int int int int int int

a declarat or [

Example:

// int // int // int // int // int // int

* *[3] (*) [3] *() (*) (double)

name resp ect ively t he typ es “ “ funct ion of ( no paramet ers)

— end example 2

The

named

where t he ident ifier would app ear if

typ e is

t hen t he

same

as

t he

typ e

of t he

i *pi *p[3] (*p3i) [3] *f() (*pf) (double)

int ” ,

int ” , “ array of 3 p oint ers int ” , and “ p oint er t o a

“ p oint er t o

ret urning p oint er t o

to

int ” ,

“ p oint er t o array of 3

funct ion of (

double )

ret urning

int ” , int ”.

]

A typ e can also b e named ( oft en more easily)

11.2 1

abstract-declarator

in a declarat ion.

by using a

typedef

( 1 0. 1 . 3) .

Ambiguity resolution

[dcl. ambig. res]

The ambiguity arising from t he similarity b etween a funct ion- style cast can also o ccur in t he cont ext

of a declarat ion.

In t hat

cont ext ,

and a declarat ion ment ioned in 9 . 8

t he choice is b etween a funct ion declarat ion

wit h a redundant set of parent heses around a paramet er name and an ob j ect declarat ion wit h a funct ion- s tyle cast as t he init ializ er. t hat

J ust as for t he amb iguit ies ment ioned in 9 . 8 ,

could p ossibly b e a declarat ion a declarat ion.

by adding parent heses around t he argument . lis t - init ializ at ion synt ax,

[

Note:

A

t he resolut ion is t o consider any const ruct

declarat ion can b e explicit ly disambiguat ed

The ambiguity can b e avoided by us e of copy- init ializ at ion or

or by use of a non- funct ion- style cast .

— end note

]

[

Example:

struct S { S(int) ; }; void foo(double a) { S w(int(a) ) ; S x(int() ) ; S y((int(a) ) ) ; S y((int) a) ; S z = int(a) ; }

— end example 2

// function declaration // function declaration // object declaration // object declaration // object declaration

]

An ambiguity can arise from t he similarity b etween a funct ion- style cast t hat [

any

const ruct

Example:

t hat

could

p ossibly

be

a

type-id

in

it s

synt act ic

template struct X {}; template struct Y {}; X a; X b; Y c; Y d;

// type-id // expression (ill-formed) // type-id (ill-formed) // expression

void foo(signed char a) { sizeof(int() ) ;

// type-id (ill-formed)

§ 11.2

© IS O / IEC

and a

cont ext

2 0 1 7 – All right s reserved

type-id

shall

be

.

The resolut ion is

considered

a

type-id

.

203

ISO/IEC 14882: 2017(E)

sizeof(int(a) ) ; sizeof(int(unsigned(a) ) ) ;

// expression // type-id (ill-formed)

(int() ) +1; (int(a) ) +1; (int(unsigned(a) ) ) +1;

// type-id (ill-formed) // expression // type-id (ill-formed)

}

— end example ] 3

Another ambiguity arises in a parameter-declaration-clause when a type-name is nested in parentheses. In this case, the choice is between the declaration of a parameter of type pointer to function and the declaration of a parameter with redundant parentheses around the declarator-id. The resolution is to consider the type-name as a simple-type-specifier rather than a declarator-id . [ Example: class C { }; void f(int(C) ) { } int g(C) ; void foo() { f(1) ; f(g) ; }

// void f(int(*fp) (C c) ) { } // not: void f(int C) { }

// error: cannot convert 1 to function pointer // OK

For another example, class C { }; void h(int *(C[10] ) ) ;

// void h(int *(*_fp) (C _parm[10] ) ) ; // not: void h(int *C[10] ) ;

— end example ] 11.3

Meaning of declarators

[dcl. meaning]

1

A declarator contains exactly one declarator-id ; it names the identifier that is declared. An unqualified-id occurring in a declarator-id shall be a simple identifier except for the declaration of some special functions (15.1, 15.3, 15.4, 16.5) and for the declaration of template specializations or partial specializations (17.7) . When the declarator-id is qualified, the declaration shall refer to a previously declared member of the class or namespace to which the qualifier refers (or, in the case of a namespace, of an element of the inline namespace set of that namespace (10.3.1) ) or to a specialization thereof; the member shall not merely have been introduced by a using-declaration in the scope of the class or namespace nominated by the nested-name-specifier of the declarator-id. The nested-name-specifier of a qualified declarator-id shall not begin with a decltype-specifier. [ Note: If the qualifier is the global : : scope resolution operator, the declarator-id refers to a name declared in the global namespace scope. — end note ] The optional attribute-specifier-seq following a declarator-id appertains to the entity that is declared.

2

A static , thread_local , extern, mutable , friend, inline , virtual , constexpr , explicit , or typedef specifier applies directly to each declarator-id in an init-declarator-list or member-declarator-list; the type specified for each declarator-id depends on both the decl-specifier-seq and its declarator .

3

Thus, a declaration of a particular identifier has the form T D

where T is of the form attribute-specifier-seq opt decl-specifier-seq and D is a declarator. Following is a recursive procedure for determining the type specified for the contained declarator-id by such a declaration. 4

First, the

decl-specifier-seq determines a type. In a declaration

T D

§ 11.3

© ISO/IEC 2017 – All rights reserved

204

ISO/IEC 14882: 2017(E)

t he

decl-specifier-seq

T

det ermines t he typ e

T.

[

Example:

In t he declarat ion

int unsigned i; t he typ e sp ecifiers

5

6

det ermine t he typ e “

attribute-specifier-seq opt

In a declarat ion “

int unsigned

T”.

TD

In a declarat ion

D

where

TD

where

D

unsigned int ”

( 1 0 . 1 . 7. 2 ) .

— end example

]

is an unadorned ident ifier t he typ e of t his ident ifier is

has t he form

( D1 ) t he typ e of t he cont ained

declarator-id

is t he same as t hat of t he cont ained

declarator-id

in t he declarat ion

T D1 Parent heses do not

alt er t he typ e of t he emb edded

declarator-id

,

but

t hey can alt er t he binding of complex

declarat ors.

1 1 . 3. 1 1

Pointers

TD

In a declarat ion

[dcl. ptr]

D

where

has t he form

* attribute-specifier-seqopt cv-qualifier-seqopt D1

T D1 derived-declarator-type-list T derived-declarator-type-list cv-qualifier-seq T cv-qualifier attribute-specifier-seq

and t he typ e of t he ident ifier in t he declarat ion ident ifier

of

D

is

p oint er and not

“

[

Example:

”,

p oint er

t o t he ob j ect

t he p oint er and not

2

is “

p oint ed t o.

t o t he ob j ect

S imilarly,

to

”.

t hen t he typ e of t he

The

t he opt ional

s

apply

( 1 0. 6. 1 )

to

t he

ap p ert ains t o

p oint ed t o.

The declarat ions

const int ci = 10, *pc = &ci, *const cpc = pc, **ppc; int i, *p, *const cp = &i; declare int eger;

ci , a const ant ppc , a p oint er

int eger; to

const ant p oint er t o int eger.

pc

can b e changed,

pc ,

a

a p oint er

p oint er

to

to

a

a const ant

The value of

ci , cpc ,

and so can t he ob j ect

const ant int eger;

and

cp

p oint ed t o by

int eger;

i,

cpc ,

an int eger;

a

p,

const ant

p oint er

a p oint er

to

to

int eger;

cannot b e changed aft er init ializ at ion.

cp .

Examples of some correct

a

const ant

and

cp ,

a

The value of

op erat ions are

i = ci; *cp = ci; pc++; pc = cpc; pc = p; ppc = &pc; Examples of ill- formed op erat ions are

// error // error // error // error // error // error // error

ci = 1; ci++; *pc = 2; cp = &ci; cpc++; p = pc; ppc = &p;

Each is unaccept able b ecause it

would eit her change t he value of an ob j ect

changed t hrough a cv- unqualified p oint er lat er,

§ 1 1 . 3. 1

const

or allow it

to be

// OK, but would make p point to ci because of previous error // clobber ci

*ppc = &ci; *p = 5;

— end example

declared

for examp le:

]

© IS O / IEC

2 0 1 7 – All right s reserved

205

ISO/IEC 1 4882:201 7(E)

3

See also 8.18 and 11.6.

4

[ Note: Forming a pointer to reference type is ill-formed; see 11.3.2. Forming a function pointer type is ill-formed if the function type has cv-qualifiers or a ref-qualifier; see 11.3.5. Since the address of a bit-field (12.2.4) cannot be taken, a pointer can never point to a bit-field. — end note ] 1 1 . 3. 2

1

References

[dcl. ref]

In a declaration T D where D has either of the forms & attribute-specifier-seqopt D1 && attribute-specifier-seqopt D1

and the type of the identifier in the declaration T D1 is “ derived-declarator-type-list T”, then the type of the identifier of D is “ derived-declarator-type-list reference to T ”. The optional attribute-specifier-seq appertains to the reference type. Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef-name (10.1.3, 17.1) or decltype-specifier (10.1.7.2) , in which case the cv-qualifiers are ignored. [ Example: typedef int& A; const A aref = 3;

// ill-formed; lvalue reference to non-const initialized with rvalue

The type of aref is “lvalue reference to int ”, not “lvalue reference to const int ”. — end example ] [ Note: A reference can be thought of as a name of an obj ect. — end note ] A declarator that specifies the type “reference to cv void” is ill-formed. 2

A reference type that is declared using & is called an lvalue reference , and a reference type that is declared using && is called an rvalue reference . Lvalue references and rvalue references are distinct types. Except where explicitly noted, they are semantically equivalent and commonly referred to as references.

3

[ Example: void f(double& a) { a += 3. 14; } // ... double d = 0; f(d) ;

declares a to be a reference parameter of f so the call f(d) will add 3. 14 to d. int v[20] ; // ... int& g(int i) { return v[i] ; } // ... g(3) = 7;

declares the function g() to return a reference to an integer so g(3) =7 will assign 7 to the fourth element of the array v. For another example, struct link { link* next; }; link* first; void h(link*& p) { p->next = first; first = p; p = 0; }

§ 11.3.2

// p is a reference to pointer

© ISO/IEC 2017 – All rights reserved

206

ISO/IEC 14882: 2017(E)

void k() { link* q = new link; h(q) ; }

declares p to be a reference to a pointer to link so h(q) will leave q with the value zero. See also 11.6.3.

— end example ] 4

It is unspecified whether or not a reference requires storage (6.7) .

5

There shall be no references to references, no arrays of references, and no pointers to references. The declaration of a reference shall contain an initializer (11.6.3) except when the declaration contains an explicit extern specifier (10.1.1) , is a class member (12.2) declaration within a class definition, or is the declaration of a parameter or a return type (11.3.5) ; see 6.1. A reference shall be initialized to refer to a valid obj ect or function. [ Note: In particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “obj ect” obtained by indirection through a null pointer, which causes undefined behavior. As described in 12.2.4, a reference cannot be bound directly to a bit-field. — end note ]

6

If a typedef-name (10.1.3, 17.1) or a decltype-specifier (10.1.7.2) denotes a type TR that is a reference to a type T, an attempt to create the type “lvalue reference to cv TR” creates the type “lvalue reference to T”, while an attempt to create the type “rvalue reference to cv TR” creates the type TR. [ Note: This rule is known as reference collapsing. — end note ] [ Example: int i; typedef int& LRI; typedef int&& RRI;

7

LRI& r1 = i; const LRI& r2 = i; const LRI&& r3 = i;

// r1 has the type int& // r2 has the type int& // r3 has the type int&

RRI& r4 = i; RRI&& r5 = 5;

// r4 has the type int& // r5 has the type int&&

decltype(r2) & r6 = i; decltype(r2) && r7 = i;

// r6 has the type int& // r7 has the type int&

— end example ] [ Note: Forming a reference to function type is ill-formed if the function type has cv-qualifier s or a ref-qualifier ; see 11.3.5. — end note ] 1 1 . 3. 3

1

Pointers to members

[dcl. mptr]

In a declaration T D where D has the form nested-name-specifier * attribute-specifier-seqopt cv-qualifier-seqopt D1

and the nested-name-specifier denotes a class, and the type of the identifier in the declaration T D1 is “ deriveddeclarator-type-list T”, then the type of the identifier of D is “ derived-declarator-type-list cv-qualifier-seq pointer to member of class nested-name-specifier of type T ”. The optional attribute-specifier-seq (10.6.1) appertains to the pointer-to-member.

2

[ Example: struct X { void f(int) ; int a; };

§ 11.3.3

© ISO/IEC 2017 – All rights reserved

207

ISO/IEC 14882: 2017(E)

struct Y; int X: : * pmi = &X: : a; void (X: : * pmf) (int) = &X: : f; double X: : * pmd; char Y: : * pmc;

declares pmi , pmf , pmd and pmc to be a pointer to a member of X of type int , a pointer to a member of X of type void(int) , a pointer to a member of X of type double and a pointer to a member of Y of type char respectively. The declaration of pmd is well-formed even though X has no members of type double . Similarly, the declaration of pmc is well-formed even though Y is an incomplete type. pmi and pmf can be used like this: X obj ;

// ... obj . *pmi = 7; (obj . *pmf) (7) ;

// assign 7 to an integer member of obj // call a function member of obj with the argument 7

— end example ] 3 A pointer to member shall not point to a static member of a class (12.2.3) , a member with reference type, or “ cv void”. [ Note: See also 8.3 and 8.5. The type “pointer to member” is distinct from the type “pointer”, that is, a

pointer to member is declared only by the pointer to member declarator syntax, and never by the pointer declarator syntax. There is no “reference-to-member” type in C++ . — end note ]

1 1 . 3. 4

Arrays

1 In a declaration T D where D has the form

[dcl. array]

D1 [ constant-expressionopt ] attribute-specifier-seqopt

and the type of the identifier in the declaration T D1 is “ derived-declarator-type-list T”, then the type of the identifier of D is an array type; if the type of the identifier of D contains the auto type-specifier , the program is ill-formed. T is called the array element type; this type shall not be a reference type, cv void, a function type or an abstract class type. If the constant-expression (8.20) is present, it shall be a converted constant expression of type std: : size_t and its value shall be greater than zero. The constant expression specifies the bound of (number of elements in) the array. If the value of the constant expression is N, the array has N elements numbered 0 to N-1 , and the type of the identifier of D is “ derived-declarator-type-list array of N T ”. An object of array type contains a contiguously allocated non-empty set of N subobjects of type T . Except as noted below, if the constant expression is omitted, the type of the identifier of D is “ derived-declarator-type-list array of unknown bound of T ”, an incomplete obj ect type. The type “ derived-declarator-type-list array of N T” is a different type from the type “ derived-declarator-type-list array of unknown bound of T”, see 6.9. Any type of the form “ cv-qualifier-seq array of N T” is adjusted to “array of N cv-qualifier-seq T ”, and similarly for “array of unknown bound of T ”. The optional attribute-specifier-seq appertains to the array. [ Example: typedef int A[5] , AA[2] [3] ; typedef const A CA; typedef const AA CAA;

// type is “array of 5 const int ” // type is “array of 2 array of 3 const int ”

— end example ] [ Note: An “array of N cv-qualifier-seq T” has cv-qualified type; see 6.9.3. — end note ]

2 An array can be constructed from one of the fundamental types (except void) , from a pointer, from a pointer to member, from a class, from an enumeration type, or from another array.

3 When several “array of” specifications are adjacent, a multidimensional array type is created; only the first of the constant expressions that specify the bounds of the arrays may be omitted. In addition to declarations in which an incomplete obj ect type is allowed, an array bound may be omitted in some cases in the declaration of a function parameter (11.3.5) . An array bound may also be omitted when the declarator is followed by an § 11.3.4

© ISO/IEC 2017 – All rights reserved

208

ISO/IEC 14882: 2017(E)

initializer (11.6) or when a declarator for a static data member is followed by a brace-or-equal-initializer (12.2) . In both cases the bound is calculated from the number of initial elements (say, N ) supplied (11.6.1) , and the type of the identifier of D is “array of N T ”. Furthermore, if there is a preceding declaration of the entity in the same scope in which the bound was specified, an omitted array bound is taken to be the same as in that earlier declaration, and similarly for the definition of a static data member of a class.

4 [ Example: float fa[17] , *afp[17] ;

declares an array of float numbers and an array of pointers to float numbers. For another example, static int x3d[3] [5] [7] ;

declares a static three-dimensional array of integers, with rank 3 × 5 × 7. In complete detail, x3d is an array of three items; each item is an array of five arrays; each of the latter arrays is an array of seven integers. Any of the expressions x3d, x3d[i] , x3d[i] [j ] , x3d[i] [j ] [k] can reasonably appear in an expression. Finally, extern int x[10] ; struct S { static int y[10] ; }; int x[] ; int S: : y[] ; void f() { extern int x[] ; int i = sizeof(x) ; }

// OK: bound is 10 // OK: bound is 10

// error: incomplete object type

— end example ] 5 [ Note: Conversions affecting expressions of array type are described in 7.2. Objects of array types cannot be modified, see 6.10. — end note ] 6 [ Note: Except where it has been declared for a class (16.5.5) , the subscript operator [] is interpreted in such a way that E1[E2] is identical to *((E1) +(E2) ) (8.2.1) . Because of the conversion rules that apply to + , if E1 is an array and E2 an integer, then E1[E2] refers to the E2 -th member of E1 . Therefore, despite its

asymmetric appearance, subscripting is a commutative operation. — end note ] 7 [ Note: A consistent rule is followed for multidimensional arrays. If E is an n-dimensional array of rank i j k , then E appearing in an expression that is subj ect to the array-to-pointer conversion (7.2) k . If the * operator, either is converted to a pointer to an ( n 1) -dimensional array with rank j ×

×

· · · ×

−

×

· · · ×

explicitly or implicitly as a result of subscripting, is applied to this pointer, the result is the pointed-to ( n − 1) -dimensional array, which itself is immediately converted into a pointer. [ Example: Consider int x[3] [5] ;

Here x is a 3 × 5 array of integers. When x appears in an expression, it is converted to a pointer to (the first of three) five-membered arrays of integers. In the expression x[i] which is equivalent to *(x+i) , x is first converted to a pointer as described; then x+i is converted to the type of x , which involves multiplying i by the length of the obj ect to which the pointer points, namely five integer obj ects. The results are added and indirection applied to yield an array (of five integers) , which in turn is converted to a pointer to the first of the integers. If there is another subscript the same argument applies again; this time the result is an integer. — end example ] — end note ]

8 [ Note: It follows from all this that arrays in C++ are stored row-wise (last subscript varies fastest) and that

the first subscript in the declaration helps determine the amount of storage consumed by an array but plays no other part in subscript calculations. — end note ]

§ 11.3.4

© ISO/IEC 2017 – All rights reserved

209

ISO/IEC 14882: 2017(E)

1 1 . 3. 5 1

Functions

[dcl. fct]

In a declaration T D where D has the form D1 ( parameter-declaration-clause ) cv-qualifier-seqopt

ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt

and the type of the contained declarator-id in the declaration T D1 is “ derived-declarator-type-list T ”, the type of the declarator-id in D is “ derived-declarator-type-list noexcept opt function of ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt returning T”, where the optional noexcept is present if and only if the exception specification (18.4) is non-throwing. The optional attribute-specifier-seq appertains to the function type. 2

In a declaration T D where D has the form D1 ( parameter-declaration-clause ) cv-qualifier-seqopt

ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt trailing-return-type

and the type of the contained declarator-id in the declaration T D1 is “ derived-declarator-type-list T ”, T shall be the single type-specifier auto . The type of the declarator-id in D is “ derived-declarator-type-list noexcept opt function of ( parameter-declaration-clause ) cv-qualifier-seq opt ref-qualifier opt returning U”, where U is the type specified by the trailing-return-type , and where the optional noexcept is present if and only if the exception specification is non-throwing. The optional attribute-specifier-seq appertains to the function type. 3

A type of either form is a

function type. 100

parameter-declaration-clause : parameter-declaration-listopt . . . opt parameter-declaration-list , . . . parameter-declaration-list : parameter-declaration parameter-declaration-list , parameter-declaration parameter-declaration : attribute-specifier-seqopt attribute-specifier-seqopt attribute-specifier-seqopt attribute-specifier-seqopt

decl-specifier-seq decl-specifier-seq decl-specifier-seq decl-specifier-seq

declarator declarator = initializer-clause abstract-declaratoropt abstract-declaratoropt = initializer-clause

attribute-specifier-seq in a parameter-declaration appertains to the parameter. The parameter-declaration-clause determines the arguments that can be specified, and their processing, when the function is called. [ Note: The parameter-declaration-clause is used to convert the arguments specified on the function call; see 8.2.2. — end note ] If the parameter-declaration-clause is empty, the function takes

The optional 4

no arguments. A parameter list consisting of a single unnamed parameter of non-dependent type void is equivalent to an empty parameter list. Except for this special case, a parameter shall not have type cv void. If the parameter-declaration-clause terminates with an ellipsis or a function parameter pack (17.5.3) , the number of arguments shall be equal to or greater than the number of parameters that do not have a default argument and are not function parameter packs. Where syntactically correct and where “ . . . ” is not part of an abstract-declarator , “ , . . . ” is synonymous with “ . . . ”. [ Example: The declaration int printf(const char*, . . . ) ;

declares a function that can be called with varying numbers and types of arguments. printf(" hello world" ) ; printf(" a=%d b=%d" , a, b) ;

However, the first argument must be of a type that can be converted to a const char* — end example ] [ Note: The standard header contains a mechanism for accessing arguments passed using the 100)

As indicat ed by synt ax, cv- qualifiers are a significant comp onent in funct ion ret urn typ es.

§ 11.3.5

© ISO/IEC 2017 – All rights reserved

210

ISO/IEC 14882: 2017(E)

ellipsis ( see 8 . 2 . 2

5

A

s ingle

name

( C lause 1 6 ) . lis t .

and 2 1 . 1 0) .

can

be

used

typ e

typ e

[

of each

are

T ”.

different

funct ions

delet ed

presence

Note:

paramet er,

or

when

any

paramet er

forming t he

absence

of t he

in

a

single

of typ e

funct ion or

typ e.

a

affect

cv-qualifier-seq

The

funct ion

of

T”

or

any t op- level

result ing lis t

paramet er

funct ion

or a

are ident ical typ es.

ref-qualifier

typ e

cv-qualifier

overloading

is

t he

Aft er det ermining

T

is

adj us t ed

paramet er

funct ion’ s

For example,

— end note

.

to

be

s mo difying a paramet er

of t ransformed

pack

typ es

and

parameter-type-list

.

int(*) (const int p,

]

typedef-name

( including a typ e named by

( 1 0. 1 . 3,

shall app ear only as:

t he funct ion typ e for a non- st at ic memb er funct ion,

(6. 2)

—

t he funct ion typ e t o which a p oint er t o memb er refers,

(6. 3)

—

t he t op- level funct ion typ e of a funct ion typ edef declarat ion or

[

is

declarator

and

of funct ion

t he typ es of t he paramet ers.

int(*) (int, const int*)

and

“ array

—

(6. 5)

t his

The typ e of each paramet er ( including

(6. 1 )

(6. 4)

scop e;

decl-specifier-seq

of paramet er typ es,

ellipsis

This t ransformat ion do es not

A funct ion typ e wit h a 1 7. 1 ) )

is det ermined from it s own

Aft er pro ducing t he list

decltype(p) *) 6

several

The typ e of a funct ion is det ermined using t he following rules.

“ p oint er t o

t he

for

]

All declarat ions for a funct ion shall agree exact ly in b ot h t he ret urn typ e and t he paramet er- typ e-

funct ion paramet er packs) t he

— end note

type-id type-id Example: —

t he

in t he default

—

t he

of a

argument

template-argument

of a

for a

type-parameter type-parameter

( 1 7. 1 ) ,

alias-declaration

,

or

( 1 7. 3 . 1 ) .

typedef int FIC(int) const; FIC f; // ill-formed: does not declare a member function struct S { FIC f; // OK }; FIC S: : *pm = &S: : f; // OK

— end example 7

The

effect

of t he

]

cv-qualifier-seq

of a

funct ion

cv-qualifier-seq

typ e. is not

In

8

in

a

lat t er

funct ion case,

a cv- qualified typ e;

typedef void F() ; struct S { const F f; };

— end example

t he

t he

not

is

not are

t he

t he default

Example:

as

adding

Note:

A

cv- qualificat ion

funct ion

typ e

— end note

]

on

t hat [

t op

has

a

Example:

]

memb er funct ions.

[

[

// OK: equivalent to: void f() ;

argument s (1 1 . 3 . 6 ) ,

ref-qualifier

are part

, t he

cv-qualifier-seq Note:

, and t he except ion sp ecificat ion,

of t he funct ion typ e.

during t he assignment s and init ializ at ions of p oint ers t o funct ions,

9

same

ignored.

t here are no cv- qualified funct ion typ es.

The ret urn typ e, t he paramet er- typ e- list , t he but

declarat or

cv- qualifiers

— end note

[

Funct ion typ es are checked

references t o funct ions,

and p oint ers t o

]

The declarat ion

int fseek(FILE*, long, int) ; declares a funct ion t aking t hree argument s of t he sp ecified typ es, and ret urning

10

Funct ions shall not

have a ret urn typ e of typ e array or funct ion,

typ e p oint er or reference t o such t hings.

int

( 1 0 . 1 . 7) .

— end example

]

alt hough t hey may have a ret urn typ e of

There shall b e no arrays of funct ions,

alt hough t here can b e arrays

of p oint ers t o funct ions.

11

Typ es shall not

§ 1 1 . 3. 5

b e defined in ret urn or paramet er typ es.

© IS O / IEC

The typ e of a paramet er or t he ret urn typ e for a

2 0 1 7 – All right s reserved

21 1

ISO/IEC 14882: 2017(E)

function definition shall not be an incomplete (possibly cv-qualified) class type in the context of the function definition unless the function is deleted (11.4.3) . 12

A typedef of function type may be used to declare a function but shall not be used to define a function (11.4) . [ Example: typedef void F() ; F fv; F fv { } void fv() { }

// OK: equivalent to void fv() ; // ill-formed // OK: definition of fv

— end example ] 13

An identifier can optionally be provided as a parameter name; if present in a function definition (11.4) , it names a parameter. [ Note: In particular, parameter names are also optional in function definitions and names used for a parameter in different declarations and the definition of a function need not be the same. If a parameter name is present in a function declaration that is not a definition, it cannot be used outside of its function declarator because that is the extent of its potential scope (6.3.4) . — end note ]

14

[ Example: The declaration int i, *pi, f() , *fpi(int) , (*pif) (const char*, const char*) , (*fpif(int) ) (int) ;

declares an integer i , a pointer pi to an integer, a function f taking no arguments and returning an integer, a function fpi taking an integer argument and returning a pointer to an integer, a pointer pif to a function which takes two pointers to constant characters and returns an integer, a function fpif taking an integer argument and returning a pointer to a function that takes an integer argument and returns an integer. It is especially useful to compare fpi and pif . The binding of *fpi(int) is *(fpi(int) ) , so the declaration suggests, and the same construction in an expression requires, the calling of a function fpi , and then using indirection through the (pointer) result to yield an integer. In the declarator (*pif) (const char*, const char*) , the extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function, which is then called. — end example ] [ Note: Typedefs and trailing-return-types are sometimes convenient when the return type of a function is complex. For example, the function fpif above could have been declared typedef int IFUNC(int) ; IFUNC* fpif(int) ;

or auto fpif(int) ->int(*) (int) ;

A trailing-return-type is most useful for a type that would be more complicated to specify before the declarator-id: template auto add(T t, U u) -> decltype(t + u) ;

rather than template decltype((*(T*) 0) + (*(U*) 0) ) add(T t, U u) ;

15

16

— end note ] A non-template function is a function that is not a function template specialization. [ Note: A function template is not a function. — end note ] A declarator-id or abstract-declarator containing an ellipsis shall only be used in a parameter-declaration . Such a parameter-declaration is a parameter pack (17.5.3) . When it is part of a parameter-declaration-clause , § 11.3.5

© ISO/IEC 2017 – All rights reserved

212

ISO/IEC 14882: 2017(E)

t he paramet er pack is a funct ion paramet er pack (1 7. 5 . 3 ) . part

note

template-parameter-list

of a ]

[

Note:

and t he paramet er pack is

O t herwise,

t he

parameter-declaration — end is

a t emplat e paramet er pack;

A funct ion paramet er pack is a pack expansion (1 7. 5 . 3 ) .

Example:

[

see 1 7. 1 .

template void f(T (* . . . t) (int, int) ) ; int add(int, int) ; float subtract(int, int) ; void g() { f(add, subtract) ; }

— end example 17

]

There is a synt act ic ambiguity when an ellip sis o ccurs at a preceding comma.

In t his case,

p aramet er eit her names a t emplat e paramet er pack t hat it

is parsed as part

1 1 . 3. 6 1

If

an

[

of t he

has not

parameter-declaration-clause 1 01

parameter-declaration-clause abstract-declarator

D efault

Example:

is

wit hout

if t he typ e of t he

b een expanded or cont ains

auto

;

ot herwise,

.

Default arguments

initializer-clause

argument .

2

of t he

t he end of a

t he ellipsis is parsed as part

[dcl. fct. default]

sp ecified

in

a

parameter-declaration

initializer-clause

t his

is

used

as

a

default

argument s will b e used in calls where t railing argument s are missing.

The declarat ion

void point(int = 3, int = 4) ; declares a funct ion t hat

can b e called wit h z ero,

one,

or two argument s of typ e

int

.

It

can b e called in any

of t hese ways:

point(1, 2) ; The last

3

point(1) ;

point() ;

two calls are equivalent

A default

argument

to

point(1, 4)

and

shall b e sp ecified only in t he

lambda-declarator template-parameter assignment-expression parameter-declaration-clause declaration 102 or

in

.

a

A default ,

it

argument

shall not

point(3, 4)

,

resp ect ively.

— end example

]

parameter-declaration-clause initializer-clause

of a funct ion declarat ion or

( 1 7. 1 ) ;

shall not

in

t he

lat t er

case,

t he

b e sp ecified for a paramet er pack.

o ccur wit hin a

declarator

or

If it

abstract-declarator

s hall

be

an

is sp ecified in a of a

parameter-

.

4

For

non- t emplat e

s ame

scop e.

funct ions,

D eclarat ions

in

default

declarat ions in inner scop es do not versa.

argument s

different

scop es

can

have

acquire default

In a given funct ion declarat ion,

be

added

complet ely

in

lat er

dist inct

declarat ions

set s

of default

of

a

funct ion

argument s.

argument s from declarat ions in out er scop es,

each paramet er subsequent

t o a paramet er wit h a default

in

t he

That

is,

and vice

argument

s hall have a default argument supp lied in t his or a previous declarat ion or shall b e a funct ion paramet er pack. A default

argument

shall not

b e redefined by a lat er declarat ion ( not

void f(int, int) ; void f(int, int = 7) ; void h() { f(3) ; 101 )

O ne

can

102)

Example:

// OK, calls f(3, 7)

explicit ly disambiguat e

parameter-declaration-clause ) declarator-id) .

[

// OK, ellipsis is not a parameter so it can follow // a parameter with a default argument

void g(int = 0, . . . ) ;

t he

even t o t he same value) .

t he

parse

eit her

by int ro ducing a comma ( so

t he

ellipsis

will

be

parsed

as

part

of

or by int ro ducing a name for t he paramet er ( so t he ellipsis will b e parsed as part of t he

This means t hat default argument s cannot app ear, for example, in declarat ions of p oint ers t o funct ions, references t o

funct ions, or

§ 1 1 . 3. 6

typedef

declarat ions.

© IS O / IEC

2 0 1 7 – All right s reserved

21 3

ISO/IEC 14882: 2017(E)

void f(int = 1, int) ; } void m() { void f(int, int) ; f(4) ; void f(int, int = 5) ; f(4) ; void f(int, int = 5) ; } void n() { f(6) ; }

// error: does not use default from surrounding scope

// has no defaults // error: wrong number of arguments // OK // OK, calls f(4, 5) ; // error: cannot redefine, even to same value

// OK, calls f(6, 7)

— end example ] For a given inline function defined in different translation units, the accumulated sets of default arguments at the end of the translation units shall be the same; see 6.2. If a friend declaration specifies a default argument expression, that declaration shall be a definition and shall be the only declaration of the function or function template in the translation unit. 5

The default argument has the same semantic constraints as the initializer in a declaration of a variable of the parameter type, using the copy-initialization semantics (11.6) . The names in the default argument are bound, and the semantic constraints are checked, at the point where the default argument appears. Name lookup and checking of semantic constraints for default arguments in function templates and in member functions of class templates are performed as described in 17.7.1. [ Example: In the following code, g will be called with the value f(2) : int a = 1; int f(int) ; int g(int x = f(a) ) ; void h() { a = 2; { int a = 3; g() ; } }

// default argument: f(: : a)

// g(f(: : a) )

— end example ] [ Note: In member function declarations, names in default arguments are looked up as described in 6.4.1. Access checking applies to names in default arguments as described in Clause 14. — end note ] 6

Except for member functions of class templates, the default arguments in a member function definition that appears outside of the class definition are added to the set of default arguments provided by the member function declaration in the class definition; the program is ill-formed if a default constructor (15.1) , copy or move constructor, or copy or move assignment operator (15.8) is so declared. Default arguments for a member function of a class template shall be specified on the initial declaration of the member function within the class template. [ Example: class C { void f(int i = 3) ; void g(int i, int j = 99) ; }; void C: : f(int i = 3) {} // error: default argument already specified in class scope void C: : g(int i = 88, int j ) {} // in this translation unit, C: : g can be called with no argument

— end example ] § 11.3.6

© ISO/IEC 2017 – All rights reserved

214

ISO/IEC 1 4882:201 7(E)

7

A lo cal variable shall not app ear as a p ot ent ially- evaluat ed exp ression in a default argument .

[

Example:

void f() { int i; extern void g(int x = i) ; // error extern void h(int x = sizeof(i) ) ; // OK // ... } 8

— end example Note:

[

]

this

The keyword

may not

app ear in a default

class A { void f(A* p = this) { } };

— end example 9

A default

— end note

]

argument

paramet er.

A

shall

not

app ear

int a; int f(int a, int b = a) ; typedef int I; int g(float I, int b = I(2) ) ; int h(int a, int b = sizeof(a) ) ; ]

A

as

a

p ot ent ially- evaluat ed

non- st at ic

argument

[

shall

not

X: : b part

.

C las ses,

X: : mem1() X: : a

The declarat ion of

int b; class X { int a; int mem1(int i = a) ; int mem2(int i = b) ; static int b; };

X: : mem2()

ob j ect s,

for t he corresp onding

in

a

default

argument .

are in scop e and can hide namespace and class

app ear

in

a

default

or unless it

argument

unless

it

app ears

as

t he

is used t o form a p oint er t o memb er

in t he following example is ill- formed b ecause no ob j ect

is

used as an init ializ er.

// error: non-static member a used as default argument // OK; use X: : b

is meaningful,

however,

since no ob j ect

and memb ers are describ ed in C lause 1 2 .

of t he typ e of a funct ion.

expression

// error: parameter I found // OK, unevaluated operand

memb er

s upplied for t he non- st at ic memb er

The declarat ion of

Example:

// error: parameter a used as default argument

of a class memb er access expression (8 . 2 . 5 )

( 8. 3. 1 ) .

[

]

Example:

— end example id-expression Example:

see 8 . 1 . 2 .

// error

Paramet ers of a funct ion declared b efore a default [

of a memb er funct ion;

is evaluat ed each t ime t he funct ion is called wit h no argument

paramet er

memb er names.

argument

[

Example:

is needed t o access t he st at ic memb er

— end example

]

A default

argument is not

int f(int = 0) ; void h() { int j = f(1) ; int k = f() ; }

// OK, means f(0)

int (*p1) (int) = &f; int (*p2) () = &f;

— end example default

]

argument

// error: type mismatch

When a declarat ion of a funct ion is int ro duced by way of a informat ion

asso ciat ed

wit h

t he

declarat ion

redeclared t hereaft er in t he namespace wit h addit ional default known at

§ 1 1 . 3. 6

any p oint

following t he redeclarat ion where t he

© IS O / IEC

is

made

argument s,

as

well.

( 1 0. 3. 3) ,

If t he

any

funct ion

is

t he addit ional argument s are also

using-declaration

2 0 1 7 – All right s reserved

using-declaration

known

is in s cop e.

21 5

ISO/IEC 14882: 2017(E)

10

A virtual function call (13.3) uses the default arguments in the declaration of the virtual function determined by the static type of the pointer or reference denoting the obj ect. An overriding function in a derived class does not acquire default arguments from the function it overrides. [ Example: struct A { virtual void f(int a = 7) ; }; struct B : public A { void f(int a) ; }; void m() { B* pb = new B; A* pa = pb; pa->f() ; // OK, calls pa->B: : f(7) pb->f() ; // error: wrong number of arguments for B: : f() }

— end example ] 11.4

Function definitions

1 1 . 4. 1 1

[dcl. fct. def]

In general

[dcl. fct. def. general]

Function definitions have the form function-definition : attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt function-body function-body : ctor-initializeropt compound-statement function-try-block = default ; = delete ;

Any informal reference to the body of a function should be interpreted as a reference to the non-terminal function-body. The optional attribute-specifier-seq in a function-definition appertains to the function. A virt-specifier-seq can be part of a function-definition only if it is a member-declaration (12.2) . 2

In a function-definition , either void declarator ; or declarator ; shall be a well-formed function declaration as described in 11.3.5. A function shall be defined only in namespace or class scope.

3

[ Example: A simple example of a complete function definition is int max(int a, int b, int c) { int m = (a > b) ? a : b; return (m > c) ? m : c; }

Here int is the decl-specifier-seq ; max(int a, int b, int c) is the function-body. — end example ] 4 5 6

declarator; { /* . . . */ } is the

ctor-initializer is used only in a constructor; see 15.1 and 15.6. [ Note: A cv-qualifier-seq affects the type of this in the body of a member function; see 11.3.2. — end note ] [ Note: Unused parameters need not be named. For example,

A

void print(int a, int) { std: : printf(" a = %d\n" , a) ; }

7

— end note ] In the function-body , a function-local predefined variable denotes a block-scope object of static storage duration § 11.4.1

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that is implicitly defined (see 6.3.3) . 8

The function-local predefined variable __func__ is defined as if a definition of the form static const char __func__[] = " function-name " ;

had been provided, where function-name is an implementation-defined string. It is unspecified whether such a variable has an address distinct from that of any other obj ect in the program. 103 [ Example: struct S { S() : s(__func__) { } const char* s; }; void f(const char* s = __func__) ;

// OK

// error: __func__ is undeclared

— end example ] 1 1 . 4. 2 1

Explicitly-defaulted functions

[dcl. fct. def. default]

A function definition of the form: attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt

is called an

= default ;

explicitly-defaulted definition. A function that is explicitly defaulted shall

(1. 1 )

— be a special member function,

(1. 2)

— have the same declared function type (except for possibly differing ref-qualifiers and except that in the case of a copy constructor or copy assignment operator, the parameter type may be “reference to non-const T ”, where T is the name of the member function’s class) as if it had been implicitly declared, and

(1. 3)

— not have default arguments.

2

An explicitly-defaulted function that is not defined as deleted may be declared constexpr only if it would have been implicitly declared as constexpr . If a function is explicitly defaulted on its first declaration, it is implicitly considered to be constexpr if the implicit declaration would be.

3

If a function that is explicitly defaulted is declared with a noexcept-specifier that does not produce the same exception specification as the implicit declaration (18.4) , then

(3. 1 )

— if the function is explicitly defaulted on its first declaration, it is defined as deleted;

(3. 2)

— otherwise, the program is ill-formed.

4

[ Example: struct S { constexpr S() = default; S(int a = 0) = default; void operator=(const S&) = default; ~ S() noexcept(false) = default; private: int i; S(S&) ; }; S: : S(S&) = default;

// ill-formed: implicit S() is not constexpr // ill-formed: default argument // ill-formed: non-matching return type // deleted: exception specification does not match

// OK: private copy constructor // OK: defines copy constructor

— end example ] 5

Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them (15.1 15.4, 15.8) , which might mean defining 103)

Implement at ions are p ermit t ed t o provide addit ional predefined variables wit h names t hat are reserved t o t he implement a-

t ion ( 5. 1 0) .

§ 11.4.2

If a predefined variable is not o dr- used ( 6. 2 ) , it s st ring value need not b e present in t he program image.

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them as deleted. A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed. [ Note: Declaring a function as defaulted after its first declaration can provide efficient execution and concise definition while enabling a stable binary interface to an evolving code base. — end note ] 6

[ Example: struct trivial { trivial() = default; trivial(const trivial&) = default; trivial(trivial&&) = default; trivial& operator=(const trivial&) = default; trivial& operator=(trivial&&) = default; ~ trivial() = default; }; struct nontrivial1 { nontrivial1() ; }; nontrivial1: : nontrivial1() = default;

// not first declaration

— end example ] 1 1 . 4. 3 1

Deleted definitions

[dcl. fct. def. delete]

A function definition of the form: attribute-specifier-seqopt decl-specifier-seqopt declarator virt-specifier-seqopt

is called a

= delete ;

deleted definition. A function with a deleted definition is also called a deleted function.

2

A program that refers to a deleted function implicitly or explicitly, other than to declare it, is ill-formed. [ Note: This includes calling the function implicitly or explicitly and forming a pointer or pointer-to-member to the function. It applies even for references in expressions that are not potentially-evaluated. If a function is overloaded, it is referenced only if the function is selected by overload resolution. The implicit odr-use (6.2) of a virtual function does not, by itself, constitute a reference. — end note ]

3

[ Example: One can enforce non-default-initialization and non-integral initialization with struct onlydouble { onlydouble() = delete; onlydouble(std: : intmax_t) = delete; onlydouble(double) ; };

// OK, but redundant

— end example ] [ Example: One can prevent use of a class in certain new-expression s by using deleted definitions of a user-declared operator new for that class.

struct sometype { void* operator new(std: : size_t) = delete; void* operator new[] (std: : size_t) = delete; }; sometype* p = new sometype; // error, deleted class operator new sometype* q = new sometype[3] ; // error, deleted class operator new[]

— end example ] [ Example: One can make a class uncopyable, i.e. move-only, by using deleted definitions of the copy § 11.4.3

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constructor and copy assignment operator, and then providing defaulted definitions of the move constructor and move assignment operator. struct moveonly { moveonly() = default; moveonly(const moveonly&) = delete; moveonly(moveonly&&) = default; moveonly& operator=(const moveonly&) = delete; moveonly& operator=(moveonly&&) = default; ~ moveonly() = default; }; moveonly* p; moveonly q(*p) ; // error, deleted copy constructor

— end example ] 4

A deleted function is implicitly an inline function (10.1.6) . [ Note: The one-definition rule (6.2) applies to deleted definitions. — end note ] A deleted definition of a function shall be the first declaration of the function or, for an explicit specialization of a function template, the first declaration of that specialization. An implicitly declared allocation or deallocation function (6.7.4) shall not be defined as deleted. [ Example: struct sometype { sometype() ; }; sometype: : sometype() = delete;

// ill-formed; not first declaration

— end example ] 11.5 1

Structured binding declarations

[dcl. struct. bind]

A structured binding declaration introduces the identifier s v0 , v1 , v2 , ... of the identifier-list as names (6.3.1) , called structured bindings. Let cv denote the cv-qualifier s in the decl-specifier-seq . First, a variable with a unique name e is introduced. If the assignment-expression in the initializer has array type A and no ref-qualifier is present, e has type cv A and each element is copy-initialized or direct-initialized from the corresponding element of the assignment-expression as specified by the form of the initializer . Otherwise, e is defined as-if by attribute-specifier-seqopt decl-specifier-seq ref-qualifieropt e initializer ;

where the declaration is never interpreted as a function declaration and the parts of the declaration other than the declarator-id are taken from the corresponding structured binding declaration. The type of the id-expression e is called E. [ Note: E is never a reference type (Clause 8) . — end note ] 2

If E is an array type with element type T , the number of elements in the identifier-list shall be equal to the number of elements of E. Each vi is the name of an lvalue that refers to the element i of the array and whose type is T ; the referenced type is T. [ Note: The top-level cv-qualifiers of T are cv. — end note ] [ Example: auto f() -> int(&) [2] ; auto [ x, y ] = f() ; auto& [ xr, yr ] = f() ;

// x and y refer to elements in a copy of the array return value // xr and yr refer to elements in the array referred to by f ’s return value

— end example ] 3

qualified-id std: : tuple_size names a complete type, the expression std: : tuple_size: : value shall be a well-formed integral constant expression and the number of elements in the identifier-list shall be equal to the value of that expression. The unqualified-id get is looked up in the scope of E by class member access lookup (6.4.5) , and if that finds at least one declaration, the initializer is e. get() . Otherwise, the initializer is get(e) , where get is looked up in the associated namespaces (6.4.2) . In either case, get is interpreted as a template-id. [ Note: Ordinary unqualified lookup (6.4.1) is not performed. Otherwise, if the

§ 11.5

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— end note ot herwise.

]

In

eit her

case,

G iven t he typ e

“ reference t o

Ti

Ti

e

is

an

lvalue

designat ed by

O t herwise, all of

E

public base class

identifier-list memb ers memb er

cv Ti

.

of

mi

if t he

” init ializ ed wit h t he init ializ er,

lvalue and an rvalue reference ot herwise;

4

e std: : tuple_element: : type typ e

of t he

ent ity

is

E

as

e

lvalue

Ti

E E ,

shall not

m0 m1 m2 ,

,

,

...

have an anonymous

( in

and whose typ e is

declarat ion

cv Ti

The lvalue is a bit - field if t hat

,

reference

vi

each

and

an

xvalue

is a variable of typ e

.

’ s non- st at ic dat a memb ers shall b e public direct memb ers of

of

,

where t he reference is an lvalue reference if t he init ializ er is an

t he referenced typ e is

union memb er,

where

order) ,

Ti

each

vi

is

t he

E

or of t he same unambiguous

and t he numb er of element s

shall b e equal t o t he numb er of non- st at ic dat a memb ers of

of

an

E

.

name

of an

lvalue

is t he declared typ e of t hat memb er;

memb er is a bit - field.

[

in t he

D esignat ing t he non- st at ic dat a

Example:

t hat

refers

to

t he

t he referenced typ e is

struct S { int x1 : 2; volatile double y1; }; S f() ; const auto [ x, y ] = f() ;

id-expression x

The typ e of t he

— end example 11.6 1

A

is “

const int

” , t he typ e of t he

id-expression y

is “

const volatile double

Initializers

declarat or

variable

”.

]

can

b eing

sp ecify

[dcl. init] an

init ializ ed.

init ial

The

value

pro cess

for

t he

ident ifier

of init ializ at ion

init ializ at ions sp ecified by ot her synt act ic cont ext s,

b eing

describ ed

declared.

in

t he

The

remainder

ident ifier of 1 1 . 6

designat es

applies

a

also

to

s uch as t he init ializ at ion of funct ion paramet ers (8 . 2 . 2 )

or

t he init ializ at ion of ret urn values (9 . 6 . 3 ) .

initializer: brace-or-equal-initializer ( expression-list ) brace-or-equal-initializer: = initializer-clause braced-init-list initializer-clause : assignment-expression braced-init-list initializer-list : initializer-clause . . . opt initializer-list , initializer-clause . . . opt braced-init-list : { initializer-list , opt } { } expr-or-braced-init-list : expression braced-init-list 2

Except of a

for ob j ect s declared wit h t he

variable

funct ions,

int int int int

3

can

§ 11.6

constexpr

of arbit rary

sp ecifier,

expres sions

for which see 1 0 . 1 . 5 ,

involving

regardles s of t he variab le’ s st orage durat ion.

[

lit erals

Example:

and

an

initializer

previously

in t he definit ion

declared

variables

and

f(int) ; a = 2; b = f(a) ; c(b) ;

— end example Note:

[

consist

D efault

]

argument s are more rest rict ed;

© IS O / IEC

see 1 1 . 3 . 6 .

— end note

]

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ISO/IEC 14882: 2017(E)

4

Note: note [

— end

The order of init ializ at ion of variables wit h st at ic st orage durat ion is describ ed in 6 . 6 and 9 . 7.

]

5

A declarat ion of a blo ck- scop e variable wit h ext ernal or int ernal linkage t hat

6

To

(6. 1 )

zero-initialize

—

if

0 (6. 2)

—

if

T

or reference of typ e

is a scalar typ e ( 6 . 9 ) ,

( z ero)

T

an ob j ect

t he ob j ect

T

initializer

has an

is ill- formed.

means:

is init ializ ed t o t he value obt ained by convert ing t he int eger lit eral

T; 1 04

to

is a ( p ossibly cv- qualified)

base class sub ob j ect ,

and,

non- union class typ e,

if t he ob j ect

is not

each non- st at ic dat a memb er,

a base class sub ob j ect ,

each non- virt ual

each virt ual base class sub ob j ect

is z ero- init ializ ed and padding is init ializ ed t o z ero bit s;

(6. 3)

—

if

T

is

a ( p ossibly cv- qualified)

union typ e,

t he ob j ect ’ s

first

non- st at ic named dat a memb er is

z ero-

init ializ ed and padding is init ializ ed t o z ero bit s;

(6. 4)

—

if

T

is an array typ e,

(6. 5)

—

if

T

is a reference typ e,

7

default-initialize

To

(7. 1 )

—

If

T

is

a

each element

no init ializ at ion is p erformed.

an ob j ect

( p ossibly

is z ero- init ializ ed;

of typ e

cv- qualified)

T

means :

class

typ e

( C lause

1 2) ,

const ruct ors

const ruct ors are enumerat ed ( 1 6 . 3 . 1 . 3) , and t he b est one for t he res olut ion ( 1 6 . 3 ) .

The const ruct or t hus select ed is called,

are

considered.

initializer ( )

The

applicable

is chosen t hrough overload

wit h an empty argument

list ,

t o init ializ e t he

ob j ect .

(7. 2)

—

If

(7. 3)

—

O t herwise,

A

clas s

T

is an array typ e,

typ e

const ruct or of

(7. 4)

—

—

no init ializ at ion is p erformed.

each direct

if

T

is

is default - init ializ ed.

T is const-default-constructible if default - init ializ at ion T ( not inherit ed from a base class) or if

class typ e

(7. 5)

each element

non- variant

X

non- st at ic dat a memb er

X

( or array t hereof) ,

a union wit h at

least

M

of

T

of

T

has a default

would

invoke

a

user- provided

memb er init ializ er or,

if

M

is of

is const - default - const ruct ib le,

one non- s t at ic dat a memb er,

exact ly one variant

memb er

has

a default

memb er init ializ er,

(7. 6)

—

if

T

is not

any) ,

(7. 7)

—

a union,

for each anonymous union memb er wit h at

exact ly one non- st at ic dat a memb er has a default

each p ot ent ially const ruct ed base class of

T

least

one non- s t at ic dat a memb er ( if

memb er init ializ er,

and

is const - default - const ruct ible.

If a program calls for t he default - init ializ at ion of an ob j ect

of a const - qualified typ e

T, T

shall b e a const -

default - const ruct ible class typ e or array t hereof.

8 (8. 1 )

To

value-initialize

—

if

T

is

—

if

T

of typ e

const ruct or t hat

—

if

(8. 4)

—

ot herwise,

9

clas s

typ e ( C lause 1 2 )

class typ e wit hout

wit h eit her

no default

t hen t he ob j ect

const ruct or

(1 5 . 1 )

or a

const ruct or,

t hen

is default - init ializ ed;

a user- provided or delet ed default

is z ero- init ializ ed and t he semant ic const raint s for default - init ializ at ion are checked,

has a non- t rivial default

(8. 3)

means:

is user- provided or delet ed,

is a ( p oss ibly cv- qualified)

t he ob j ect

T

T

a ( p oss ibly cv- qualified)

default

(8. 2)

an ob j ect

is an array typ e,

t he ob j ect

const ruct or,

t he ob j ect

t hen each element

and if

T

is default - init ializ ed;

is value- init ializ ed;

is z ero- init ializ ed.

A p rogram t hat calls for default - init ializ at ion or value- init ializ at ion of an ent ity of reference typ e is ill- formed.

104)

As sp ecified in 7. 1 1 , convert ing an int eger lit eral whose value is

§ 11.6

© IS O / IEC

0

t o a p oint er typ e result s in a null p oint er value.

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10

[ Note: Every obj ect of static storage duration is zero-initialized at program startup before any other initialization takes place. In some cases, additional initialization is done later. — end note ]

11

An obj ect whose initializer is an empty set of parentheses, i.e., () , shall be value-initialized. [ Note: Since () is not permitted by the syntax for

initializer,

X a() ;

is not the declaration of an obj ect of class X , but the declaration of a function taking no argument and returning an X . The form () is permitted in certain other initialization contexts (8.3.4, 8.2.3, 15.6.2) . — end note ] 12

If no initializer is specified for an obj ect, the obj ect is default-initialized. When storage for an obj ect with automatic or dynamic storage duration is obtained, the obj ect has an indeterminate value , and if no initialization is performed for the obj ect, that obj ect retains an indeterminate value until that value is replaced (8.18) . [ Note: Obj ects with static or thread storage duration are zero-initialized, see 6.6.2. — end note ] If an indeterminate value is produced by an evaluation, the behavior is undefined except in the following cases:

(12. 1)

— If an indeterminate value of unsigned narrow character type (6.9.1) or std: : byte type (21.2.1) is produced by the evaluation of:

(12. 1. 1)

— the second or third operand of a conditional expression (8.16) ,

(12. 1. 2)

— the right operand of a comma expression (8.19) ,

(12. 1. 3)

— the operand of a cast or conversion (7.8, 8.2.3, 8.2.9, 8.4) to an unsigned narrow character type or std: : byte type (21.2.1) , or

(12. 1. 4)

— a discarded-value expression (Clause 8) , then the result of the operation is an indeterminate value.

(12. 2)

— If an indeterminate value of unsigned narrow character type or std: : byte type is produced by the evaluation of the right operand of a simple assignment operator (8.18) whose first operand is an lvalue of unsigned narrow character type or std: : byte type, an indeterminate value replaces the value of the obj ect referred to by the left operand.

(12. 3)

— If an indeterminate value of unsigned narrow character type is produced by the evaluation of the initialization expression when initializing an obj ect of unsigned narrow character type, that obj ect is initialized to an indeterminate value.

(12. 4)

— If an indeterminate value of unsigned narrow character type or std: : byte type is produced by the evaluation of the initialization expression when initializing an obj ect of std: : byte type, that obj ect is initialized to an indeterminate value. [ Example: int f(bool b) { unsigned char c; unsigned char d = c; int e = d; return b ? d : 0; }

// OK, d has an indeterminate value // undefined behavior // undefined behavior if b is true

— end example ] 13

An initializer for a static member is in the scope of the member’s class. [ Example: int a; struct X { static int a;

§ 11.6

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ISO/IEC 14882: 2017(E)

static int b; }; int X: : a = 1; int X: : b = a;

// X: : b

= X: : a

— end example ] expression-list in a parenthesized initializer shall

14

If the entity being initialized does not have class type, the be a single expression.

15

The initialization that occurs in the = form of a brace-or-equal-initializer or condition (9.4) , as well as in argument passing, function return, throwing an exception (18.1) , handling an exception (18.3) , and aggregate member initialization (11.6.1) , is called copy-initialization . [ Note: Copy-initialization may invoke a move (15.8) . — end note ]

16

The initialization that occurs in the forms T x(a) ; T x{a};

as well as in new expressions (8.3.4) , static_cast expressions (8.2.9) , functional notation type conversions (8.2.3) , mem-initializer s (15.6.2) , and the braced-init-list form of a condition is called direct-initialization . 17

The semantics of initializers are as follows. The destination type is the type of the obj ect or reference being initialized and the source type is the type of the initializer expression. If the initializer is not a single (possibly parenthesized) expression, the source type is not defined.

braced-init-list or is = braced-init-list, the object or reference is

(17. 1)

— If the initializer is a (non-parenthesized) list-initialized (11.6.4) .

(17. 2)

— If the destination type is a reference type, see 11.6.3.

(17. 3)

— If the destination type is an array of characters, an array of char16_t , an array of char32_t , or an array of wchar_t , and the initializer is a string literal, see 11.6.2.

(17. 4)

— If the initializer is () , the obj ect is value-initialized.

(17. 5)

— Otherwise, if the destination type is an array, the program is ill-formed.

(17. 6)

— If the destination type is a (possibly cv-qualified) class type:

(17. 6. 1)

— If the initializer expression is a prvalue and the cv-unqualified version of the source type is the same class as the class of the destination, the initializer expression is used to initialize the destination obj ect. [ Example: T x = T(T(T() ) ) ; calls the T default constructor to initialize x. — end example ]

(17. 6. 2)

— Otherwise, if the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified version of the source type is the same class as, or a derived class of, the class of the destination, constructors are considered. The applicable constructors are enumerated (16.3.1.3) , and the best one is chosen through overload resolution (16.3) . The constructor so selected is called to initialize the obj ect, with the initializer expression or expression-list as its argument(s) . If no constructor applies, or the overload resolution is ambiguous, the initialization is ill-formed.

(17. 6. 3)

— Otherwise (i.e., for the remaining copy-initialization cases) , user-defined conversion sequences that can convert from the source type to the destination type or (when a conversion function is used) to a derived class thereof are enumerated as described in 16.3.1.4, and the best one is chosen through overload resolution (16.3) . If the conversion cannot be done or is ambiguous, the initialization is ill-formed. The function selected is called with the initializer expression as its argument; if the function is a constructor, the call is a prvalue of the cv-unqualified version of the destination type whose result object is initialized by the constructor. The call is used to direct-initialize, according § 11.6

© ISO/IEC 2017 – All rights reserved

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to the rules above, the obj ect that is the destination of the copy-initialization. (17. 7)

— Otherwise, if the source type is a (possibly cv-qualified) class type, conversion functions are considered. The applicable conversion functions are enumerated (16.3.1.5) , and the best one is chosen through overload resolution (16.3) . The user-defined conversion so selected is called to convert the initializer expression into the obj ect being initialized. If the conversion cannot be done or is ambiguous, the initialization is ill-formed.

(17. 8)

— Otherwise, the initial value of the obj ect being initialized is the (possibly converted) value of the initializer expression. Standard conversions (Clause 7) will be used, if necessary, to convert the initializer expression to the cv-unqualified version of the destination type; no user-defined conversions are considered. If the conversion cannot be done, the initialization is ill-formed. When initializing a bitfield with a value that it cannot represent, the resulting value of the bit-field is implementation-defined. [ Note: An expression of type “ cv1 T” can initialize an obj ect of type “ cv2 T ” independently of the cv-qualifiers cv1 and cv2. int a; const int b = a; int c = b;

18 19

— end note ] An initializer-clause followed by an ellipsis is a pack expansion (17.5.3) . If the initializer is a parenthesized expression-list, the expressions are evaluated in the order specified for function calls (8.2.2) .

20

An obj ect whose initialization has completed is deemed to be constructed, even if no constructor of the obj ect’s class is invoked for the initialization. [ Note: Such an obj ect might have been value-initialized or initialized by aggregate initialization (11.6.1) or by an inherited constructor (15.6.3) . — end note ]

21

A declaration that specifies the initialization of a variable, whether from an explicit initializer or by defaultinitialization, is called the initializing declaration of that variable. [ Note: In most cases this is the defining declaration (6.1) of the variable, but the initializing declaration of a non-inline static data member (12.2.3.2) might be the declaration within the class definition and not the definition at namespace scope. — end note ] 1 1 . 6. 1

1

An

Aggregates

[dcl. init. aggr]

aggregate is an array or a class (Clause 12) with

(1. 1 )

— no user-provided, explicit , or inherited constructors (15.1) ,

(1. 2)

— no private or protected non-static data members (Clause 14) ,

(1. 3)

— no virtual functions (13.3) , and

(1. 4)

— no virtual, private, or protected base classes (13.1) . [ Note: Aggregate initialization does not allow accessing protected and private base class’ members or constructors. — end note ]

2

The

elements of an aggregate are:

(2. 1 )

— for an array, the array elements in increasing subscript order, or

(2. 2)

— for a class, the direct base classes in declaration order, followed by the direct non-static data members (12.2) that are not members of an anonymous union, in declaration order.

3

When an aggregate is initialized by an initializer list as specified in 11.6.4, the elements of the initializer list are taken as initializers for the elements of the aggregate, in order. Each element is copy-initialized from the corresponding initializer-clause . If the initializer-clause is an expression and a narrowing conversion (11.6.4) is required to convert the expression, the program is ill-formed. [ Note: If an initializer-clause is itself an initializer list, the element is list-initialized, which will result in a recursive application of the rules in this § 11.6.1

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section if the element is an aggregate.

— end note ] [ Example:

struct A { int x; struct B { int i; int j ; } b; } a = { 1, { 2, 3 } };

initializes a. x with 1, a. b. i with 2, a. b. j with 3. struct base1 { int b1, b2 = 42; }; struct base2 { base2() { b3 = 42; } int b3; }; struct derived : base1, base2 { int d; }; derived d1{{1, 2}, {}, 4}; derived d2{{}, {}, 4};

initializes d1. b1 with 1, d1. b2 with 2, d1. b3 with 42, d1. d with 4, and d2. b1 with 0, d2. b2 with 42, d2. b3 with 42, d2. d with 4. — end example ] 4

An aggregate that is a class can also be initialized with a single expression not enclosed in braces, as described in 11.6.

5

An array of unknown bound initialized with a brace-enclosed initializer-list containing n where n shall be greater than zero, is defined as having n elements (11.3.4) . [ Example:

initializer-clauses,

int x[] = { 1, 3, 5 };

declares and initializes x as a one-dimensional array that has three elements since no size was specified and there are three initializers. — end example ] An empty initializer list {} shall not be used as the initializer-clause for an array of unknown bound. 1 05 [ Note: A default member initializer does not determine the bound for a member array of unknown bound. Since the default member initializer is ignored if a suitable mem-initializer is present (15.6.2) , the default member initializer is not considered to initialize the array of unknown bound. [ Example: struct S { int y[] = { 0 }; };

6

// error: non-static data member of incomplete type

— end example ] — end note ] [ Note: Static data members and unnamed bit-fields are not considered elements of the aggregate. [ Example: struct A { int i; static int s; int j ; int : 17; int k; } a = { 1, 2, 3 }; 105)

The synt ax provides for empty

§ 11.6.1

initializer-list s,

but nonet heless C

++

do es not have zero lengt h arrays.

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Here, the second initializer 2 initializes a. j and not the static data member A: : s , and the third initializer 3 initializes a. k and not the unnamed bit-field before it. — end example ] — end note ] 7

An initializer-list is ill-formed if the number of initializer-clause s exceeds the number of elements to initialize. [ Example: char cv[4] = { ’ a’ , ’ s’ , ’ d’ , ’ f’ , 0 };

// error

— end example ] If there are fewer initializer-clause s in the list than there are elements in a non-union aggregate, then each is ill-formed.

8

element not explicitly initialized is initialized as follows: (8. 1 )

— If the element has a default member initializer (12.2) , the element is initialized from that initializer.

(8. 2)

— Otherwise, if the element is not a reference, the element is copy-initialized from an empty initializer list (11.6.4) .

(8. 3)

— Otherwise, the program is ill-formed. If the aggregate is a union and the initializer list is empty, then

(8. 4)

— if any variant member has a default member initializer, that member is initialized from its default member initializer;

(8. 5)

— otherwise, the first member of the union (if any) is copy-initialized from an empty initializer list. [ Example: struct S { int a; const char* b; int c; int d = b[a] ; }; S ss = { 1, " asdf" };

initializes ss. a with 1, ss. b with " asdf" , ss. c with the value of an expression of the form int{} (that is, 0 ) , and ss. d with the value of ss. b[ss. a] (that is, ’ s’ ) , and in struct X { int i, j , k = 42; }; X a[] = { 1, 2, 3, 4, 5, 6 }; X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };

a and b have the same value — end example ] 9

If a reference member is initialized from its default member initializer and a potentially-evaluated subexpression thereof is an aggregate initialization that would use that default member initializer, the program is ill-formed. [ Example: struct A; extern A a; struct A { const A& a1 { A{a, a} }; const A& a2 { A{} }; }; A a{a, a};

// OK // error // OK

— end example ] 10

If an aggregate class C contains a subaggregate element e with no elements, the initializer-clause for e shall not be omitted from an initializer-list for an obj ect of type C unless the initializer-clause s for all elements of C following e are also omitted. [ Example: struct S { } s; struct A { S s1; int i1; S s2; int i2;

§ 11.6.1

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S s3; int i3; } a = { { }, 0, s, 0 };

// Required initialization // Required initialization // Initialization not required for A: : s3 because A: : i3 is also not initialized

— end example ] 11

When initializing a multi-dimensional array, the initializer-clauses initialize the elements with the last (rightmost) index of the array varying the fastest (11.3.4) . [ Example: int x[2] [2] = { 3, 1, 4, 2 };

initializes x[0] [0] to 3 , x[0] [1] to 1 , x[1] [0] to 4, and x[1] [1] to 2 . On the other hand, float y[4] [3] = { { 1 }, { 2 }, { 3 }, { 4 } };

initializes the first column of y (regarded as a two-dimensional array) and leaves the rest zero.

example ] 12

— end

Braces can be elided in an initializer-list as follows. If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializer-clause s initializes the elements of a subaggregate; it is erroneous for there to be more initializer-clause s than elements. If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializer-clause s from the list are taken to initialize the elements of the subaggregate; any remaining initializer-clause s are left to initialize the next element of the aggregate of which the current subaggregate is an element. [ Example: float y[4] [3] = { { 1, 3, 5 }, { 2, 4, 6 }, { 3, 5, 7 }, };

is a completely-braced initialization: 1, 3, and 5 initialize the first row of the array y[0] , namely y[0] [0] , y[0] [1] , and y[0] [2] . Likewise the next two lines initialize y[1] and y[2] . The initializer ends early and therefore y[3] s elements are initialized as if explicitly initialized with an expression of the form float() , that is, are initialized with 0. 0 . In the following example, braces in the initializer-list are elided; however the initializer-list has the same effect as the completely-braced initializer-list of the above example, float y[4] [3] = { 1, 3, 5, 2, 4, 6, 3, 5, 7 };

The initializer for y begins with a left brace, but the one for y[0] does not, therefore three elements from the list are used. Likewise the next three are taken successively for y[1] and y[2] . — end example ] 13

All implicit type conversions (Clause 7) are considered when initializing the element with an assignmentexpression. If the assignment-expression can initialize an element, the element is initialized. Otherwise, if the element is itself a subaggregate, brace elision is assumed and the assignment-expression is considered for the initialization of the first element of the subaggregate. [ Note: As specified above, brace elision cannot apply to subaggregates with no elements; an initializer-clause for the entire subobj ect is required. — end note ] [ Example: struct A { int i;

§ 11.6.1

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operator int() ; }; struct B { A a1, a2; int z; }; A a; B b = { 4, a, a };

Braces are elided around the initializer-clause for b. a1. i . b. a1. i is initialized with 4, b. a2 is initialized with a, b. z is initialized with whatever a. operator int() returns. — end example ] 14

[ Note: An aggregate array or an aggregate class may contain elements of a class type with a user-provided constructor (15.1) . Initialization of these aggregate obj ects is described in 15.6.1. — end note ]

15

[ Note: Whether the initialization of aggregates with static storage duration is static or dynamic is specified in 6.6.2, 6.6.3, and 9.7. — end note ]

16

When a union is initialized with a brace-enclosed initializer, the braces shall only contain an for the first non-static data member of the union. [ Example: union u a = u b = u c = u d = u e =

17

u { int a; const char* b; }; { 1 }; a; 1; // error { 0, " asdf" }; // error { " asdf" }; // error

— end example ] [ Note: As described above, the braces around the initializer-clause for a union member can be omitted if the union is a member of another aggregate. — end note ] 1 1 . 6. 2

1

initializer-clause

Character arrays

[dcl. init. string]

An array of narrow character type (6.9.1) , char16_t array, char32_t array, or wchar_t array can be initialized by a narrow string literal, char16_t string literal, char32_t string literal, or wide string literal, respectively, or by an appropriately-typed string literal enclosed in braces (5.13.5) . Successive characters of the value of the string literal initialize the elements of the array. [ Example: char msg[] = " Syntax error on line %s\n" ;

shows a character array whose members are initialized with a string-literal . Note that because ’ \n’ is a single character and because a trailing ’ \0’ is appended, sizeof(msg) is 25 . — end example ] 2

There shall not be more initializers than there are array elements. [ Example: char cv[4] = " asdf" ;

// error

is ill-formed since there is no space for the implied trailing ’ \0’ . 3

If there are fewer initializers than there are array elements, each element not explicitly initialized shall be zero-initialized (11.6) . 1 1 . 6. 3

1

— end example ]

References

[dcl. init. ref]

A variable whose declared type is “reference to type T ” (11.3.2) shall be initialized. [ Example: int g(int) noexcept; void f() { int i; int& r = i; r = 1;

§ 11.6.3

// r refers to i // the value of i becomes 1

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int* p = &r; int& rr = r; int (&rg) (int) = g; rg(i) ; int a[3] ; int (&ra) [3] = a; ra[1] = i; }

// p points to i // rr refers to what r refers to, that is, to i // rg refers to the function g // calls function g // ra refers to the array a // modifies a[1]

— end example ] 2

A reference cannot be changed to refer to another object after initialization. [ Note: Assignment to a reference assigns to the object referred to by the reference (8.18) . — end note ] Argument passing (8.2.2) and function value return (9.6.3) are initializations.

3

The initializer can be omitted for a reference only in a parameter declaration (11.3.5) , in the declaration of a function return type, in the declaration of a class member within its class definition (12.2) , and where the extern specifier is explicitly used. [ Example: int& r1; extern int& r2;

4

// error: initializer missing // OK

— end example ] Given types “ cv1

T1 ” and “ cv2 T2 ”, “ cv1 T1 ” is reference-related to “ cv2 T2 ” if T1 is the same type as T2 , or T1 is a base class of T2 . “ cv1 T1 ” is reference-compatible with “ cv2 T2 ” if

(4. 1 )

— T1 is reference-related to T2 , or

(4. 2)

— T2 is “ noexcept function” and T1 is “function”, where the function types are otherwise the same, and cv1 is the same cv-qualification as, or greater cv-qualification than, cv2. In all cases where the referencerelated or reference-compatible relationship of two types is used to establish the validity of a reference binding, and T1 is a base class of T2 , a program that necessitates such a binding is ill-formed if T1 is an inaccessible (Clause 14) or ambiguous (13.2) base class of T2 .

5 (5. 1 )

A reference to type “ cv1 T1 ” is initialized by an expression of type “ cv2 T2 ” as follows: — If the reference is an lvalue reference and the initializer expression

(5. 1. 1 )

— is an lvalue (but is not a bit-field) , and “ cv1 T1 ” is reference-compatible with “ cv2 T2 ”, or

(5. 1. 2)

— has a class type (i.e., T2 is a class type) , where T1 is not reference-related to T2 , and can be converted to an lvalue of type “ cv3 T3 ”, where “ cv1 T1 ” is reference-compatible with “ cv3 T3 ” 1 06 (this conversion is selected by enumerating the applicable conversion functions (16.3.1.6) and choosing the best one through overload resolution (16.3) ) , then the reference is bound to the initializer expression lvalue in the first case and to the lvalue result of the conversion in the second case (or, in either case, to the appropriate base class subobj ect of the obj ect) . [ Note: The usual lvalue-to-rvalue (7.1) , array-to-pointer (7.2) , and function-to-pointer (7.3) standard conversions are not needed, and therefore are suppressed, when such direct bindings to lvalues are done. — end note ] [ Example: double d = 2. 0; double& rd = d; const double& rcd = d;

// rd refers to d // rcd refers to d

struct A { }; struct B : A { operator int&() ; } b; 106)

This requires a conversion funct ion ( 1 5. 3. 2 ) ret urning a reference typ e.

§ 11.6.3

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// ra refers to A subobject in b // rca refers to A subobject in b // ir refers to the result of B: : operator int&

A& ra = b; const A& rca = b; int& ir = B() ;

— end example ] (5. 2)

— Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., const ) , or the reference shall be an rvalue reference. [ Example:

cv1 shall be

// error: not an lvalue and reference not const

double& rd2 = 2. 0; int i = 2; double& rd3 = i;

// error: type mismatch and reference not const

— end example ] (5. 2. 1 )

— If the initializer expression

(5. 2. 1. 1)

— is an rvalue (but not a bit-field) or function lvalue and “ cv1 T1 ” is reference-compatible with “ cv2 T2 ”, or

(5. 2. 1. 2)

— has a class type (i.e., T2 is a class type) , where T1 is not reference-related to T2 , and can be converted to an rvalue or function lvalue of type “ cv3 T3 ”, where “ cv1 T1 ” is referencecompatible with “ cv3 T3 ” (see 16.3.1.6) , then the value of the initializer expression in the first case and the result of the conversion in the second case is called the converted initializer. If the converted initializer is a prvalue, its type T4 is adj usted to type “ cv1 T4 ” (7.5) and the temporary materialization conversion (7.4) is applied. In any case, the reference is bound to the resulting glvalue (or to an appropriate base class subobj ect) . [ Example: struct A { }; struct B : A { } b; extern B f() ; const A& rca2 = f() ; A&& rra = f() ; struct X { operator B() ; operator int&() ; } x; const A& r = x; int i2 = 42; int&& rri = static_cast(i2) ; B&& rrb = x;

// bound to the A subobject of the B rvalue. // same as above

// bound to the A subobject of the result of the conversion // bound directly to i2 // bound directly to the result of operator B

— end example ] (5. 2. 2)

— Otherwise:

(5. 2. 2. 1)

— If T1 or T2 is a class type and T1 is not reference-related to T2 , user-defined conversions are considered using the rules for copy-initialization of an obj ect of type “ cv1 T1 ” by userdefined conversion (11.6, 16.3.1.4, 16.3.1.5) ; the program is ill-formed if the corresponding non-reference copy-initialization would be ill-formed. The result of the call to the conversion function, as described for the non-reference copy-initialization, is then used to direct-initialize the reference. For this direct-initialization, user-defined conversions are not considered.

(5. 2. 2. 2)

— Otherwise, the initializer expression is implicitly converted to a prvalue of type “cv1 T1 ”. The temporary materialization conversion is applied and the reference is bound to the result. If T1 is reference-related to T2 :

§ 11.6.3

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cv1 shall be the same cv-qualification as, or greater cv-qualification than, cv2 ; and

(5. 2. 2. 3)

—

(5. 2. 2. 4)

— if the reference is an rvalue reference, the initializer expression shall not be an lvalue. [ Example: struct Banana { }; struct Enigma { operator const Banana() ; }; struct Alaska { operator Banana&() ; }; void enigmatic() { typedef const Banana ConstBanana; Banana &&banana1 = ConstBanana() ; // ill-formed Banana &&banana2 = Enigma() ; // ill-formed Banana &&banana3 = Alaska() ; // ill-formed } const double& rcd2 = 2; // rcd2 refers to temporary with value 2. 0 double&& rrd = 2; // rrd refers to temporary with value 2. 0 const volatile int cvi = 1; const int& r2 = cvi; // error: cv-qualifier dropped struct A { operator volatile int&() ; } a; const int& r3 = a; // error: cv-qualifier dropped // from result of conversion function double d2 = 1. 0; double&& rrd2 = d2; // error: initializer is lvalue of related type struct X { operator int&() ; }; int&& rri2 = X() ; // error: result of conversion function is lvalue of related type int i3 = 2; double&& rrd3 = i3; // rrd3 refers to temporary with value 2. 0

— end example ] In all cases except the last (i.e., implicitly converting the initializer expression to the underlying type of the reference) , the reference is said to bind directly to the initializer expression. 6

[ Note: 15.2 describes the lifetime of temporaries bound to references. 1 1 . 6. 4

1

— end note ]

List-initialization

[dcl. init. list]

List-initialization is initialization of an obj ect or reference from a braced-init-list. Such an initializer is called an initializer list, and the comma-separated initializer-clause s of the list are called the elements of

the initializer list. An initializer list may be empty. List-initialization can occur in direct-initialization or copy-initialization contexts; list-initialization in a direct-initialization context is called direct-list-initialization and list-initialization in a copy-initialization context is called copy-list-initialization . [ Note: List-initialization can be used (1. 1 )

— as the initializer in a variable definition (11.6)

(1. 2)

— as the initializer in a

(1. 3)

— in a return statement (9.6.3)

(1. 4)

— as a

(1. 5)

— as a function argument (8.2.2)

(1. 6)

— as a subscript (8.2.1)

(1. 7)

— as an argument to a constructor invocation (11.6, 8.2.3)

(1. 8)

— as an initializer for a non-static data member (12.2)

(1. 9)

— in a § 11.6.4

new-expression (8.3.4)

for-range-initializer (9.5)

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(1. 10)

— on the right-hand side of an assignment (8.18) [ Example: int a = {1}; std: : complex z{1, 2}; new std: : vector{" once" , " upon" , " a" , " time" }; // 4 string elements f( {" Nicholas" , " Annemarie" } ) ; // pass list of two elements return { " Norah" }; // return list of one element int* e {}; // initialization to zero / null pointer x = double{1}; // explicitly construct a double std: : map anim = { {" bear" , 4}, {" cassowary" , 2}, {" tiger" , 7} };

2

— end example ] — end note ] A constructor is an initializer-list constructor if its first parameter is of type std: : initializer_list or

reference to possibly cv-qualified std: : initializer_list for some type E, and either there are no other parameters or else all other parameters have default arguments (11.3.6) . [ Note: Initializer-list constructors are favored over other constructors in list-initialization (16.3.1.7) . Passing an initializer list as the argument to the constructor template template C(T) of a class C does not create an initializer-list constructor, because an initializer list argument causes the corresponding parameter to be a non-deduced context (17.8.2.1) . — end note ] The template std: : initializer_list is not predefined; if the header is not included prior to a use of std: : initializer_list — even an implicit use in which the type is not named (10.1.7.4) — the program is ill-formed. 3

List-initialization of an obj ect or reference of type T is defined as follows:

(3. 1 )

— If T is an aggregate class and the initializer list has a single element of type cv U, where U is T or a class derived from T, the obj ect is initialized from that element (by copy-initialization for copy-listinitialization, or by direct-initialization for direct-list-initialization) .

(3. 2)

— Otherwise, if T is a character array and the initializer list has a single element that is an appropriatelytyped string literal (11.6.2) , initialization is performed as described in that section.

(3. 3)

— Otherwise, if T is an aggregate, aggregate initialization is performed (11.6.1) . [ Example: double ad[] = { 1, 2. 0 }; int ai[] = { 1, 2. 0 };

// OK // error: narrowing

struct S2 { int m1; double m2, m3; }; S2 s21 = { 1, 2, 3. 0 }; S2 s22 { 1. 0, 2, 3 }; S2 s23 { };

// OK // error: narrowing // OK: default to 0,0,0

— end example ] (3. 4)

— Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the obj ect is value-initialized.

(3. 5)

— Otherwise, if T is a specialization of std: : initializer_list , the object is constructed as described below.

(3. 6)

— Otherwise, if T is a class type, constructors are considered. The applicable constructors are enumerated and the best one is chosen through overload resolution (16.3, 16.3.1.7) . If a narrowing conversion (see below) is required to convert any of the arguments, the program is ill-formed. [ Example: § 11.6.4

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struct S { S(std: : initializer_list) ; S(std: : initializer_list) ; S() ; // ... }; S s1 = { 1. 0, 2. 0, 3. 0 }; S s2 = { 1, 2, 3 }; S s3 = { };

// #1 // #2 // #3

// invoke #1 // invoke #2 // invoke #3

— end example ] [ Example: struct Map { Map(std: : initializer_list) ; }; Map ship = {{" Sophie" , 14}, {" Surprise" , 28}};

— end example ] [ Example: struct S { // no initializer-list constructors S(int, double, double) ; S() ; // ... }; S s1 = { 1, 2, 3. 0 }; S s2 { 1. 0, 2, 3 }; S s3 { };

// #1 // #2

// OK: invoke #1 // error: narrowing // OK: invoke #2

— end example ] (3. 7)

— Otherwise, if T is an enumeration with a fixed underlying type (10.2) , the initializer-list has a single element v, and the initialization is direct-list-initialization, the obj ect is initialized with the value T(v) (8.2.3) ; if a narrowing conversion is required to convert v to the underlying type of T, the program is ill-formed. [ Example: enum byte byte byte byte

byte : unsigned char { }; b { 42 }; c = { 42 }; d = byte{ 42 }; e { -1 };

// OK // error // OK; same value as b // error

struct A { byte b; }; A a1 = { { 42 } }; A a2 = { byte{ 42 } };

// error // OK

void f(byte) ; f({ 42 }) ;

// error

enum class Handle : uint32_t { Invalid = 0 }; Handle h { 42 }; // OK

— end example ] (3. 8)

— Otherwise, if the initializer list has a single element of type E and either T is not a reference type or its referenced type is reference-related to E, the obj ect or reference is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization) ; if a § 11.6.4

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narrowing conversion (see below) is required to convert the element to T , the program is ill-formed. [ Example: // OK // error: narrowing

int x1 {2}; int x2 {2. 0};

— end example ] (3. 9)

— Otherwise, if T is a reference type, a prvalue of the type referenced by T is generated. The prvalue initializes its result obj ect by copy-list-initialization or direct-list-initialization, depending on the kind of initialization for the reference. The prvalue is then used to direct-initialize the reference. [ Note: As usual, the binding will fail and the program is ill-formed if the reference type is an lvalue reference to a non-const type. — end note ] [ Example: struct S { S(std: : initializer_list) ; S(const std: : string&) ; // ... }; const S& r1 = { 1, 2, 3. 0 }; const S& r2 { " Spinach" }; S& r3 = { 1, 2, 3 }; const int& i1 = { 1 }; const int& i2 = { 1. 1 }; const int (&iar) [2] = { 1, 2 };

// #1 // #2

// OK: invoke #1 // OK: invoke #2 // error: initializer is not an lvalue // OK // error: narrowing // OK: iar is bound to temporary array

— end example ] (3. 10)

— Otherwise, if the initializer list has no elements, the obj ect is value-initialized. [ Example: // initialized to null pointer

int** pp {};

— end example ] (3. 11 )

— Otherwise, the program is ill-formed. [ Example:

4

struct A { int i; int j ; }; A a1 { 1, 2 }; A a2 { 1. 2 }; struct B { B(std: : initializer_list) ; }; B b1 { 1, 2 }; B b2 { 1, 2. 0 }; struct C { C(int i, double j ) ; }; C c1 = { 1, 2. 2 }; C c2 = { 1. 1, 2 };

// calls constructor with arguments (1, 2.2) // error: narrowing

int j { 1 }; int k { };

// initialize to 1 // initialize to 0

// aggregate initialization // error: narrowing

// creates initializer_list and calls constructor // error: narrowing

— end example ] initializer-list of a braced-init-list, the initializer-clauses, including any that result from pack

Within the § 11.6.4

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expansions (17.5.3) , are evaluated in the order in which they appear. That is, every value computation and side effect associated with a given initializer-clause is sequenced before every value computation and side effect associated with any initializer-clause that follows it in the comma-separated list of the initializer-list . [ Note: This evaluation ordering holds regardless of the semantics of the initialization; for example, it applies when the elements of the initializer-list are interpreted as arguments of a constructor call, even though ordinarily there are no sequencing constraints on the arguments of a call. — end note ] 5

An object of type std: : initializer_list is constructed from an initializer list as if the implementation generated and materialized (7.4) a prvalue of type “array of const E”, where is the number of elements in the initializer list. Each element of that array is copy-initialized with the corresponding element of the initializer list, and the std: : initializer_list obj ect is constructed to refer to that array. [ Note: A constructor or conversion function selected for the copy shall be accessible (Clause 14) in the context of the initializer list. — end note ] If a narrowing conversion is required to initialize any of the elements, the program is ill-formed. [ Example:

N

N

struct X { X(std: : initializer_list v) ; }; X x{ 1, 2, 3 };

The initialization will be implemented in a way roughly equivalent to this: const double __a[3] = {double{1}, double{2}, double{3}}; X x(std: : initializer_list(__a, __a+3) ) ;

assuming that the implementation can construct an initializer_list obj ect with a pair of pointers.

example ] 6

— end

The array has the same lifetime as any other temporary object (15.2) , except that initializing an initializer_list obj ect from the array extends the lifetime of the array exactly like binding a reference to a temporary. [ Example: typedef std: : complex cmplx; std: : vector v1 = { 1, 2, 3 }; void f() { std: : vector v2{ 1, 2, 3 }; std: : initializer_list i3 = { 1, 2, 3 }; } struct A { std: : initializer_list i4; A() : i4{ 1, 2, 3 } {} // ill-formed, would create a dangling reference };

For v1 and v2 , the initializer_list obj ect is a parameter in a function call, so the array created for { 1, 2, 3 } has full-expression lifetime. For i3 , the initializer_list object is a variable, so the array persists for the lifetime of the variable. For i4 , the initializer_list obj ect is initialized in the constructor’s ctor-initializer as if by binding a temporary array to a reference member, so the program is ill-formed (15.6.2) . — end example ] [ Note: The implementation is free to allocate the array in read-only memory if an explicit array with the same initializer could be so allocated. — end note ] 7

A

narrowing conversion is an implicit conversion

(7. 1 )

— from a floating-point type to an integer type, or

(7. 2)

— from long double to double or float , or from double to float , except where the source is a constant expression and the actual value after conversion is within the range of values that can be represented (even if it cannot be represented exactly) , or § 11.6.4

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(7. 3)

— from an integer type or unscoped enumeration type to a floating-point type, except where the source is a constant expression and the actual value after conversion will fit into the target type and will produce the original value when converted back to the original type, or

(7. 4)

— from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where the source is a constant expression whose value after integral promotions will fit into the target type. [ Note: As indicated above, such conversions are not allowed at the top level in list-initializations. — end note ] [ Example: int x = 999; const int y = 999; const int z = 99; char c1 = x; char c2{x}; char c3{y}; char c4{z}; unsigned char uc1 = {5}; unsigned char uc2 = {-1}; unsigned int ui1 = {-1}; signed int si1 = { (unsigned int) -1 }; int ii = {2. 0}; float f1 { x }; float f2 { 7 }; int f(int) ; int a[] = { 2, f(2) , f(2. 0) };

// x is not a constant expression

// OK, though it might narrow (in this case, it does narrow) // error: might narrow // error: narrows (assuming char is 8 bits) // OK: no narrowing needed // OK: no narrowing needed // error: narrows // error: narrows // error: narrows // error: narrows // error: might narrow // OK: 7 can be exactly represented as a float

// OK: the double -to-int conversion is not at the top level

— end example ]

§ 11.6.4

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12 Classes 1

A clas s is a typ e.

[class]

It s name b ecomes a

class-name

(1 2. 1 )

wit hin it s s cop e.

class-name : identifier simple-template-id

Class-specifier

s and

elaborated-type-specifier

consist s of a ( p ossibly empty)

s ( 1 0 . 1 . 7. 3 )

are us ed t o make

class-name

s.

An ob j ect

of a class

sequence of memb ers and bas e class ob j ect s.

class-specifier: class-head { member-specificationopt } class-head : class-key attribute-specifier-seqopt class-head-name class-virt-specifieropt base-clauseopt class-key attribute-specifier-seqopt base-clauseopt class-head-name : nested-name-specifieropt class-name class-virt-specifier: final class-key : class struct union A

class-specifier

class t hus can’ t

2

A

whose be

class-name class-name

is

The

class-head

omit s t he

final — end note .

insert ed

int o

t he

class-head-name

defines an unnamed class.

scop e

in

which

it

is

declared

is commonly referred t o as a class definit ion.

brace of it s

app ert ains t o t he class;

t hereaft er considered at t ribut es of t he class whenever it

If a class is marked wit h t he

final

,

Note:

An unnamed

class-name injected-class-name

t he

is

and a colon or left

struct A; struct A final {};

brace,

.

A

A class is considered defined aft er t he clos ing

t he at t ribut es in t he

yet

defined.

attribute-specifier-seq

are

is named.

class-virt-specifier final class-or-decltype class-key class-head-name final class-virt-specifier Example: and it

t he program is ill- formed.

seen.

were a public memb er name.

has b een seen even t hough it s memb er funct ions are in general not

The opt ional

( C lause 1 3 ) ,

aft er

t his is known as t he

t he inj ect ed- class- name is t reat ed as if it

class-specifier class-specifier attribute-specifier-seq 3

immediat ely

is also ins ert ed int o t he scop e of t he class it self;

For purp oses of access checking,

[

]

Whenever a

app ears as a

in a

is followed by a

is int erpret ed as a

.

,

base-clause identifier

t he

[

// OK: definition of struct A , // not value-initialization of variable final

struct X { struct C { constexpr operator int() { return 5; } }; struct B final : C{}; // OK: definition of nested class B , // not declaration of a bit-field member final };

— end example

C lasses

]

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4

Complete obj ects and member subobj ects of class type shall have nonzero size. 107 [ Note: Class obj ects can be assigned, passed as arguments to functions, and returned by functions (except objects of classes for which copying or moving has been restricted; see 15.8) . Other plausible operators, such as equality comparison, can be defined by the user; see 16.5. — end note ]

5

A union is a class defined with the class-key union; it holds at most one data member at a time (12.3) . [ Note: Aggregates of class type are described in 11.6.1. — end note ]

6

A

trivially copyable class is a class:

(6. 1 )

— where each copy constructor, move constructor, copy assignment operator, and move assignment operator (15.8, 16.5.3) is either deleted or trivial,

(6. 2)

— that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator, and

(6. 3)

— that has a trivial, non-deleted destructor (15.4) . A trivial class is a class that is trivially copyable and has one or more default constructors (15.1) , all of which are either trivial or deleted and at least one of which is not deleted. [ Note: In particular, a trivially copyable or trivial class does not have virtual functions or virtual base classes. — end note ]

7

A class S is a

standard-layout class if it:

(7. 1 )

— has no non-static data members of type non-standard-layout class (or array of such types) or reference,

(7. 2)

— has no virtual functions (13.3) and no virtual base classes (13.1) ,

(7. 3)

— has the same access control (Clause 14) for all non-static data members,

(7. 4)

— has no non-standard-layout base classes,

(7. 5)

— has at most one base class subobj ect of any given type,

(7. 6)

— has all non-static data members and bit-fields in the class and its base classes first declared in the same class, and

(7. 7)

— has no element of the set M

M

( S ) of types (defined below) as a base class. 1 08

( X) is defined as follows:

(7. 8)

— If X is a non-union class type with no (possibly inherited (Clause 13) ) non-static data members, the set M ( X ) is empty.

(7. 9)

— If X is a non-union class type whose first non-static data member has type X 0 (where said member may be an anonymous union) , the set M ( X) consists of X 0 and the elements of M ( X 0 ) .

(7. 10)

— If X is a union type, the set M ( X) is the union of all is the type of the i th non-static data member of X.

(7. 11 )

— If X is an array type with element type Xe , the set

(7. 12)

— If X is a non-class, non-array type, the set

M

M

M

( Ui ) and the set containing all Ui , where each Ui

( X ) consists of X e and the elements of M ( X e ) .

( X) is empty.

[ Note: M ( X ) is the set of the types of all non-base-class subobjects that are guaranteed in a standard-layout class to be at a zero offset in X . — end note ] [ Example: struct struct struct struct 107) 108)

B C D E

{ : : :

int B { C { D {

i; }; }; }; char : 4; };

// standard-layout class // standard-layout class // standard-layout class // not a standard-layout class

B ase class sub ob j ect s are not so const rained. This ensures t hat two sub ob j ect s t hat have t he same class typ e and t hat b elong t o t he same most derived ob j ect are not

allo cat ed at t he same address ( 8. 1 0) .

Classes

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struct struct struct struct

Q S T U

{}; : Q { }; : Q { }; : S, T { };

— end example standard-layout struct standard-layout union Note:

// not a standard-layout class

]

8

9

A

is a st andard- layout

A

is a st andard- layout

[

S t andard- layout

languages.

10

A

Their layout

POD struct109

is

classes

are

useful

is sp ecified in 1 2 . 2 .

a non- union clas s

class defined wit h t he class defined wit h t he

for

communicat ing

— end note

t hat

is

union

is a union t hat

of typ e non- P O D POD

[

s t ruct

or a P O D

non- P O D

wit h

co de

writ t en

in

class-key class

ot her

and

a st andard- layout

non- P O D union ( or array of such typ es) .

is b ot h a t rivial class and a st andard- layout

st ruct ,

or t he

.

.

programming

]

b ot h a t rivial class

non- st at ic dat a memb ers of typ e non- P O D st ruct ,

class-key struct class-key union

class,

union ( or array of such typ es) .

A

class,

and

S imilarly,

has a

no

POD

and has no non- st at ic dat a memb ers

POD class

is a clas s t hat

is eit her a

union.

Example: struct N { int i; int j ; virtual ~ N() ; };

// neither trivial nor standard-layout

struct T { int i; private: int j ; };

// trivial but not standard-layout

struct SL { int i; int j ; ~ SL() ; };

// standard-layout but not trivial

struct POD { int i; int j ; };

// both trivial and standard-layout

— end example class-head-name ]

11

If a

p reviously declared element

s uch

cases,

1 2. 1

),

t he

decltype-specifier 1

a

nested-name-specifier

of t he inline names pace set

using-declaration In

cont ains

direct ly in t he

class

(1 0 . 3 . 1 )

class-specifier nested-name-specifier and t he

of t hat

,

t he to

class-specifier nested-name-specifier shall

refer

to

which t he

names pace ( i. e. ,

not

a

class

t hat

refers,

or

was

in an

merely inherit ed or int ro duced by a

shall app ear in a namespace enclosing t he previous declarat ion. of t he

class-head-name

of t he

definit ion

shall

not

b egin

wit h

a

.

Class names

[class. name]

A class definit ion int ro duces a new typ e.

109)

or namespace

[

Example:

The acronym P O D st ands for “ plain old dat a”.

§ 1 2. 1

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struct X { int a; }; struct Y { int a; }; X a1; Y a2; int a3;

declares three variables of three different types. This implies that // error: Y assigned to X // error: int assigned to X

a1 = a2; a1 = a3;

are type mismatches, and that int f(X) ; int f(Y) ;

declare an overloaded (Clause 16) function f( ) and not simply a single function f( ) twice. For the same reason, struct S { int a; }; struct S { int a; };

// error, double definition

is ill-formed because it defines S twice. 2

— end example ]

A class declaration introduces the class name into the scope where it is declared and hides any class, variable, function, or other declaration of that name in an enclosing scope (6.3) . If a class name is declared in a scope where a variable, function, or enumerator of the same name is also declared, then when both declarations are in scope, the class can be referred to only using an elaborated-type-specifier (6.4.4) . [ Example: struct stat { // ... }; stat gstat;

// use plain stat to define variable

int stat(struct stat*) ;

// redeclare stat as function

void f() { struct stat* ps; stat(ps) ; }

// struct prefix needed to name struct stat // call stat()

— end example ] A declaration consisting solely of class-key identifier ; is either a redeclaration of the name in the current scope or a forward declaration of the identifier as a class name. It introduces the class name into the current scope. [ Example: struct s { int a; }; void g() struct s* p; struct struct }

{ s; s { char* p; }; s;

// hide global struct s with a block-scope declaration // refer to local struct s // define local struct s // redeclaration, has no effect

— end example ] [ Note: Such declarations allow definition of classes that refer to each other. [ Example: class Vector; class Matrix { // ... friend Vector operator*(const Matrix&, const Vector&) ;

§ 12.1

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}; class Vector { // ... friend Vector operator*(const Matrix&, const Vector&) ; };

— end example ] — end note ] [ Note: An elaborated-type-specifier (10.1.7.3) can also be used as a type-specifier as part of a declaration. It

Declaration of friends is described in 14.3, operator functions in 16.5. 3

differs from a class declaration in that if a class of the elaborated name is in scope the elaborated name will refer to it. — end note ] [ Example: struct s { int a; }; void g(int s) { struct s* p = new struct s; p->a = s; } 4

// global s // parameter s

— end example ] [ Note: The declaration of a class name takes effect immediately after the identifier is seen in the class definition or elaborated-type-specifier . For example, class A * A;

first specifies A to be the name of a class and then redefines it as the name of a pointer to an obj ect of that class. This means that the elaborated form class A must be used to refer to the class. Such artistry with names can be confusing and is best avoided. — end note ] 5

A typedef-name (10.1.3) that names a class type, or a cv-qualified version thereof, is also a class-name . If a typedef-name that names a cv-qualified class type is used where a class-name is required, the cv-qualifiers are ignored. A typedef-name shall not be used as the identifier in a class-head. 1 2. 2

Class members

[class. mem]

member-specification : member-declaration member-specificationopt access-specifier : member-specificationopt member-declaration : attribute-specifier-seqopt decl-specifier-seqopt member-declarator-listopt ; function-definition using-declaration static_assert-declaration template-declaration deduction-guide alias-declaration empty-declaration member-declarator-list : member-declarator member-declarator-list , member-declarator member-declarator: declarator virt-specifier-seqopt pure-specifieropt declarator brace-or-equal-initializeropt identifieropt attribute-specifier-seqopt : constant-expression

§ 12.2

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virt-specifier-seq : virt-specifier virt-specifier-seq virt-specifier virt-specifier: override final pure-specifier: = 0 1

The member-specification in a class definition declares the full set of members of the class; no member can be added elsewhere. A direct member of a class X is a member of X that was first declared within the member-specification of X , including anonymous union obj ects (12.3.1) and direct members thereof. Members of a class are data members, member functions (12.2.1) , nested types, enumerators, and member templates (17.5.2) and specializations thereof. [ Note: A specialization of a static data member template is a static data member. A specialization of a member function template is a member function. A specialization of a member class template is a nested class. — end note ]

2

A

(2. 1 ) (2. 2) (2. 3) (2. 4)

member-declaration does not declare new members of the class if it is — a friend declaration (14.3) ,

static_assert-declaration, — a using-declaration (10.3.3) , or — an empty-declaration . For any other member-declaration , each declared entity that is not an unnamed bit-field (12.2.4) is a member of the class, and each such member-declaration shall either declare at least one member name of the class or — a

declare at least one unnamed bit-field. 3

A data member is a non-function member introduced by a member-declarator. A member function is a member that is a function. Nested types are classes (12.1, 12.2.5) and enumerations (10.2) declared in the class and arbitrary types declared as members by use of a typedef declaration (10.1.3) or alias-declaration . The enumerators of an unscoped enumeration (10.2) defined in the class are members of the class.

4

A data member or member function may be declared static in its member-declaration , in which case it is a static member (see 12.2.3) (a static data member (12.2.3.2) or static member function (12.2.3.1) , respectively) of the class. Any other data member or member function is a non-static member (a non-static data member or non-static member function (12.2.2) , respectively) . [ Note: A non-static data member of non-reference type is a member subobj ect of a class obj ect (4.5) . — end note ] A member shall not be declared twice in the member-specification , except that

5 (5. 1 )

— a nested class or member class template can be declared and then later defined, and

(5. 2)

— an enumeration can be introduced with an enum-specifier.

opaque-enum-declaration and later redeclared with an

[ Note: A single name can denote several member functions provided their types are sufficiently different (Clause 16) . — end note ] 6

7

A class is considered a completely-defined obj ect type (6.9) (or complete type) at the closing } of the

class-specifier. Within the class member-specification, the class is regarded as complete within function bodies, default arguments, noexcept-specifiers, and default member initializers (including such things in nested classes) . Otherwise it is regarded as incomplete within its own class member-specification . In a member-declarator , an = immediately following the declarator is interpreted as introducing a pure-specifier if the declarator-id has function type, otherwise it is interpreted as introducing a brace-or-equal-initializer . [ Example: § 12.2

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struct S { using T = void() ; T * p = 0; // OK: brace-or-equal-initializer virtual T f = 0; // OK: pure-specifier };

8

— end example ] A brace-or-equal-initializer shall appear only in the declaration of a data member. (For static data members, see 12.2.3.2; for non-static data members, see 15.6.2 and 11.6.1) . A brace-or-equal-initializer for a non-static data member specifies a default member initializer for the member, and shall not directly or indirectly cause the implicit definition of a defaulted default constructor for the enclosing class or the exception specification of that constructor.

9

A member shall not be declared with the extern storage-class-specifier . Within a class definition, a member shall not be declared with the thread_local storage-class-specifier unless also declared static .

10

The decl-specifier-seq may be omitted in constructor, destructor, and conversion function declarations only; when declaring another kind of member the decl-specifier-seq shall contain a type-specifier that is not a cv-qualifier. The member-declarator-list can be omitted only after a class-specifier or an enum-specifier or in a friend declaration (14.3) . A pure-specifier shall be used only in the declaration of a virtual function (13.3) that is not a friend declaration.

11

The optional attribute-specifier-seq in a member-declaration appertains to each of the entities declared by the member-declarators; it shall not appear if the optional member-declarator-list is omitted. A virt-specifier-seq shall contain at most one of each virt-specifier . A virt-specifier-seq shall appear only in

12

the declaration of a virtual member function (13.3) .

13

Non-static data members shall not have incomplete types. In particular, a class C shall not contain a non-static member of class C , but it can contain a pointer or reference to an obj ect of class C .

14

[ Note: See 8.1 for restrictions on the use of non-static data members and non-static member functions. — end note ] [ Note: The type of a non-static member function is an ordinary function type, and the type of a non-static

15

data member is an ordinary obj ect type. There are no special member function types or data member types.

16

— end note ] [ Example: A simple example of a class definition is struct tnode { char tword[20] ; int count; tnode* left; tnode* right; };

which contains an array of twenty characters, an integer, and two pointers to objects of the same type. Once this definition has been given, the declaration tnode s, *sp;

declares s to be a tnode and sp to be a pointer to a tnode . With these declarations, sp->count refers to the count member of the obj ect to which sp points; s. left refers to the left subtree pointer of the obj ect s ; and s. right->tword[0] refers to the initial character of the tword member of the right subtree of s . — end example ] 17

Non-static data members of a (non-union) class with the same access control (Clause 14) are allocated so that later members have higher addresses within a class obj ect. The order of allocation of non-static data members with different access control is unspecified (Clause 14) . Implementation alignment requirements § 12.2

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might cause two adj acent members not to be allocated immediately after each other; so might requirements for space for managing virtual functions (13.3) and virtual base classes (13.1) . 18

If T is the name of a class, then each of the following shall have a name different from T :

(18. 1)

— every static data member of class T;

(18. 2)

— every member function of class T [ Note: This restriction does not apply to constructors, which do not have names (15.1) — end note ] ;

(18. 3)

— every member of class T that is itself a type;

(18. 4)

— every member template of class T;

(18. 5)

— every enumerator of every member of class T that is an unscoped enumerated type; and

(18. 6)

— every member of every anonymous union that is a member of class T.

19

In addition, if class T has a user-declared constructor (15.1) , every non-static data member of class T shall have a name different from T.

20

The common initial sequence of two standard-layout struct (Clause 12) types is the longest sequence of non-static data members and bit-fields in declaration order, starting with the first such entity in each of the structs, such that corresponding entities have layout-compatible types and either neither entity is a bit-field or both are bit-fields with the same width. [ Example: struct struct struct struct struct

A B C D E

{ { { { {

int a; char b; }; const int b1; volatile char b2; }; int c; unsigned : 0; char b; }; int d; char b : 4; }; unsigned int e; char b; };

The common initial sequence of A and B comprises all members of either class. The common initial sequence of A and C and of A and D comprises the first member in each case. The common initial sequence of A and E is empty. — end example ] 21

Two standard-layout struct (Clause 12) types are layout-compatible comprises all members and bit-fields of both classes (6.9) .

classes if their common initial sequence

22

Two standard-layout unions are layout-compatible if they have the same number of non-static data members and corresponding non-static data members (in any order) have layout-compatible types (6.9) .

23

In a standard-layout union with an active member (12.3) of struct type T1 , it is permitted to read a non-static data member m of another union member of struct type T2 provided m is part of the common initial sequence of T1 and T2 ; the behavior is as if the corresponding member of T1 were nominated. [ Example: struct T1 { int a, b; }; struct T2 { int c; double d; }; union U { T1 t1; T2 t2; }; int f() { U u = { { 1, 2 } }; // active member is t1 return u. t2. c; // OK, as if u. t1. a were nominated }

— end example ] [ Note: Reading a volatile obj ect through a non-volatile glvalue has undefined behavior — end note ]

(10.1.7.1) . 24

If a standard-layout class obj ect has any non-static data members, its address is the same as the address of its first non-static data member. Otherwise, its address is the same as the address of its first base class subobject (if any) . [ Note: There might therefore be unnamed padding within a standard-layout struct object, but not at its beginning, as necessary to achieve appropriate alignment. — end note ] [ Note: The obj ect and its first subobj ect are pointer-interconvertible (6.9.2, 8.2.9) . — end note ] § 12.2

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1 2. 2. 1

Member functions

[class. mfct]

1

A member function may be defined (11.4) in its class definition, in which case it is an inline member function (10.1.6) , or it may be defined outside of its class definition if it has already been declared but not defined in its class definition. A member function definition that appears outside of the class definition shall appear in a namespace scope enclosing the class definition. Except for member function definitions that appear outside of a class definition, and except for explicit specializations of member functions of class templates and member function templates (17.7) appearing outside of the class definition, a member function shall not be redeclared.

2

An inline member function (whether static or non-static) may also be defined outside of its class definition provided either its declaration in the class definition or its definition outside of the class definition declares the function as inline or constexpr . [ Note: Member functions of a class in namespace scope have the linkage of that class. Member functions of a local class (12.4) have no linkage. See 6.5. — end note ]

3

[ Note: There can be at most one definition of a non-inline member function in a program. There may be more than one inline member function definition in a program. See 6.2 and 10.1.6. — end note ]

4

If the definition of a member function is lexically outside its class definition, the member function name shall be qualified by its class name using the : : operator. [ Note: A name used in a member function definition (that is, in the parameter-declaration-clause including the default arguments (11.3.6) or in the member function body) is looked up as described in 6.4. — end note ] [ Example: struct X { typedef int T; static T count; void f(T) ; }; void X: : f(T t = count) { }

The member function f of class X is defined in global scope; the notation X: : f specifies that the function f is a member of class X and in the scope of class X . In the function definition, the parameter type T refers to the typedef member T declared in class X and the default argument count refers to the static data member count declared in class X . — end example ] 5

[ Note: A static local variable or local type in a member function always refers to the same entity, whether or not the member function is inline . — end note ]

6

Previously declared member functions may be mentioned in friend declarations.

7

Member functions of a local class shall be defined inline in their class definition, if they are defined at all.

8

[ Note: A member function can be declared (but not defined) using a typedef for a function type. The resulting member function has exactly the same type as it would have if the function declarator were provided explicitly, see 11.3.5. For example, typedef void fv() ; typedef void fvc() const; struct S { fv memfunc1; // equivalent to: void memfunc1() ; void memfunc2() ; fvc memfunc3; // equivalent to: void memfunc3() const; }; fv S: : * pmfv1 = &S: : memfunc1; fv S: : * pmfv2 = &S: : memfunc2; fvc S: : * pmfv3 = &S: : memfunc3;

Also see 17.3.

§ 12.2.1

— end note ]

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1 2. 2. 2

Non-static member functions

[class. mfct. non-static]

1

A non-static member function may be called for an object of its class type, or for an object of a class derived (Clause 13) from its class type, using the class member access syntax (8.2.5, 16.3.1.1) . A non-static member function may also be called directly using the function call syntax (8.2.2, 16.3.1.1) from within the body of a member function of its class or of a class derived from its class.

2

If a non-static member function of a class X is called for an obj ect that is not of type X , or of a type derived from X , the behavior is undefined.

3

When an id-expression (8.1) that is not part of a class member access syntax (8.2.5) and not used to form a pointer to member (8.3.1) is used in a member of class X in a context where this can be used (8.1.2) , if name lookup (6.4) resolves the name in the id-expression to a non-static non-type member of some class C , and if either the id-expression is potentially evaluated or C is X or a base class of X, the id-expression is transformed into a class member access expression (8.2.5) using (*this) (12.2.2.1) as the postfix-expression to the left of the . operator. [ Note: If C is not X or a base class of X , the class member access expression is ill-formed. — end note ] Similarly during name lookup, when an unqualified-id (8.1) used in the definition of a member function for class X resolves to a static member, an enumerator or a nested type of class X or of a base class of X , the unqualified-id is transformed into a qualified-id (8.1) in which the nested-name-specifier names the class of the member function. These transformations do not apply in the template definition context (17.6.2.1) . [ Example: struct tnode { char tword[20] ; int count; tnode* left; tnode* right; void set(const char*, tnode* l, tnode* r) ; }; void tnode: : set(const char* w, tnode* l, tnode* r) { count = strlen(w) +1; if (sizeof(tword) x = i; // OK: assign to enclose: : x } }; }; inner* p = 0;

§ 1 2. 2. 5

// error: inner not in scope

© IS O / IEC

2 0 1 7 – All right s reserved

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— end example ] 2

Member functions and static data members of a nested class can be defined in a namespace scope enclosing the definition of their class. [ Example: struct enclose struct inner static int void f(int }; };

{ { x; i) ;

int enclose: : inner: : x = 1; void enclose: : inner: : f(int i) { /* . . . */ }

— end example ] 3

If class X is defined in a namespace scope, a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in a namespace scope enclosing the definition of class X. [ Example: class E { class I1; class I2; class I1 { }; }; class E: : I2 { };

// forward declaration of nested class // definition of nested class // definition of nested class

— end example ] 4

Like a member function, a friend function (14.3) defined within a nested class is in the lexical scope of that class; it obeys the same rules for name binding as a static member function of that class (12.2.3) , but it has no special access rights to members of an enclosing class. 1 2. 2. 6

1

Nested type names

[class. nested. type]

Type names obey exactly the same scope rules as other names. In particular, type names defined within a class definition cannot be used outside their class without qualification. [ Example: struct X { typedef int I; class Y { /* . . . */ }; I a; }; I b; Y c; X: : Y d; X: : I e;

// error // error // OK // OK

— end example ] 1 2. 3 1

Unions

[class. union]

In a union, a non-static data member is active if its name refers to an obj ect whose lifetime has begun and has not ended (6.8) . At most one of the non-static data members of an object of union type can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time. [ Note: One special guarantee is made in order to simplify the use of unions: If a standard-layout union contains several standard-layout structs that share a common initial sequence (12.2) , and if a non-static data member of an object of this standard-layout union type is active and is one of the standard-layout structs, it § 12.3

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is permitted to inspect the common initial sequence of any of the standard-layout struct members; see 12.2.

— end note ] 2

The size of a union is sufficient to contain the largest of its non-static data members. Each non-static data member is allocated as if it were the sole member of a struct. [ Note: A standard-layout union object and its non-static data members are pointer-interconvertible (6.9.2, 8.2.9) . As a consequence, all non-static data members of such a union obj ect have the same address. — end note ]

3

A union can have member functions (including constructors and destructors) , but it shall not have virtual (13.3) functions. A union shall not have base classes. A union shall not be used as a base class. If a union contains a non-static data member of reference type the program is ill-formed. [ Note: Absent default member initializers (12.2) , if any non-static data member of a union has a non-trivial default constructor (15.1) , copy constructor (15.8) , move constructor (15.8) , copy assignment operator (15.8) , move assignment operator (15.8) , or destructor (15.4) , the corresponding member function of the union must be user-provided or it will be implicitly deleted (11.4.3) for the union. — end note ]

4

[ Example: Consider the following union: union U { int i; float f; std: : string s; };

Since std: : string (24.3) declares non-trivial versions of all of the special member functions, U will have an implicitly deleted default constructor, copy/move constructor, copy/move assignment operator, and destructor. To use U, some or all of these member functions must be user-provided. — end example ] 5

(5. 1 )

When the left operand of an assignment operator involves a member access expression (8.2.5) that nominates a union member, it may begin the lifetime of that union member, as described below. For an expression E, define the set S( E) of subexpressions of E as follows: — If E is of the form A. B , S( E) contains the elements of S( A ) , and also contains A. B if B names a union member of a non-class, non-array type, or of a class type with a trivial default constructor that is not deleted, or an array of such types.

(5. 2)

— If E is of the form A[B] and is interpreted as a built-in array subscripting operator, of array type, S( B ) if B is of array type, and empty otherwise.

(5. 3)

— Otherwise,

S ( E)

S ( E)

is

S( A)

if A is

is empty.

In an assignment expression of the form E1 = E2 that uses either the built-in assignment operator (8.18) or a trivial assignment operator (15.8) , for each element X of S( E1 ) , if modification of X would have undefined behavior under 6.8, an object of the type of X is implicitly created in the nominated storage; no initialization is performed and the beginning of its lifetime is sequenced after the value computation of the left and right operands and before the assignment. [ Note: This ends the lifetime of the previously-active member of the union, if any (6.8) . — end note ] [ Example: union A { int x; int y[4] ; }; struct B { A a; }; union C { B b; int k; }; int f() { C c; // does not start lifetime of any union member c. b. a. y[3] = 4; // OK: S( c. b. a. y[3] ) contains c. b and c. b. a. y; // creates objects to hold union members c. b and c. b. a. y return c. b. a. y[3] ; // OK: c. b. a. y refers to newly created object (see 6.8) } struct X { const int a; int b; };

§ 12.3

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union Y { X x; int k; }; void g() { Y y = { { 1, 2 } }; // OK, y. x is active union member (12.2) int n = y. x. a; y. k = 4; // OK: ends lifetime of y. x , y. k is active member of union y. x. b = n; // undefined behavior: y. x. b modified outside its lifetime, // S ( y. x. b ) is empty because X ’s default constructor is deleted, // so union member y. x ’s lifetime does not implicitly start }

6

— end example ] [ Note: In general, one must use explicit destructor calls and placement new-expression to change the active member of a union. — end note ] [ Example: Consider an object u of a union type U having non-static data

members m of type M and n of type N . If M has a non-trivial destructor and N has a non-trivial constructor (for instance, if they declare or inherit virtual functions) , the active member of u can be safely switched from m to n using the destructor and placement new-expression as follows: u. m. ~ M() ; new (&u. n) N;

— end example ] 1 2. 3. 1 1

Anonymous unions

[class. union. anon]

A union of the form union { member-specification } ;

is called an anonymous union; it defines an unnamed type and an unnamed obj ect of that type called an anonymous union object. Each member-declaration in the member-specification of an anonymous union shall either define a non-static data member or be a static_assert-declaration . [ Note: Nested types, anonymous unions, and functions cannot be declared within an anonymous union. — end note ] The names of the

members of an anonymous union shall be distinct from the names of any other entity in the scope in which the anonymous union is declared. For the purpose of name lookup, after the anonymous union definition, the members of the anonymous union are considered to have been defined in the scope in which the anonymous union is declared. [ Example: void f() { union { int a; const char* p; }; a = 1; p = " Jennifer" ; }

Here a and p are used like ordinary (non-member) variables, but since they are union members they have the same address. — end example ] 2

Anonymous unions declared in a named namespace or in the global namespace shall be declared static . Anonymous unions declared at block scope shall be declared with any storage class allowed for a block-scope variable, or with no storage class. A storage class is not allowed in a declaration of an anonymous union in a class scope. An anonymous union shall not have private or protected members (Clause 14) . An anonymous union shall not have member functions.

3

A union for which obj ects, pointers, or references are declared is not an anonymous union. [ Example: void f() { union { int aa; char* p; } obj , *ptr = &obj ; aa = 1; // error ptr->aa = 1; // OK }

§ 12.3.1

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The assignment to plain aa is ill-formed since the member name is not visible outside the union, and even if it were visible, it is not associated with any particular obj ect. — end example ] [ Note: Initialization of unions with no user-declared constructors is described in 11.6.1. — end note ] 4

A union-like class is a union or a class that has an anonymous union as a direct member. A union-like class X has a set of variant members . If X is a union, a non-static data member of X that is not an anonymous union is a variant member of X . In addition, a non-static data member of an anonymous union that is a member of X is also a variant member of X . At most one variant member of a union may have a default member initializer. [ Example: union U int x union union int int }; };

{ = 0; { int k; }; { z; y = 1; // error: initialization for second variant member of U

— end example ] 1 2. 4 1

Local class declarations

[class. local]

A class can be declared within a function definition; such a class is called a local class. The name of a local class is local to its enclosing scope. The local class is in the scope of the enclosing scope, and has the same access to names outside the function as does the enclosing function. Declarations in a local class shall not odr-use (6.2) a variable with automatic storage duration from an enclosing scope. [ Example: int x; void f() { static int s; int x; const int N = 5; extern int q() ; struct local { int g() { return x; } int h() { return s; } int k() { return : : x; } int l() { return q() ; } int m() { return N; } int* n() { return &N; } };

// error: odr-use of automatic variable x // OK // OK // OK // OK: not an odr-use // error: odr-use of automatic variable N

} local* p = 0;

// error: local not in scope

— end example ] 2

An enclosing function has no special access to members of the local class; it obeys the usual access rules (Clause 14) . Member functions of a local class shall be defined within their class definition, if they are defined at all.

3

If class X is a local class a nested class Y may be declared in class X and later defined in the definition of class X or be later defined in the same scope as the definition of class X. A class nested within a local class is a local class.

4

A local class shall not have static data members.

§ 12.4

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13 1

Derived classes

[class.derived]

A list of base classes can be specified in a class definition using the notation: base-clause : : base-specifier-list base-specifier-list : base-specifier . . . opt base-specifier-list , base-specifier . . . opt base-specifier: attribute-specifier-seqopt class-or-decltype attribute-specifier-seqopt virtual access-specifieropt class-or-decltype attribute-specifier-seqopt access-specifier virtual opt class-or-decltype class-or-decltype : nested-name-specifieropt class-name nested-name-specifier template simple-template-id decltype-specifier access-specifier: private protected public

attribute-specifier-seq appertains to the base-specifier. A class-or-decltype shall denote a class type that is not an incompletely defined class (Clause 12) . The class denoted by the class-or-decltype of a base-specifier is called a direct base class for the class being defined.

The optional 2

During the lookup for a base class name, non-type names are ignored (6.3.10) . If the name found is not a B is a base class of a class D if it is a direct base class of D or a direct base class of one of D ’s base classes. A class is an indirect base class of another if it is a base class but not a direct base class. A class is said to be (directly or indirectly) derived from its (direct or indirect) base classes. [ Note: See Clause 14 for the meaning of access-specifier. — end note ] Unless redeclared in the derived class, members of a base class are also considered to be members of the derived class. Members of a base class other than constructors are said to be inherited by the derived class. Constructors of a base class can also be inherited as described in 10.3.3. Inherited members can be referred to in expressions in the same manner as other members of the derived class, unless their names are hidden or ambiguous (13.2) . [ Note: The scope resolution operator : : (8.1) can be used to refer to a direct or indirect base member explicitly. This allows access to a name that has been redeclared in the derived class. A derived class can itself serve as a base class subj ect to access control; see 14.2. A pointer to a derived class can be implicitly converted to a pointer to an accessible unambiguous base class (7.11) . An lvalue of a derived class type can be bound to a reference to an accessible unambiguous base class (11.6.3) . — end note ]

class-name, the program is ill-formed. A class

3

The base-specifier-list specifies the type of the class type. [ Example:

base class subobjects contained in an obj ect of the derived

struct Base { int a, b, c; }; struct Derived : Base { int b; };

Derived classes

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struct Derived2 : Derived { int c; };

Here, an object of class Derived2 will have a subobject of class Derived which in turn will have a subobject of class Base . — end example ]

base-specifier followed by an ellipsis is a pack expansion (17.5.3) .

4

A

5

The order in which the base class subobj ects are allocated in the most derived obj ect (4.5) is unspecified. [ Note: A derived class and its base class subobj ects can be represented by a directed acyclic graph (DAG) where an arrow means “directly derived from”. An arrow need not have a physical representation in memory. A DAG of subobj ects is often referred to as a “subobj ect lattice”.

Base Derived1 Derived2 Figure 2 — Directed acyclic graph

6

7

— end note ] [ Note: Initialization of obj ects representing base classes can be specified in constructors; see 15.6.2. — end note ] [ Note: A base class subobj ect might have a layout (6.7) different from the layout of a most derived obj ect

of the same type. A base class subobj ect might have a polymorphic behavior (15.7) different from the polymorphic behavior of a most derived obj ect of the same type. A base class subobj ect may be of zero size (Clause 12) ; however, two subobjects that have the same class type and that belong to the same most derived obj ect must not be allocated at the same address (8.10) . — end note ] 1 3. 1 1

class class class class

2

3

Multiple base classes

[class. mi]

A class can be derived from any number of base classes. [ Note: The use of more than one direct base class is often called multiple inheritance. — end note ] [ Example: A B C D

{ { { :

/* . . . /* . . . /* . . . public

*/ */ */ A,

}; }; }; public B, public C { /* . . . */ };

— end example ] [ Note: The order of derivation is not significant except as specified by the semantics of initialization by constructor (15.6.2) , cleanup (15.4) , and storage layout (12.2, 14.1) . — end note ] A class shall not be specified as a direct base class of a derived class more than once. [ Note: A class can be an indirect base class more than once and can be a direct and an indirect base class. There are limited things that can be done with such a class. The non-static data members and member functions of the direct base class cannot be referred to in the scope of the derived class. However, the static members, enumerations and types can be unambiguously referred to. — end note ] [ Example: class X { /* . . . */ }; class Y : public X, public X { /* . . . */ };

§ 13.1

// ill-formed

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class class class class class

L A B C D

{ : : : :

public: int next; /* . . . */ }; public L { /* . . . */ }; public L { /* . . . */ }; public A, public B { void f() ; /* . . . */ }; public A, public L { void f() ; /* . . . */ };

// well-formed // well-formed

— end example ] 4

A base class specifier that does not contain the keyword virtual specifies a non-virtual base class. A base class specifier that contains the keyword virtual specifies a virtual base class. For each distinct occurrence of a non-virtual base class in the class lattice of the most derived class, the most derived obj ect (4.5) shall contain a corresponding distinct base class subobject of that type. For each distinct base class that is specified virtual, the most derived obj ect shall contain a single base class subobj ect of that type.

5

[ Note: For an obj ect of class type C , each distinct occurrence of a (non-virtual) base class L in the class lattice of C corresponds one-to-one with a distinct L subobj ect within the object of type C . Given the class C defined above, an obj ect of class C will have two subobj ects of class L as shown in Figure 3.

L

L

A

B C

Figure 3 — Non-virtual base In such lattices, explicit qualification can be used to specify which subobject is meant. The body of function C: : f could refer to the member next of each L subobj ect: void C: : f() { A: : next = B: : next; }

// well-formed

Without the A: : or B: : qualifiers, the definition of C: : f above would be ill-formed because of ambiguity (13.2) .

6

— end note ] [ Note: In contrast, consider the case with a virtual base class: class class class class

V A B C

{ : : :

/* . . . */ }; virtual public V { /* . . . */ }; virtual public V { /* . . . */ }; public A, public B { /* . . . */ };

V B

A C

Figure 4 — Virtual base For an obj ect c of class type C , a single subobj ect of type V is shared by every base class subobj ect of c § 13.1

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that has a virtual base class of type V. Given the class C defined above, an obj ect of class C will have one subobj ect of class V, as shown in Figure 4. — end note ] 7

[ Note: A class can have both virtual and non-virtual base classes of a given type. class class class class class

B { /* . . . */ }; X : virtual public B { /* . . . */ }; Y : virtual public B { /* . . . */ }; Z : public B { /* . . . */ }; AA : public X, public Y, public Z { /* . . . */ };

For an obj ect of class AA , all virtual occurrences of base class B in the class lattice of AA correspond to a single B subobj ect within the obj ect of type AA , and every other occurrence of a (non-virtual) base class B in the class lattice of AA corresponds one-to-one with a distinct B subobject within the object of type AA . Given the class AA defined above, class AA has two subobj ects of class B : Z ’s B and the virtual B shared by X and Y, as shown in Figure 5.

B

B Y

X

Z

AA Figure 5 — Virtual and non-virtual base

— end note ] 1 3. 2

Member name lookup

[class. member. lookup]

1

Member name lookup determines the meaning of a name ( id-expression) in a class scope (6.3.7) . Name lookup can result in an ambiguity , in which case the program is ill-formed. For an id-expression, name lookup begins in the class scope of this ; for a qualified-id, name lookup begins in the scope of the nested-name-specifier . Name lookup takes place before access control (6.4, Clause 14) .

2

The following steps define the result of name lookup for a member name f in a class scope C .

3

4

5

The lookup set for f in C , called S( f, C) , consists of two component sets: the declaration set, a set of members named f ; and the subobject set, a set of subobj ects where declarations of these members (possibly including using-declarations) were found. In the declaration set, using-declarations are replaced by the set of designated members that are not hidden or overridden by members of the derived class (10.3.3) , and type declarations (including inj ected-class-names) are replaced by the types they designate. S( f, C) is calculated as follows: If C contains a declaration of the name f , the declaration set contains every declaration of f declared in C that satisfies the requirements of the language construct in which the lookup occurs. [ Note: Looking up a name in an elaborated-type-specifier (6.4.4) or base-specifier (Clause 13) , for instance, ignores all non-type declarations, while looking up a name in a nested-name-specifier (6.4.3) ignores function, variable, and enumerator declarations. As another example, looking up a name in a using-declaration (10.3.3) includes the declaration of a class or enumeration that would ordinarily be hidden by another declaration of that name in the same scope. — end note ] If the resulting declaration set is not empty, the subobj ect set contains C itself, and calculation is complete. Otherwise (i.e., C does not contain a declaration of f or the resulting declaration set is empty) , S( f, C) is initially empty. If C has base classes, calculate the lookup set for f in each direct base class subobj ect Bi , and merge each such lookup set S( f, Bi ) in turn into S( f, C) . § 13.2

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6

The following st eps define t he result

(6. 1)

—

If each of t he sub ob j ect memb ers of

S f, C (

memb ers of

(6. 2)

—

O t herwise,

S f, C (

—

(

memb ers of

or if

O t herwise,

t he new

(

S f, C (

of name lo okup for

ill- formed.

[

Example:

f

struct A { int x; }; struct B { float x; }; struct C: public A, public struct D: public virtual C struct E: public virtual C struct F: public D, public int main() { F f; f. x = 0; }

8

(

i)

)

(

of

of at

i)

least

S f, C (

S f, C S f, C

and

(

(

)

)

of at

is a copy of

differ,

t he

least

S f, B (

)

is cons idered different

is a lo okup set

):

one of t he sub ob j ect

) is unchanged and t he merge is complet e.

t he new

S f, B (

int o t he int ermediat e

is a base class sub ob j ect

is empty,

set s

i)

is a base class sub ob j ect

(

)

(

C onversely,

one of t he sub ob j ect

i).

merge

is

ambiguous:

t he

new

In subsequent

from any ot her.

wit h t he shared set

of declarat ions and t he union of t he

set s .

The result

S x, F S x, D

S f, C

declarat ion

S f, B S f, C S f, C

i ) is empty,

an invalid declarat ion set

sub ob j ect

7

if t he

i),

S f, B

S f, B

) is a lo okup set wit h an invalid declarat ion set and t he union of t he sub ob j ect set s .

merges ,

(6. 3)

S f, B (

memb ers of

) , or if

if each of t he sub ob j ect

of merging lo okup s et

C

in

B { { E

is t he declarat ion s et

of

S f, C (

// S(x,A) = { { A: : x }, // S(x,B) = { { B: : x }, { }; // S(x,C) = { invalid, { }; // S(x,D) = S(x,C) char x; }; // S(x,E) = { { E: : x }, { }; // S(x,F) = S(x,E)

).

If it

is an invalid set ,

t he program is

{A}} {B}} A in C , B in C } } {E}}

// OK, lookup finds E: : x

(

)

is unambiguous b ecaus e t he

(

)

is discarded in t he first

A

and

B

merge st ep.

base class s ub ob j ect s of

— end example

D

are also base clas s sub ob j ect s of

E,

so

]

If t he name of an overloaded funct ion is unambiguous ly found, overload resolut ion (1 6 . 3 ) also t akes place b efore access cont rol.

Ambiguit ies can oft en b e resolved by qualifying a name wit h it s class name.

[

Example:

struct A { int f() ; }; struct B { int f() ; }; struct C : A, B { int f() { return A: : f() + B: : f() ; } };

9

— end example Note:

[

]

A st at ic memb er,

found even if an ob j ect

a nest ed typ e or an enumerat or defined in a base class

has more t han one base class sub ob j ect

of typ e

t he non- st at ic memb er sub ob j ect s of t heir common virt ual base class es.

struct V int v; }; struct A int a; static enum { }; struct B

§ 1 3. 2

T.

T

can unambiguously b e

Two base class sub ob j ect s share

— end note

]

[

Example:

{

{ int e };

s;

: A, virtual V { };

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struct C : A, virtual V { }; struct D : B, C { }; void f(D* pd) { pd->v++; pd->s++; int i = pd->e; pd->a++; }

10

— end example Note:

[

// OK: only one v (virtual) // OK: only one s (static) // OK: only one e (enumerator) // error, ambiguous: two as in D

]

When virt ual base classes are used,

s ub ob j ect

lat t ice t hat

do es not

a hidden declarat ion can b e reached along a pat h t hrough t he

pass t hrough t he hiding declarat ion.

us e wit h non- virt ual bas e classes is an ambiguity; hides all t he ot hers.

struct V { struct W { struct B : int f() ; int g() ; }; struct C :

— end note

]

[

Example:

in t hat

This is not

an ambiguity.

The ident ical

case t here is no unique inst ance of t he name t hat

int f() ; int x; }; int g() ; int y; }; virtual V, W { int x; int y; virtual V, W { };

struct D : B, C { void glorp() ; };

W

V

W

B

C D

F igure 6 — Name lo okup

The names declared in

V

and t he left - hand inst ance of

t he right - hand inst ance of

void D: : glorp() { x++; f() ; y++; g() ; }

— end example 11

An exp licit

W

are not

hidden at

W

are hidden by t hose in

B

,

but

t he names declared in

all.

// OK: B: : x hides V: : x // OK: B: : f() hides V: : f() // error: B: : y and C ’s W: : y // error: B: : g() and C ’s W: : g()

]

or implicit

conversion from a p oint er t o or an expression designat ing an ob j ect

of a derived class

t o a p oint er or reference t o one of it s base clas ses shall unambiguously refer t o a unique ob j ect t he base class.

struct struct struct struct § 1 3. 2

V A B C

{ { : :

[

Example:

represent ing

}; }; A, virtual V { }; A, virtual V { }; © IS O / IEC

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struct D : B, C { }; void g() { D d; B* pb = &d; A* pa = &d; V* pv = &d; }

12

// error, ambiguous: C ’s A or B ’s A ? // OK: only one V subobject

— end example ] [ Note: Even if the result of name lookup is unambiguous, use of a name found in multiple subobj ects might still be ambiguous (7.12, 8.2.5, 14.2) . — end note ] [ Example: struct B1 { void f() ; static void f(int) ; int i; }; struct B2 { void f(double) ; }; struct I1: B1 { }; struct I2: B1 { }; struct D: I1, I2, B2 { using B1: : f; using B2: : f; void g() { f() ; f(0) ; f(0. 0) ; int B1: : * mpB1 = &D: : i; int D: : * mpD = &D: : i; } };

// Ambiguous conversion of this // Unambiguous (static) // Unambiguous (only one B2 ) // Unambiguous // Ambiguous conversion

— end example ] 1 3. 3

Virtual functions

[class. virtual]

1

[ Note: Virtual functions support dynamic binding and obj ect-oriented programming. that declares or inherits a virtual function is called a polymorphic class.

— end note ] A class

2

If a virtual member function vf is declared in a class Base and in a class Derived, derived directly or indirectly from Base , a member function vf with the same name, parameter-type-list (11.3.5) , cv-qualification, and ref-qualifier (or absence of same) as Base: : vf is declared, then Derived: : vf is also virtual (whether or not it is so declared) and it overrides11 1 Base: : vf . For convenience we say that any virtual function overrides itself. A virtual member function C: : vf of a class object S is a final overrider unless the most derived class (4.5) of which S is a base class subobj ect (if any) declares or inherits another member function that overrides vf . In a derived class, if a virtual member function of a base class subobj ect has more than one final overrider the program is ill-formed. [ Example: struct A { virtual void f() ; 111 )

A funct ion wit h t he same name but a different paramet er list ( C lause 1 6) as a virt ual funct ion is not necessarily virt ual

and do es not override. empty semant ics) .

§ 13.3

The use of t he

virtual

sp ecifier in t he declarat ion of an overriding funct ion is legal but redundant ( has

Access cont rol ( C lause 1 4) is not considered in det ermining overriding.

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}; struct B : virtual A { virtual void f() ; }; struct C : B , virtual A { using A: : f; }; void foo() { C c; c. f() ; c. C: : f() ; }

— end example Example:

// calls B: : f , the final overrider // calls A: : f because of the using-declaration ]

[

struct struct struct struct

3

A B C D

{ : : :

virtual void f() ; }; A { }; A { void f() ; }; B, C { }; // OK: A: : f and C: : f are the final overriders // for the B and C subobjects, respectively

— end example Note:

[

]

A virt ual memb er funct ion do es not

have t o b e visible t o b e overridden,

for example,

struct B { virtual void f() ; }; struct D : B { void f(int) ; }; struct D2 : D { void f() ; }; t he funct ion funct ion.

f(int) f()

in class

However,

D

hides t he virt ual funct ion

declared in class

t herefore is a virt ual funct ion t hat

D2 — end note .

4

f()

in it s b ase class

B D: : f(int) ;

has t he same name and t he same paramet er list

overrides t he funct ion

B: : f()

even t hough

B: : f()

is not

is not as

a virt ual

B: : f()

,

and

visible in class

]

If a virt ual funct ion a funct ion

D2

D: : f

f

in some clas s

overrides

B: : f

,

B

is marked wit h t he

virt-specifier final Example:

t he program is ill- formed.

and in a class

D

derived from

B

[

struct B { virtual void f() const final; }; struct D : B { void f() const; // error: D: : f attempts to override final B: : f };

— end example 5

]

If a virt ual funct ion is marked wit h t he bas e class,

t he program is ill- formed.

[

virt-specifier override Example:

and do es not override a memb er funct ion of a

struct B { virtual void f(int) ; };

§ 1 3. 3

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struct D : B { virtual void f(long) override; virtual void f(int) override; };

// error: wrong signature overriding B: : f // OK

— end example ] 6

Even though destructors are not inherited, a destructor in a derived class overrides a base class destructor declared virtual; see 15.4 and 15.5.

7

The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions. If a function D: : f overrides a function B: : f , the return types of the functions are covariant if they satisfy the following criteria:

(7. 1 )

— both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes 112

(7. 2)

— the class in the return type of B: : f is the same class as the class in the return type of D: : f , or is an unambiguous and accessible direct or indirect base class of the class in the return type of D: : f

(7. 3)

— both pointers or references have the same cv-qualification and the class type in the return type of D: : f has the same cv-qualification as or less cv-qualification than the class type in the return type of B: : f .

8

If the class type in the covariant return type of D: : f differs from that of B: : f , the class type in the return type of D: : f shall be complete at the point of declaration of D: : f or shall be the class type D . When the overriding function is called as the final overrider of the overridden function, its result is converted to the type returned by the (statically chosen) overridden function (8.2.2) . [ Example: class B { }; class D : private B { friend class Derived; }; struct Base { virtual void vf1() ; virtual void vf2() ; virtual void vf3() ; virtual B* vf4() ; virtual B* vf5() ; void f() ; }; struct No_good : public Base { D* vf4() ; // error: B (base class of D ) inaccessible }; class A; struct Derived : public Base { void vf1() ; // virtual and overrides Base: : vf1() void vf2(int) ; // not virtual, hides Base: : vf2() char vf3() ; // error: invalid difference in return type only D* vf4() ; // OK: returns pointer to derived class A* vf5() ; // error: returns pointer to incomplete class void f() ; }; void g() { Derived d; Base* bp = &d; 112)

// standard conversion: // Derived* to Base*

Mult i- level p oint ers t o classes or references t o mult i- level p oint ers t o classes are not allowed.

§ 13.3

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bp->vf1() ; bp->vf2() ; bp->f() ; B* p = bp->vf4() ; Derived* dp = &d; D* q = dp->vf4() ; dp->vf2() ; }

9

// calls Derived: : vf1() // calls Base: : vf2() // calls Base: : f() (not virtual) // calls Derived: : pf() and converts the // result to B* // calls Derived: : pf() and does not // convert the result to B* // ill-formed: argument mismatch

— end example ] [ Note: The interpretation of the call of a virtual function depends on the type of the obj ect for which it is called (the dynamic type) , whereas the interpretation of a call of a non-virtual member function depends only on the type of the pointer or reference denoting that obj ect (the static type) (8.2.2) . — end note ]

10

[ Note: The virtual specifier implies membership, so a virtual function cannot be a non-member (10.1.2) function. Nor can a virtual function be a static member, since a virtual function call relies on a specific obj ect for determining which function to invoke. A virtual function declared in one class can be declared a friend in another class. — end note ]

11

A virtual function declared in a class shall be defined, or declared pure (13.4) in that class, or both; no diagnostic is required (6.2) .

12

[ Example: Here are some uses of virtual functions with multiple base classes: struct A { virtual void f() ; }; struct B1 : A { void f() ; };

// note non-virtual derivation

struct B2 : A { void f() ; }; struct D : B1, B2 { };

// D has two separate A subobjects

void foo() { D d; // A* ap = &d; // would be ill-formed: ambiguous B1* b1p = &d; A* ap = b1p; D* dp = &d; ap->f() ; // calls D: : B1: : f dp->f() ; // ill-formed: ambiguous }

In class D above there are two occurrences of class A and hence two occurrences of the virtual member function A: : f . The final overrider of B1: : A: : f is B1: : f and the final overrider of B2: : A: : f is B2: : f . — end example ]

§ 13.3

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13

[ Example: The following example shows a function that does not have a unique final overrider: struct A { virtual void f() ; }; struct VB1 : virtual A { void f() ; };

// note virtual derivation

struct VB2 : virtual A { void f() ; }; struct Error : VB1, VB2 { };

// ill-formed

struct Okay : VB1, VB2 { void f() ; };

Both VB1: : f and VB2: : f override A: : f but there is no overrider of both of them in class Error . This example is therefore ill-formed. Class Okay is well formed, however, because Okay: : f is a final overrider. — end example ] 14

[ Example: The following example uses the well-formed classes from above. struct VB1a : virtual A { }; struct Da : VB1a, VB2 { }; void foe() { VB1a* vb1ap = new Da; vb1ap->f() ; }

// does not declare f

// calls VB2: : f

— end example ] 15

Explicit qualification with the scope operator (8.1) suppresses the virtual call mechanism. [ Example: class B { public: virtual void f() ; }; class D : public B { public: void f() ; }; void D: : f() { /* . . . */ B: : f() ; }

Here, the function call in D: : f really does call B: : f and not D: : f . 16

— end example ]

A function with a deleted definition (11.4) shall not override a function that does not have a deleted definition. Likewise, a function that does not have a deleted definition shall not override a function with a deleted definition. 1 3. 4

Abstract classes

[class. abstract]

1

[ Note: The abstract class mechanism supports the notion of a general concept, such as a shape , of which only more concrete variants, such as circle and square , can actually be used. An abstract class can also be used to define an interface for which derived classes provide a variety of implementations. — end note ]

2

An abstract class is a class that can be used only as a base class of some other class; no objects of an abstract class can be created except as subobj ects of a class derived from it. A class is abstract if it has at least one pure virtual function. [ Note: Such a function might be inherited: see below. — end note ] A virtual function is specified pure by using a pure-specifier (12.2) in the function declaration in the class definition. A § 13.4

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pure virtual function need be defined only if called with, or as if with (15.4) , the [ Example:

qualified-id syntax (8.1) .

class point { /* . . . */ }; class shape { // abstract class point center; public: point where() { return center; } void move(point p) { center=p; draw() ; } virtual void rotate(int) = 0; // pure virtual virtual void draw() = 0; // pure virtual };

— end example ] [ Note: A function declaration cannot provide both a pure-specifier and a definition — end note ] [ Example: struct C { virtual void f() = 0 { }; };

// ill-formed

— end example ] 3

An abstract class shall not be used as a parameter type, as a function return type, or as the type of an explicit conversion. Pointers and references to an abstract class can be declared. [ Example: shape x; shape* p; shape f() ; void g(shape) ; shape& h(shape&) ;

// error: object of abstract class // OK // error // error // OK

— end example ] 4

A class is abstract if it contains or inherits at least one pure virtual function for which the final overrider is pure virtual. [ Example: class ab_circle : public shape { int radius; public: void rotate(int) { } // ab_circle: : draw() is a pure virtual };

Since shape: : draw() is a pure virtual function ab_circle: : draw() is a pure virtual by default. The alternative declaration, class circle : public shape { int radius; public: void rotate(int) { } void draw() ; };

5

6

// a definition is required somewhere

would make class circle non-abstract and a definition of circle: : draw() must be provided. — end example ] [ Note: An abstract class can be derived from a class that is not abstract, and a pure virtual function may override a virtual function which is not pure. — end note ] Member functions can be called from a constructor (or destructor) of an abstract class; the effect of making a virtual call (13.3) to a pure virtual function directly or indirectly for the obj ect being created (or destroyed) from such a constructor (or destructor) is undefined. § 13.4

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14 1

Member access control

[class.access]

A member of a class can be

(1. 1)

— private ; that is, its name can be used only by members and friends of the class in which it is declared.

(1. 2)

— protected; that is, its name can be used only by members and friends of the class in which it is declared, by classes derived from that class, and by their friends (see 14.4) .

(1. 3)

— public ; that is, its name can be used anywhere without access restriction.

2

A member of a class can also access all the names to which the class has access. A local class of a member function may access the same names that the member function itself may access. 11 3

3

Members of a class defined with the keyword class are private by default. Members of a class defined with the keywords struct or union are public by default. [ Example: class X { int a; };

// X: : a is private by default

struct S { int a; };

// S: : a is public by default

— end example ] 4

Access control is applied uniformly to all names, whether the names are referred to from declarations or expressions. [ Note: Access control applies to names nominated by friend declarations (14.3) and usingdeclarations (10.3.3) . — end note ] In the case of overloaded function names, access control is applied to the function selected by overload resolution. [ Note: Because access control applies to names, if access control is applied to a typedef name, only the accessibility of the typedef name itself is considered. The accessibility of the entity referred to by the typedef is not considered. For example, class A { class B { }; public: typedef B BB; }; void f() { A: : BB x; A: : B y; }

// OK, typedef name A: : BB is public // access error, A: : B is private

— end note ] 5

It should be noted that it is access to members and base classes that is controlled, not their visibility . Names of members are still visible, and implicit conversions to base classes are still considered, when those members and base classes are inaccessible. The interpretation of a given construct is established without regard to access control. If the interpretation established makes use of inaccessible member names or base classes, the construct is ill-formed.

6

All access controls in Clause 14 affect the ability to access a class member name from the declaration of a particular entity, including parts of the declaration preceding the name of the entity being declared and, if 113)

Access p ermissions are t hus t ransit ive and cumulat ive t o nest ed and lo cal classes.

Member access control

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the entity is a class, the definitions of members of the class appearing outside the class’s member-specification . [ Note: This access also applies to implicit references to constructors, conversion functions, and destructors. — end note ] 7

[ Example: class A { typedef int I; // private member I f() ; friend I g(I) ; static I x; template struct Q; template friend struct R; protected: struct B { }; }; A: : I A: : f() { return 0; } A: : I g(A: : I p = A: : x) ; A: : I g(A: : I p) { return 0; } A: : I A: : x = 0; template struct A: : Q { }; template struct R { }; struct D: A: : B, A { };

Here, all the uses of A: : I are well-formed because A: : f , A: : x, and A: : Q are members of class A and g and R are friends of class A. This implies, for example, that access checking on the first use of A: : I must be deferred until it is determined that this use of A: : I is as the return type of a member of class A . Similarly, the use of A: : B as a base-specifier is well-formed because D is derived from A , so checking of base-specifiers must be deferred until the entire base-specifier-list has been seen. — end example ] 8

The names in a default argument (11.3.6) are bound at the point of declaration, and access is checked at that point rather than at any points of use of the default argument. Access checking for default arguments in function templates and in member functions of class templates is performed as described in 17.7.1.

9

The names in a default template-argument (17.1) have their access checked in the context in which they appear rather than at any points of use of the default template-argument . [ Example: class B { }; template class C { protected: typedef T TT; }; template class D : public U { }; D * d;

// access error, C: : TT is protected

— end example ] 1 4. 1 1

Access specifiers

[class. access. spec]

Member declarations can be labeled by an

access-specifier (Clause 13) :

access-specifier : member-specificationopt

An access-specifier specifies the access rules for members following it until the end of the class or until another access-specifier is encountered. [ Example: § 14.1

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class X { int a; public: int b; int c; };

// X: : a is private by default: class used // X: : b is public // X: : c is public

— end example ] 2

Any number of access specifiers is allowed and no particular order is required. [ Example: struct S { int a; protected: int b; private: int c; public: int d; };

3

4

// S: : a is public by default: struct used // S: : b is protected // S: : c is private // S: : d is public

— end example ] [ Note: The effect of access control on the order of allocation of data members is described in 12.2. — end note ] When a member is redeclared within its class definition, the access specified at its redeclaration shall be the same as at its initial declaration. [ Example: struct S { class A; enum E : int; private: class A { }; // error: cannot change access enum E: int { e0 }; // error: cannot change access };

5

— end example ] [ Note: In a derived class, the lookup of a base class name will find the inj ected-class-name instead of the

name of the base class in the scope in which it was declared. The injected-class-name might be less accessible than the name of the base class in the scope in which it was declared. — end note ] [ Example: class A { }; class B : private A { }; class C : public B { A* p; // error: injected-class-name A is inaccessible : : A* q; // OK };

— end example ] 1 4. 2 1

Accessibility of base classes and base class members

[class. access. base]

If a class is declared to be a base class (Clause 13) for another class using the public access specifier, the public members of the base class are accessible as public members of the derived class and protected members of the base class are accessible as protected members of the derived class. If a class is declared to be a base class for another class using the protected access specifier, the public and protected members of the base class are accessible as protected members of the derived class. If a class is declared to be a base § 14.2

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class for another class using the private access specifier, the public and protected members of the base class are accessible as private members of the derived class11 4 . 2

In the absence of an access-specifier for a base class, public is assumed when the derived class is defined with the class-key struct and private is assumed when the class is defined with the class-key class . [ Example: class B { /* . . . */ }; class D1 : private B { /* . . . */ }; class D2 : public B { /* . . . */ }; class D3 : B { /* . . . */ }; struct D4 : public B { /* . . . */ }; struct D5 : private B { /* . . . */ }; struct D6 : B { /* . . . */ }; class D7 : protected B { /* . . . */ }; struct D8 : protected B { /* . . . */ };

// B private by default

// B public by default

Here B is a public base of D2 , D4 , and D6 , a private base of D1 , D3 , and D5 , and a protected base of D7 and D8 . — end example ] 3

[ Note: A member of a private base class might be inaccessible as an inherited member name, but accessible directly. Because of the rules on pointer conversions (7.11) and explicit casts (8.4) , a conversion from a pointer to a derived class to a pointer to an inaccessible base class might be ill-formed if an implicit conversion is used, but well-formed if an explicit cast is used. For example, class B { public: int mi; static int si; }; class D : private B { }; class DD : public D { void f() ; }; void DD: : f() { mi = 3; si = 3; : : B b; b. mi = 3; b. si = 3; : : B: : si = 3; : : B* bp1 = this; : : B* bp2 = (: : B*) this; bp2->mi = 3; }

// non-static member // static member

// error: mi is private in D // error: si is private in D // OK ( b. mi is different from this->mi ) // OK ( b. si is different from this->si ) // OK // error: B is a private base class // OK with cast // OK: access through a pointer to B .

— end note ] 4

A base class B of N is

accessible at R, if

(4. 1 )

— an invented public member of B would be a public member of N, or

(4. 2)

—

R occurs in a member or friend of class N, and an invented public member of B would be a private or

—

R occurs in a member or friend of a class P derived from N, and an invented public member of B would

(4. 3)

114)

protected member of N, or

As sp ecified previously in C lause 1 4, privat e memb ers of a base class remain inaccessible even t o derived classes unless

f riend

§ 14.2

declarat ions wit hin t he base class definit ion are used t o grant access explicit ly.

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be a private or protected member of P , or (4. 4)

— there exists a class S such that B is a base class of S accessible at at R.

R and S is a base class of N accessible

[ Example: class B { public: int m; }; class S: private B { friend class N; }; class N: private S { void f() { B* p = this; // OK because class S satisfies the fourth condition above: B is a base class of N // accessible in f() because B is an accessible base class of S and S is an accessible // base class of N. } };

— end example ] 5

If a base class is accessible, one can implicitly convert a pointer to a derived class to a pointer to that base class (7.11, 7.12) . [ Note: It follows that members and friends of a class X can implicitly convert an X* to a pointer to a private or protected immediate base class of X. — end note ] The access to a member is affected by the class in which the member is named. This naming class is the class in which the member name was looked up and found. [ Note: This class can be explicit, e.g., when a qualified-id is used, or implicit, e.g., when a class member access operator (8.2.5) is used (including cases where an implicit “ this-> ” is added) . If both a class member access operator and a qualified-id are used to name the member (as in p->T: : m) , the class naming the member is the class denoted by the nested-name-specifier of the qualified-id (that is, T ) . — end note ] A member m is accessible at the point R when named in class N if

(5. 1 )

— m as a member of N is public, or

(5. 2)

— m as a member of N is private, and

(5. 3)

— m as a member of N class P derived from N, where m as a member of P is public, private, or protected, or

(5. 4)

— there exists a base class B of N that is accessible at [ Example:

R occurs in a member or friend of class N, or is protected, and R occurs in a member or friend of class N , or in a member of a R, and

m is accessible at

R when named in class B .

class B; class A { private: int i; friend void f(B*) ; }; class B : public A { }; void f(B* p) { p->i = 1; // OK: B* can be implicitly converted to A* , and f has access to i in A }

— end example ]

§ 14.2

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6

If a class member access operator, including an implicit “ this-> ”, is used to access a non-static data member or non-static member function, the reference is ill-formed if the left operand (considered as a pointer in the “ . ” operator case) cannot be implicitly converted to a pointer to the naming class of the right operand. [ Note: This requirement is in addition to the requirement that the member be accessible as named. — end note ] 1 4. 3

1

Friends

[class. friend]

A friend of a class is a function or class that is given permission to use the private and protected member names from the class. A class specifies its friends, if any, by way of friend declarations. Such declarations give special access rights to the friends, but they do not make the nominated friends members of the befriending class. [ Example: The following example illustrates the differences between members and friends: class X { int a; friend void friend_set(X*, int) ; public: void member_set(int) ; }; void friend_set(X* p, int i) { p->a = i; } void X: : member_set(int i) { a = i; } void f() { X obj ; friend_set(&obj , 10) ; obj . member_set(10) ; }

— end example ] 2

Declaring a class to be a friend implies that the names of private and protected members from the class granting friendship can be accessed in the base-specifiers and member declarations of the befriended class. [ Example: class A { class B { }; friend class X; }; struct X : A: : B { A: : B mx; class Y { A: : B my; }; };

// OK: A: : B accessible to friend // OK: A: : B accessible to member of friend // OK: A: : B accessible to nested member of friend

— end example ] [ Example: class X { enum { a=100 }; friend class Y; }; class Y { int v[X: : a] ; };

§ 14.3

// OK, Y is a friend of X

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class Z { int v[X: : a] ; };

// error: X: : a is private

— end example ] A class shall not be defined in a friend declaration. [ Example: class A { friend class B { }; };

// error: cannot define class in friend declaration

— end example ] 3

A friend declaration that does not declare a function shall have one of the following forms: friend elaborated-type-specifier ; friend simple-type-specifier ; friend typename-specifier ;

[ Note: A friend declaration may be the declaration in a template-declaration (Clause 17, 17.5.4) . — end note ] If the type specifier in a friend declaration designates a (possibly cv-qualified) class type, that class is declared as a friend; otherwise, the friend declaration is ignored. [ Example: class C; typedef C Ct; class X1 { friend C; };

// OK: class C is a friend

class X2 friend friend friend };

// OK: class C is a friend // error: no type-name D in scope // OK: elaborated-type-specifier declares new class

{ Ct; D; class D;

template class R { friend T; }; R rc; R Ri;

// class C is a friend of R // OK: " friend int; " is ignored

— end example ] 4

A function first declared in a friend declaration has the linkage of the namespace of which it is a member (6.5) . Otherwise, the function retains its previous linkage (10.1.1) .

5

When a friend declaration refers to an overloaded name or operator, only the function specified by the parameter types becomes a friend. A member function of a class X can be a friend of a class Y. [ Example: class Y { friend char* X: : foo(int) ; friend X: : X(char) ; friend X: : ~ X() ; };

// constructors can be friends // destructors can be friends

— end example ] 6

A function can be defined in a friend declaration of a class if and only if the class is a non-local class (12.4) , the function name is unqualified, and the function has namespace scope. [ Example: § 14.3

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class M { friend void f() { } };

// definition of global f , a friend of M, // not the definition of a member function

— end example ] 7

Such a function is implicitly an inline function (10.1.6) . A friend function defined in a class is in the (lexical) scope of the class in which it is defined. A friend function defined outside the class is not (6.4.1) .

8

No

9

A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration. The meaning of the friend declaration is the same whether the friend declaration appears in the private , protected or public (12.2) portion of the class member-specification.

10

storage-class-specifier shall appear in the decl-specifier-seq of a friend declaration.

Friendship is neither inherited nor transitive. [ Example: class A { friend class B; int a; }; class B { friend class C; }; class C { void f(A* p) { p->a++; } }; class D : public B void f(A* p) { p->a++; } };

// error: C is not a friend of A despite being a friend of a friend

{

// error: D is not a friend of A despite being derived from a friend

— end example ] 11

If a friend declaration appears in a local class (12.4) and the name specified is an unqualified name, a prior declaration is looked up without considering scopes that are outside the innermost enclosing non-class scope. For a friend function declaration, if there is no prior declaration, the program is ill-formed. For a friend class declaration, if there is no prior declaration, the class that is specified belongs to the innermost enclosing non-class scope, but if it is subsequently referenced, its name is not found by name lookup until a matching declaration is provided in the innermost enclosing non-class scope. [ Example: class X; void a() ; void f() { class Y; extern void b() ; class A { friend class X; friend class Y; friend class Z; friend void a() ; friend void b() ;

§ 14.3

// OK, but X is a local class, not : : X // OK // OK, introduces local class Z // error, : : a is not considered // OK

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friend void c() ; }; X* px; Z* pz; }

// error // OK, but : : X is found // error, no Z is found

— end example ] 1 4. 4 1

Protected member access

[class. protected]

An additional access check beyond those described earlier in Clause 14 is applied when a non-static data member or non-static member function is a protected member of its naming class (14.2) . 115 As described earlier, access to a protected member is granted because the reference occurs in a friend or member of some class C . If the access is to form a pointer to member (8.3.1) , the nested-name-specifier shall denote C or a class derived from C . All other accesses involve a (possibly implicit) obj ect expression (8.2.5) . In this case, the class of the obj ect expression shall be C or a class derived from C . [ Example: class B { protected: int i; static int j ; }; class D1 : public B { }; class D2 : public B { friend void fr(B*, D1*, D2*) ; void mem(B*, D1*) ; }; void fr(B* pb, D1* p1, D2* p2) { pb->i = 1; // ill-formed p1->i = 2; // ill-formed p2->i = 3; // OK (access through a D2 ) p2->B: : i = 4; // OK (access through a D2 , even though naming class is B ) int B: : * pmi_B = &B: : i; // ill-formed int B: : * pmi_B2 = &D2: : i; // OK (type of &D2: : i is int B: : * ) B: : j = 5; // ill-formed (not a friend of naming class B ) D2: : j = 6; // OK (because refers to static member) } void D2: : mem(B* pb, D1* p1) { pb->i = 1; p1->i = 2; i = 3; B: : i = 4; int B: : * pmi_B = &B: : i; int B: : * pmi_B2 = &D2: : i; j = 5; B: : j = 6; }

// ill-formed // ill-formed // OK (access through this ) // OK (access through this , qualification ignored) // ill-formed // OK // OK (because j refers to static member) // OK (because B: : j refers to static member)

void g(B* pb, D1* p1, D2* p2) { pb->i = 1; // ill-formed 115)

This addit ional check do es not apply t o ot her memb ers, e. g. , st at ic dat a memb ers or enumerat or memb er const ant s.

§ 14.4

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// ill-formed // ill-formed

p1->i = 2; p2->i = 3; }

— end example ] 1 4. 5 1

Access to virtual functions

[class. access. virt]

The access rules (Clause 14) for a virtual function are determined by its declaration and are not affected by the rules for a function that later overrides it. [ Example: class B { public: virtual int f() ; }; class D : public B { private: int f() ; }; void f() { D d; B* pb = &d; D* pd = &d;

// OK: B: : f() is public, D: : f() is invoked // error: D: : f() is private

pb->f() ; pd->f() ; }

— end example ] 2

Access is checked at the call point using the type of the expression used to denote the obj ect for which the member function is called ( B* in the example above) . The access of the member function in the class in which it was defined ( D in the example above) is in general not known. 1 4. 6

1

Multiple access

[class. paths]

If a name can be reached by several paths through a multiple inheritance graph, the access is that of the path that gives most access. [ Example: class W { public: void f() ; }; class A : private virtual W { }; class B : public virtual W { }; class C : public A, public B { void f() { W: : f() ; } // OK };

Since 1 4. 7 1

W: : f ( )

is available to

C: : f ( )

along the public path through B , access is allowed.

Nested classes

— end example ]

[class. access. nest]

A nested class is a member and as such has the same access rights as any other member. The members of an enclosing class have no special access to members of a nested class; the usual access rules (Clause 14) shall be obeyed. [ Example: class E { int x; class B { };

§ 14.7

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class I { B b; int y; void f(E* p, int i) { p->x = i; } }; int g(I* p) { return p->y; }

// OK: E: : I can access E: : B

// OK: E: : I can access E: : x

// error: I: : y is private

};

— end example ]

§ 14.7

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ISO/IEC 1 4882:201 7(E)

15

Special member functions

[special]

1

The default constructor (15.1) , copy constructor and copy assignment operator (15.8) , move constructor and move assignment operator (15.8) , and destructor (15.4) are special member functions . [ Note: The implementation will implicitly declare these member functions for some class types when the program does not explicitly declare them. The implementation will implicitly define them if they are odr-used (6.2) . See 15.1, 15.4 and 15.8. — end note ] An implicitly-declared special member function is declared at the closing } of the class-specifier . Programs shall not define implicitly-declared special member functions.

2

Programs may explicitly refer to implicitly-declared special member functions. [ Example: A program may explicitly call, take the address of, or form a pointer to member to an implicitly-declared special member function. struct A { }; struct B : A { B& operator=(const B &) ; }; B& B: : operator=(const B& s) { this->A: : operator=(s) ; return *this; }

3

// implicitly declared A: : operator=

// well formed

— end example ] [ Note: The special member functions affect the way obj ects of class type are created, copied, moved, and destroyed, and how values can be converted to values of other types. Often such special member functions are called implicitly. — end note ]

4

Special member functions obey the usual access rules (Clause 14) . [ Example: Declaring a constructor protected ensures that only derived classes and friends can create obj ects using it. — end example ]

5

For a class, its non-static data members, its non-virtual direct base classes, and, if the class is not abstract (13.4) , its virtual base classes are called its potentially constructed subobjects. 1 5. 1

1

Constructors

[class. ctor]

Constructors do not have names. In a declaration of a constructor, the tor (11.3.5) of the form

declarator is a function declara-

ptr-declarator ( parameter-declaration-clause ) noexcept-specifieropt attribute-specifier-seqopt

where the ptr-declarator consists solely of an id-expression , an optional attribute-specifier-seq , and optional surrounding parentheses, and the id-expression has one of the following forms: (1. 1 )

— in a member-declaration that belongs to the member-specification of a class but is not a friend declaration (14.3) , the id-expression is the inj ected-class-name (Clause 12) of the immediately-enclosing class;

(1. 2)

— in a member-declaration that belongs to the member-specification of a class template but is not a friend declaration, the id-expression is a class-name that names the current instantiation (17.6.2.1) of the immediately-enclosing class template; or

(1. 3)

— in a declaration at namespace scope or in a friend declaration, the names a constructor (6.4.3.1) . The

id-expression is a qualified-id that

class-name shall not be a typedef-name . In a constructor declaration, each decl-specifier in the optional

§ 15.1

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decl-specifier-seq shall be friend, inline , explicit , or constexpr . [ Example: struct S { S() ; };

// declares the constructor

S: : S() { }

// defines the constructor

— end example ] 2

A constructor is used to initialize objects of its class type. Because constructors do not have names, they are never found during name lookup; however an explicit type conversion using the functional notation (8.2.3) will cause a constructor to be called to initialize an obj ect. [ Note: For initialization of obj ects of class type see 15.6. — end note ]

3

A constructor can be invoked for a const , volatile or const volatile obj ect. const and volatile semantics (10.1.7.1) are not applied on an obj ect under construction. They come into effect when the constructor for the most derived obj ect (4.5) ends.

4

A default constructor for a class X is a constructor of class X for which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters) . If there is no user-declared constructor for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted (11.4) . An implicitly-declared default constructor is an inline public member of its class.

5

A defaulted default constructor for class X is defined as deleted if:

(5. 1 )

— X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,

(5. 2)

— X is a non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer,

(5. 3)

— any non-static data member with no default member initializer (12.2) is of reference type,

(5. 4)

— any non-variant non-static data member of const-qualified type (or array thereof) with no equal-initializer does not have a user-provided default constructor,

(5. 5)

— X is a union and all of its variant members are of const-qualified type (or array thereof) ,

(5. 6)

— X is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof) ,

(5. 7)

— any potentially constructed subobj ect, except for a non-static data member with a brace-or-equalinitializer, has class type M (or array thereof) and either M has no default constructor or overload resolution (16.3) as applied to find M’s corresponding constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor, or

(5. 8)

— any potentially constructed subobj ect has a type with a destructor that is deleted or inaccessible from the defaulted default constructor.

6

A default constructor is

brace-or-

trivial if it is not user-provided and if:

(6. 1 )

— its class has no virtual functions (13.3) and no virtual base classes (13.1) , and

(6. 2)

— no non-static data member of its class has a default member initializer (12.2) , and

(6. 3)

— all the direct base classes of its class have trivial default constructors, and

(6. 4)

— for all the non-static data members of its class that are of class type (or array thereof) , each such class has a trivial default constructor. Otherwise, the default constructor is

7

non-trivial.

A default constructor that is defaulted and not defined as deleted is § 15.1

implicitly defined when it is odr-used (6.2)

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to create an obj ect of its class type (4.5) or when it is explicitly defaulted after its first declaration. The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer (15.6.2) and an empty compoundstatement. If that user-written default constructor would be ill-formed, the program is ill-formed. If that user-written default constructor would satisfy the requirements of a constexpr constructor (10.1.5) , the implicitly-defined default constructor is constexpr . Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members shall have been implicitly defined. [ Note: An implicitly-declared default constructor has an exception specification (18.4) . An explicitly-defaulted definition might have an implicit exception specification, see 11.4. — end note ] 8

Default constructors are called implicitly to create class obj ects of static, thread, or automatic storage duration (6.7.1, 6.7.2, 6.7.3) defined without an initializer (11.6) , are called to create class obj ects of dynamic storage duration (6.7.4) created by a new-expression in which the new-initializer is omitted (8.3.4) , or are called when the explicit type conversion syntax (8.2.3) is used. A program is ill-formed if the default constructor for an obj ect is implicitly used and the constructor is not accessible (Clause 14) .

9

[ Note: 15.6.2 describes the order in which constructors for base classes and non-static data members are called and describes how arguments can be specified for the calls to these constructors. — end note ]

10

A return statement in the body of a constructor shall not specify a return value. The address of a constructor shall not be taken.

11

A functional notation type conversion (8.2.3) can be used to create new obj ects of its type. [ Note: The syntax looks like an explicit call of the constructor. — end note ] [ Example: complex zz = complex(1, 2. 3) ; cprint( complex(7. 8, 1. 2) ) ;

— end example ] 12

13

14

An obj ect created in this way is unnamed. [ Note: 15.2 describes the lifetime of temporary obj ects. — end note ] [ Note: Explicit constructor calls do not yield lvalues, see 6.10. — end note ] [ Note: Some language constructs have special semantics when used during construction; see 15.6.2 and 15.7. — end note ] During the construction of an obj ect, if the value of the obj ect or any of its subobj ects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor’s this pointer, the value of the obj ect or subobj ect thus obtained is unspecified. [ Example: struct C; void no_opt(C*) ; struct C { int c; C() : c(0) { no_opt(this) ; } }; const C cobj ; void no_opt(C* cptr) { int i = cobj . c * 100; cptr->c = 1; cout ~ B() ; B_ptr-> ~ B_alias() ; B_ptr->B_alias: : ~ B() ; B_ptr->B_alias: : ~ B_alias() ; }

// calls // calls // calls // calls // calls

B ’s D ’s D ’s B ’s B ’s

destructor destructor destructor destructor destructor

— end example ] [ Note: An explicit destructor call must always be written using a member access operator (8.2.5) or a qualified-id (8.1) ; in particular, the unary-expression ~ X() in a member function is not an explicit destructor call (8.3.1) . — end note ] 15

[ Note: Explicit calls of destructors are rarely needed. One use of such calls is for obj ects placed at specific addresses using a placement new-expression . Such use of explicit placement and destruction of objects can be necessary to cope with dedicated hardware resources and for writing memory management facilities. For example, void* operator new(std: : size_t, void* p) { return p; } struct X { X(int) ; ~ X() ; }; void f(X* p) ;

§ 15.4

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void g() { // rare, specialized use: char* buf = new char[sizeof(X) ] ; X* p = new(buf) X(222) ; // use buf[] and initialize f(p) ; // cleanup p->X: : ~ X() ; }

— end note ] 16

Once a destructor is invoked for an obj ect, the obj ect no longer exists; the behavior is undefined if the destructor is invoked for an obj ect whose lifetime has ended (6.8) . [ Example: If the destructor for an automatic obj ect is explicitly invoked, and the block is subsequently left in a manner that would ordinarily invoke implicit destruction of the obj ect, the behavior is undefined. — end example ]

17

[ Note: The notation for explicit call of a destructor can be used for any scalar type name (8.2.4) . Allowing this makes it possible to write code without having to know if a destructor exists for a given type. For example: typedef int I; I* p; p->I: : ~ I() ;

— end note ] 1 5. 5 1 2

Free store

[class. free]

Any allocation function for a class T is a static member (even if not explicitly declared static ) . [ Example: class Arena; struct B { void* operator new(std: : size_t, Arena*) ; }; struct D1 : B { }; Arena* ap; void foo(int i) { new (ap) D1; new D1[i] ; new D1; }

// calls B: : operator new(std: : size_t, Arena*) // calls : : operator new[] (std: : size_t) // ill-formed: : : operator new(std: : size_t) hidden

— end example ] 3

When an obj ect is deleted with a delete-expression (8.3.5) , a deallocation function ( operator delete() for non-array obj ects or operator delete[] () for arrays) is (implicitly) called to reclaim the storage occupied by the obj ect (6.7.4.2) .

4

Class-specific deallocation function lookup is a part of general deallocation function lookup (8.3.5) and occurs as follows. If the delete-expression is used to deallocate a class obj ect whose static type has a virtual destructor, the deallocation function is the one selected at the point of definition of the dynamic type’s virtual destructor (15.4) . 118 Otherwise, if the delete-expression is used to deallocate an obj ect of class T or array thereof, the static and dynamic types of the obj ect shall be identical and the deallocation function’s name is looked up in the scope of T . If this lookup fails to find the name, general deallocation function 118)

A similar provision is not needed for t he array version of

operator delete

b ecause 8. 3. 5 requires t hat in t his sit uat ion, t he

st at ic typ e of t he ob j ect t o b e delet ed b e t he same as it s dynamic typ e.

§ 15.5

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lo okup ( 8 . 3 . 5 ) placement

5

cont inues .

If t he result

deallo cat ion funct ion,

of t he lo okup is ambiguous or inaccess ible,

or if t he lo okup select s a

t he program is ill- formed.

Any deallo cat ion funct ion for a class

X

is a st at ic memb er ( even if not explicit ly declared

static

).

[

Example:

class X { void operator delete(void*) ; void operator delete[] (void*, std: : size_t) ; }; class Y { void operator delete(void*, std: : size_t) ; void operator delete[] (void*) ; };

— end example 6

S ince

memb er

when t he

]

allo cat ion

and

cast-expression

of a

deallo cat ion

funct ions

delete-expression

are

refers

static

t hey

t o an ob j ect

funct ion act ually called is lo oked up in t he scop e of t he class t hat dest ruct or is virt ual,

t he effect

is t he same.

cannot

of class

be

typ e,

virt ual.

[

Note:

However,

b ecause t he deallo cat ion

is t he dynamic typ e of t he ob j ect ,

if t he

For example,

struct B { virtual ~ B() ; void operator delete(void*, std: : size_t) ; }; struct D : B { void operator delete(void*) ; }; void f() { B* bp = new D; delete bp; } Here,

// 1: uses D: : operator delete(void*)

st orage for t he non- array ob j ect of class

when t he

D

— end note Note: cast-expression delete-expression

dest ruct or.

]

[

is deallo cat ed by

D: : operator delete()

,

due t o t he virt ual

Virt ual dest ruct ors have no effect on t he deallo cat ion funct ion act ually called

of a

refers t o an array of ob j ect s of clas s typ e.

For example,

struct B { virtual ~ B() ; void operator delete[] (void*, std: : size_t) ; }; struct D : B { void operator delete[] (void*, std: : size_t) ; }; void f(int i) { D* dp = new D[i] ; delete [] dp; // uses D: : operator delete[] (void*, std: : size_t) B* bp = new D[i] ; delete[] bp; // undefined behavior }

— end note 7

]

Access t o t he deallo cat ion funct ion is checked st at ically.

§ 1 5. 5

© IS O / IEC

Hence,

even t hough a different

2 0 1 7 – All right s reserved

one might

act ually

290

ISO/IEC 14882: 2017(E)

be executed, the statically visible deallocation function is required to be accessible. [ Example: For the call on line “// 1” above, if B: : operator delete() had been private , the delete expression would have been ill-formed. — end example ] 8

[ Note: If a deallocation function has no explicit tion (18.4) . — end note ] 1 5. 6

noexcept-specifier, it has a non-throwing exception specifica-

Initialization

[class. init]

1

When no initializer is specified for an obj ect of (possibly cv-qualified) class type (or array thereof) , or the initializer has the form () , the obj ect is initialized as specified in 11.6.

2

An obj ect of class type (or array thereof) can be explicitly initialized; see 15.6.1 and 15.6.2.

3

When an array of class obj ects is initialized (either explicitly or implicitly) and the elements are initialized by constructor, the constructor shall be called for each element of the array, following the subscript order; see 11.3.4. [ Note: Destructors for the array elements are called in reverse order of their construction. — end note ] 1 5. 6. 1

1

Explicit initialization

[class. expl. init]

An obj ect of class type can be initialized with a parenthesized expression-list, where the expression-list is construed as an argument list for a constructor that is called to initialize the obj ect. Alternatively, a single assignment-expression can be specified as an initializer using the = form of initialization. Either direct-initialization semantics or copy-initialization semantics apply; see 11.6. [ Example: struct complex { complex() ; complex(double) ; complex(double, double) ; }; complex sqrt(complex, complex) ; complex a(1) ; complex b = a; complex c = complex(1, 2) ; complex complex complex complex

2

d = sqrt(b, c) ; e; f = 3; g = { 1, 2 };

// initialize by a call of complex(double) // initialize by a copy of a // construct complex(1, 2) using complex(double, double) , // copy/move it into c // call sqrt(complex, complex) and copy/move the result into d // initialize by a call of complex() // construct complex(3) using complex(double) , copy/move it into f // initialize by a call of complex(double, double)

— end example ] [ Note: Overloading of the assignment operator (16.5.3) has no effect on initialization. — end note ] An object of class type can also be initialized by a braced-init-list . List-initialization semantics apply; see 11.6 and 11.6.4. [ Example: complex v[6] = { 1, complex(1, 2) , complex() , 2 };

Here, complex: : complex(double) is called for the initialization of v[0] and v[3] , complex: : complex( double, double) is called for the initialization of v[1] , complex: : complex() is called for the initialization v[2] , v[4] , and v[5] . For another example, struct X { int i; float f; complex c; } x = { 99, 88. 8, 77. 7 };

§ 15.6.1

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x. i

Here,

x. f x. c — end example

is init ializ ed wit h 9 9 ,

init ializ at ion of even

if t he

note Note:

is init ializ ed wit h 8 8 . 8 ,

.

aggregat e

has

memb ers

]

[

of a

Note:

and

complex: : complex(double)

initializer-list

B races can b e elided in t he

clas s

typ e

wit h

user- defined

typ e

conversions;

is called for t he

for any aggregat e, see 1 1 . 6 . 1 .

— end

]

3

[

4

[

Note:

t he

a

class

typ e

wit h

no

initializer

default

const ruct or,

any

declarat ion

of an

ob j ect

of typ e

— end note

is explicit ly sp ecified ( see 1 5 . 6 and 1 1 . 6) .

T

( or

array

]

The order in which ob j ect s wit h st at ic or t hread st orage durat ion are init ializ ed is des crib ed in 6 . 6 . 3

1 5. 6. 2 In

is

is ill- formed if no

and 9 . 7.

1

T

If

t hereof)

— end note

]

Initializing bases and members definit ion

of a

const ruct or

for

a

class,

[class. base. init]

init ializ ers

non- st at ic dat a memb ers can b e sp ecified by a

for

ctor-initializer

direct ,

and

virt ual

b ase

class

sub ob j ect s

and

which has t he form

ctor-initializer: : mem-initializer-list mem-initializer-list : mem-initializer . . . opt mem-initializer-list , mem-initializer . . . opt mem-initializer: mem-initializer-id ( expression-listopt ) mem-initializer-id braced-init-list mem-initializer-id : class-or-decltype identifier 2

In a and,

mem-initializer-id if not

If t he

an init ial unqualified

found in t hat

const ruct or’ s

class

scop e,

it

cont ains

a

memb er

mem-initializer-id mem-initializer-id — end note mem-initializer-id class,

a

naming t he

t he class memb er. ]

A

3

or a direct

mem-initializer-list Example:

A [

is lo oked up in t he scop e of t he const ruct or’ s class

wit h

memb er

or

t he

same

b ase

name

class

and

as

a

direct

comp osed

or

virt ual

of a s ingle

base

class

ident ifier

[

Note:

of t he

refers

to

for t he hidden b ase class may b e sp ecified using a qualified name.

Unless t he

const ruct or’ s class,

identifier

is lo oked up in t he s cop e cont aining t he const ruct or’ s definit ion.

names t he const ruct or’ s class,

or virt ual base of t hat

can init ializ e a base class using any

a non- st at ic dat a memb er of t he

mem-initializer class-or-decltype

class,

t he

t hat

is ill- formed.

denot es t hat

base class typ e.

struct A { A() ; }; typedef A global_A; struct B { }; struct C: public A, public B { C() ; }; C: : C() : global_A() { } // mem-initializer for base A

— end example mem-initializer-id ]

4

If a

virt ual base clas s,

t he

is ambiguous b ecause it designat es b ot h a direct non- virt ual base class and an inherit ed

mem-initializer

is ill- formed.

[

Example:

struct A { A() ; }; struct B: public virtual A { }; struct C: public A, public B { C() ; }; C: : C() : A() { } // ill-formed: which A ?

— end example ctor-initializer mem-initializer ]

5

A

t han one

§ 1 5. 6. 2

may init ializ e a variant memb er of t he const ruct or’ s class. for t he same memb er or for t he same bas e class,

© IS O / IEC

2 0 1 7 – All right s reserved

If a t he

ctor-initializer ctor-initializer

sp ecifies more is ill- formed.

292

ISO/IEC 14882: 2017(E)

6

A mem-initializer-list can delegate to another constructor of the constructor’s class using any class-or-decltype that denotes the constructor’s class itself. If a mem-initializer-id designates the constructor’s class, it shall be the only mem-initializer ; the constructor is a delegating constructor , and the constructor selected by the mem-initializer is the target constructor. The target constructor is selected by overload resolution. Once the target constructor returns, the body of the delegating constructor is executed. If a constructor delegates to itself directly or indirectly, the program is ill-formed, no diagnostic required. [ Example: struct C { C( int ) { } C() : C(42) { } C( char c ) : C(42. 0) { } C( double d ) : C(’ a’ ) { } };

7

// #1: // #2: // #3: // #4:

non-delegating constructor delegates to #1 ill-formed due to recursion with #4 ill-formed due to recursion with #3

— end example ] The expression-list or braced-init-list in a mem-initializer is used to initialize the designated subobject (or, in

the case of a delegating constructor, the complete class object) according to the initialization rules of 11.6 for direct-initialization. [ Example: struct B1 { B1(int) ; /* . . . */ }; struct B2 { B2(int) ; /* . . . */ }; struct D : B1, B2 { D(int) ; B1 b; const int c; }; D: : D(int a) : B2(a+1) , B1(a+2) , c(a+3) , b(a+4) { /* . . . */ } D d(10) ;

— end example ] [ Note: The initialization performed by each mem-initializer constitutes a full-expression (4.6) . Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization. — end note ] A mem-initializer where the mem-initializer-id denotes a virtual base class is ignored during execution of a constructor of any class that is not the most derived class. 8

A temporary expression bound to a reference member in a struct A { A() : v(42) { } const int& v; };

mem-initializer is ill-formed. [ Example:

// error

— end example ] 9

(9. 1 )

In a non-delegating constructor, if a given potentially constructed subobj ect is not designated by a meminitializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer) , then — if the entity is a non-static data member that has a default member initializer (12.2) and either

(9. 1. 1 )

— the constructor’s class is a union (12.3) , and no other variant member of that union is designated by a mem-initializer-id or

(9. 1. 2)

— the constructor’s class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id, the entity is initialized from its default member initializer as specified in 11.6;

(9. 2)

— otherwise, if the entity is an anonymous union or a variant member (12.3.1) , no initialization is performed; § 15.6.2

© ISO/IEC 2017 – All rights reserved

293

ISO/IEC 1 4882:201 7(E)

(9. 3)

— otherwise, the entity is default-initialized (11.6) . [ Note: An abstract class (13.4) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted. — end note ] An attempt to initialize more than one non-static data member of a union renders the program ill-formed. [ Note: After the call to a constructor for class X for an obj ect with automatic or dynamic storage duration has completed, if the constructor was not invoked as part of value-initialization and a member of X is neither initialized nor given a value during execution of the compound-statement of the body of the constructor, the member has an indeterminate value. — end note ] [ Example: struct A { A() ; }; struct B { B(int) ; }; struct C { C() { } A a; const B b; int i; int j = 5; };

// initializes members as follows: // OK: calls A: : A() // error: B has no default constructor // OK: i has indeterminate value // OK: j has the value 5

— end example ] 10

If a given non-static data member has both a default member initializer and a mem-initializer , the initialization specified by the mem-initializer is performed, and the non-static data member’s default member initializer is ignored. [ Example: Given struct A { int i = /∗ some integer expression with side effects ∗/ ; A(int arg) : i(arg) { } // ... };

the A(int) constructor will simply initialize i to the value of arg, and the side effects in i ’s default member initializer will not take place. — end example ] 11

A temporary expression bound to a reference member from a default member initializer is ill-formed. [ Example: struct A { A() = default; A(int v) : v(v) { } const int& v = 42; }; A a1; A a2(1) ;

// OK // OK // OK // error: ill-formed binding of temporary to reference // OK, unfortunately

— end example ] 12

In a non-delegating constructor, the destructor for each potentially constructed subobj ect of class type is potentially invoked (15.4) . [ Note: This provision ensures that destructors can be called for fully-constructed subobj ects in case an exception is thrown (18.2) . — end note ]

13

In a non-delegating constructor, initialization proceeds in the following order:

§ 15.6.2

© ISO/IEC 2017 – All rights reserved

294

ISO/IEC 14882: 2017(E)

(13. 1)

— First, and only for the constructor of the most derived class (4.5) , virtual base classes are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base classes in the derived class base-specifier-list .

(13. 2)

— Then, direct base classes are initialized in declaration order as they appear in the (regardless of the order of the mem-initializer s) .

(13. 3)

— Then, non-static data members are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializer s) .

(13. 4)

— Finally, the

base-specifier-list

compound-statement of the constructor body is executed.

[ Note: The declaration order is mandated to ensure that base and member subobj ects are destroyed in the reverse order of initialization. — end note ] 14

[ Example: struct V { V() ; V(int) ; }; struct A : virtual V { A() ; A(int) ; }; struct B : virtual V { B() ; B(int) ; }; struct C : A, B, virtual V { C() ; C(int) ; }; A: : A(int i) : V(i) { /* . . . */ } B: : B(int i) { /* . . . */ } C: : C(int i) { /* . . . */ } V A B C

15

v(1) ; a(2) ; b(3) ; c(4) ;

// use // use // use // use

V(int) V(int) V() V()

— end example ] Names in the expression-list or braced-init-list of a mem-initializer are evaluated in the scope of the constructor for which the mem-initializer is specified. [ Example: class X { int a; int b; int i; int j ; public: const int& r; X(int i) : r(a) , b(i) , i(i) , j (this->i) { }

§ 15.6.2

© ISO/IEC 2017 – All rights reserved

295

ISO/IEC 14882: 2017(E)

}; init ializ es

X: : r

t o refer t o

X: : a

, init ializ es

X: : b i

wit h t he value of t he const ruct or paramet er each t ime an ob j ect

of class

X

is creat ed.

in t he scop e of t he const ruct or, refer t o t he ob j ect

16

t he

this

,

and init ializ es

— end example

cast or

an ob j ect

( 8 . 2 . 7) .

indirect ly

Note:

1 3. 3)

a

ctor-initializer Example: )

b efore

all

, init ializ es

X: : i

t his t akes place

mem-initializer expression-list mem-initializer

B ecause t he

are evaluat ed

of a

typeid

to

t he

s

for

under const ruct ion.

op erat or ( 8 . 2 . 8 )

ctor-initializer mem-initializer

if t hese op erat ions are p erformed in a

program has undefined b ehavior.

;

can b e called for an ob j ect

under const ruct ion can b e t he op erand of t he

However, from

[

wit h t he value of

]

Memb er funct ions ( including virt ual memb er funct ions, S imilarly,

]

X: : j

p oint er can b e used in t he

— end note

b eing init ializ ed.

i X: : i

wit h t he value of t he const ruct or paramet er

or of a

dynamic_-

( or in a funct ion called direct ly

b ase

classes

have

complet ed,

t he

[

class A { public: A(int) ; }; class B : public A { int j ; public: int f() ; B() : A(f() ) , // undefined: calls member function but base A not yet initialized j (f() ) { } // well-defined: bases are all initialized }; class C { public: C(int) ; }; class D : public B, C { int i; public: D() : C(f() ) , // undefined: calls member function but base C not yet initialized i(f() ) { } // well-defined: bases are all initialized };

17

— end example Note:

[

]

1 5 . 7 describ es t he result

t he well- defined cases;

note mem-initializer

t hat

is,

of virt ual funct ion calls, describ es t he

typeid

and

polymorphic behavior

dynamic_cast

of an ob j ect

s during const ruct ion for

under const ruct ion.

— end

]

18

A

followed by an ellipsis is a pack expansion (1 7. 5 . 3 )

by a pack expansion in t he

base-specifier-list

for t he class.

[

t hat init ializ es t he base classes s p ecified

Example:

template class X : public Mixins. . . { public: X(const Mixins&. . . mixins) : Mixins(mixins) . . . { } };

— end example

§ 1 5. 6. 2

]

© IS O / IEC

2 0 1 7 – All right s reserved

296

ISO/IEC 1 4882:201 7(E)

1 5. 6. 3 1

When

a

Initialization by inherited constructor const ruct or

for

typ e

B

is

const ruct or was inherit ed (1 0 . 3 . 3 ) ) , init ializ e t he t he

B

D

ob j ect

sub ob j ect

invoked

to

init ializ e

an

[class. inhctor. init] ob j ect

of a

different

typ e

init ializ at ion pro ceeds as if a default ed default

and each b ase class sub ob j ect

D

( t hat

is,

from which t he const ruct or was inherit ed,

is init ializ ed by t he invo cat ion of t he inherit ed const ruct or.

when

t he

const ruct or were used t o except

t hat

The complet e init ializ at ion is

considered t o b e a single funct ion call; in part icular, t he init ializ at ion of t he inherit ed const ruct or’ s paramet ers is s equenced b efore t he init ializ at ion of any part

of t he

D

ob j ect .

[

Example:

struct B1 { B1(int, . . . ) { } }; struct B2 { B2(double) { } }; int get() ; struct D1 : B1 { using B1: : B1; int x; int y = get() ; };

// inherits B1(int, . . . )

void test() { D1 d(2, 3, 4) ;

// OK: B1 is initialized by calling B1(2, 3, 4) , // then d. x is default-initialized (no initialization is performed), // then d. y is initialized by calling get() // error: D1 has a deleted default constructor

D1 e; } struct D2 : B2 { using B2: : B2; B1 b; };

// error: B1 has a deleted default constructor

D2 f(1. 0) ; struct W struct X struct Y struct Z Z z(0) ;

{ : : :

W(int) ; }; virtual W { using W: : W; X() = delete; }; X { using X: : X; }; Y, virtual W { using Y: : Y; }; // OK: initialization of Y does not invoke default constructor of X

template struct Log : T { using T: : T; // inherits all constructors from class T ~ Log() { std: : clog f() ; }

— end example 4

The

typeid

// calls V: : f , not A: : f // calls B: : g, not D: : g // v is base of B , the call is well-defined, calls B: : g // undefined behavior, a’s type not a base of B

]

op erat or ( 8 . 2 . 8)

a const ruct or ( including t he or in a des t ruct or, op erand of ob j ect

typeid

can b e us ed during cons t ruct ion or dest ruct ion (1 5 . 6 . 2 ) .

mem-initializer

When

typeid

is used in

or default memb er init ializ er (1 2 . 2 ) for a non- st at ic dat a memb er)

or used in a funct ion called ( direct ly or indirect ly)

from a const ruct or or dest ruct or,

refers t o t he ob j ect under const ruct ion or dest ruct ion,

represent ing t he const ruct or or des t ruct or’ s class.

typeid typeid

If t he op erand of

yields t he

if t he

std: : type_info

refers t o t he ob j ect

under

const ruct ion or des t ruct ion and t he st at ic typ e of t he op erand is neit her t he const ruct or or des t ruct or’ s class nor one of it s bas es,

5

dynamic_cast

t he b ehavior is undefined.

s ( 8 . 2 . 7) can b e us ed during const ruct ion or dest ruct ion (1 5 . 6 . 2 ) .

in a const ruct or ( including t he or in a dest ruct or, op erand of t he t o b e a most

not

or default

t hat

dynamic_cast

[

if t he

t his ob j ect is considered

has t he typ e of t he const ruct or or dest ruct or’ s class.

If t he op erand of t he

under cons t ruct ion or dest ruct ion and t he st at ic typ e of t he op erand is

of t he const ruct or or dest ruct or’ s own clas s or one of it s bases,

result s in undefined b ehavior.

is us ed

from a const ruct or or dest ruct or,

refers t o t he ob j ect under const ruct ion or dest ruct ion,

refers t o t he ob j ect

a p oint er t o or ob j ect

When a

memb er init ializ er for a non- st at ic dat a memb er)

or used in a funct ion called ( direct ly or indirect ly)

dynamic_cast

derived ob j ect

dynamic_cast

mem-initializer

Example:

t he

dynamic_cast

struct V { virtual void f() ; }; struct A : virtual V { }; struct B : virtual V { B(V*, A*) ; }; struct D : A, B { D() : B((A*) this, this) { } }; B: : B(V* v, A* a) { typeid(*this) ; typeid(*v) ; typeid(*a) ; dynamic_cast(v) ; dynamic_cast(a) ; }

§ 1 5. 7

// type_info for B // well-defined: *v has type V, a base of B yields type_info for B // undefined behavior: type A not a base of B // well-defined: v of type V* , V base of B results in B* // undefined behavior, a has type A* , A not a base of B

© IS O / IEC

2 0 1 7 – All right s reserved

300

ISO/IEC 14882: 2017(E)

— end example ] 1 5. 8

Copying and moving class obj ects

[class. copy]

1

A class obj ect can be copied or moved in two ways: by initialization (15.1, 11.6) , including for function argument passing (8.2.2) and for function value return (9.6.3) ; and by assignment (8.18) . Conceptually, these two operations are implemented by a copy/move constructor (15.1) and copy/move assignment operator (16.5.3) .

2

A program is ill-formed if the copy/move constructor or the copy/move assignment operator for an obj ect is implicitly odr-used and the special member function is not accessible (Clause 14) . [ Note: Copying/moving one obj ect into another using the copy/move constructor or the copy/move assignment operator does not change the layout or size of either obj ect. — end note ] 1 5. 8. 1

1

Copy/move constructors

[class. copy. ctor]

A non-template constructor for class X is a copy constructor if its first parameter is of type X& , const X& , volatile X& or const volatile X& , and either there are no other parameters or else all other parameters have default arguments (11.3.6) . [ Example: X: : X(const X&) and X: : X(X&, int=1) are copy constructors. struct X { X(int) ; X(const X&, int = 1) ; }; X a(1) ; // calls X(int) ; X b(a, 0) ; // calls X(const X&, int) ; X c = b; // calls X(const X&, int) ;

— end example ] 2

A non-template constructor for class X is a move constructor if its first parameter is of type X&& , const X&& , volatile X&& , or const volatile X&& , and either there are no other parameters or else all other parameters have default arguments (11.3.6) . [ Example: Y: : Y(Y&&) is a move constructor. struct Y { Y(const Y&) ; Y(Y&&) ; }; extern Y f(int) ; Y d(f(1) ) ; Y e = d;

3

— end example ] [ Note: All forms of copy/move constructor may be declared for a class. [ Example: struct X { X(const X&) ; X(X&) ; X(X&&) ; X(const X&&) ; };

4

// calls Y(Y&&) // calls Y(const Y&)

// OK // OK, but possibly not sensible

— end example ] — end note ] [ Note: If a class X only has a copy constructor with a parameter of type X& , an initializer of type const X or volatile X cannot initialize an obj ect of type (possibly cv-qualified) X. [ Example: struct X { X() ; X(X&) ;

§ 15.8.1

// default constructor // copy constructor with a non-const parameter

© ISO/IEC 2017 – All rights reserved

301

ISO/IEC 14882: 2017(E)

}; const X cx; X x = cx;

// error: X: : X(X&) cannot copy cx into x

— end example ] — end note ] 5

A declaration of a constructor for a class X is ill-formed if its first parameter is of type (optionally cv-qualified) X and either there are no other parameters or else all other parameters have default arguments. A member function template is never instantiated to produce such a constructor signature. [ Example: struct S { template S(T) ; S() ; }; S g; void h() { S a(g) ; }

// does not instantiate the member template to produce S: : S(S) ; // uses the implicitly declared copy constructor

— end example ] 6

If the class definition does not explicitly declare a copy constructor, a non-explicit one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (11.4) . The latter case is deprecated if the class has a user-declared copy assignment operator or a user-declared destructor.

7

The implicitly-declared copy constructor for a class X will have the form X: : X(const X&)

if each potentially constructed subobject of a class type M (or array thereof) has a copy constructor whose first parameter is of type const M& or const volatile M& . 119 Otherwise, the implicitly-declared copy constructor will have the form X: : X(X&) 8

If the definition of a class X does not explicitly declare a move constructor, a non-explicit one will be implicitly declared as defaulted if and only if

(8. 1 )

— X does not have a user-declared copy constructor,

(8. 2)

— X does not have a user-declared copy assignment operator,

(8. 3)

— X does not have a user-declared move assignment operator, and

(8. 4)

— X does not have a user-declared destructor. [ Note: When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor may instead invoke a copy constructor. — end note ]

9

The implicitly-declared move constructor for class X will have the form X: : X(X&&)

10

(10. 1)

An implicitly-declared copy/move constructor is an inline public member of its class. copy/move constructor for a class X is defined as deleted (11.4.3) if X has:

A defaulted

— a variant member with a non-trivial corresponding constructor and X is a union-like class, 119)

This implies t hat t he reference paramet er of t he implicit ly- declared copy const ruct or cannot bind t o a

volatile

lvalue;

see C . 1 . 9.

§ 15.8.1

© ISO/IEC 2017 – All rights reserved

302

ISO/IEC 14882: 2017(E)

(10. 2)

— a potentially constructed subobj ect type M (or array thereof) that cannot be copied/moved because overload resolution (16.3) , as applied to find M’s corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor,

(10. 3)

— any potentially constructed subobj ect of a type with a destructor that is deleted or inaccessible from the defaulted constructor, or,

(10. 4)

— for the copy constructor, a non-static data member of rvalue reference type. A defaulted move constructor that is defined as deleted is ignored by overload resolution (16.3, 16.4) . [ Note: A deleted move constructor would otherwise interfere with initialization from an rvalue which can use the copy constructor instead. — end note ]

11

A copy/move constructor for class X is trivial if it is not user-provided and if:

(11. 1)

— class X has no virtual functions (13.3) and no virtual base classes (13.1) , and

(11. 2)

— the constructor selected to copy/move each direct base class subobj ect is trivial, and

(11. 3)

— for each non-static data member of X that is of class type (or array thereof) , the constructor selected to copy/move that member is trivial; otherwise the copy/move constructor is

non-trivial.

12

A copy/move constructor that is defaulted and not defined as deleted is implicitly defined if it is odrused (6.2) or when it is explicitly defaulted after its first declaration. [ Note: The copy/move constructor is implicitly defined even if the implementation elided its odr-use (6.2, 15.2) . — end note ] If the implicitlydefined constructor would satisfy the requirements of a constexpr constructor (10.1.5) , the implicitly-defined constructor is constexpr .

13

Before the defaulted copy/move constructor for a class is implicitly defined, all non-user-provided copy/move constructors for its potentially constructed subobj ects shall have been implicitly defined. [ Note: An implicitly-declared copy/move constructor has an implied exception specification (18.4) . — end note ]

14

The implicitly-defined copy/move constructor for a non-union class X performs a memberwise copy/move of its bases and members. [ Note: Default member initializers of non-static data members are ignored. See also the example in 15.6.2. — end note ] The order of initialization is the same as the order of initialization of bases and members in a user-defined constructor (see 15.6.2) . Let x be either the parameter of the constructor or, for the move constructor, an xvalue referring to the parameter. Each base or non-static data member is copied/moved in the manner appropriate to its type:

(14. 1)

— if the member is an array, each element is direct-initialized with the corresponding subobj ect of x;

(14. 2)

— if a member m has rvalue reference type T&& , it is direct-initialized with static_cast(x. m) ;

(14. 3)

— otherwise, the base or member is direct-initialized with the corresponding base or member of x. Virtual base class subobj ects shall be initialized only once by the implicitly-defined copy/move constructor (see 15.6.2) .

15

The implicitly-defined copy/move constructor for a union X copies the obj ect representation (6.9) of X. 1 5. 8. 2

1

Copy/move assignment operator

[class. copy. assign]

A user-declared copy assignment operator X: : operator= is a non-static non-template member function of class X with exactly one parameter of type X , X& , const X& , volatile X& or const volatile X& . 120 [ Note: An overloaded assignment operator must be declared to have only one parameter; see 16.5.3. — end note ] [ Note: More than one form of copy assignment operator may be declared for a class. — end note ] [ Note: If 120)

B ecause a t emplat e assignment

op erat or or an assignment

op erat or t aking an rvalue reference paramet er is never a

copy assignment op erat or, t he presence of such an assignment op erat or do es not suppress t he implicit declarat ion of a copy assignment op erat or.

S uch assignment op erat ors part icipat e in overload resolut ion wit h ot her assignment op erat ors, including

copy assignment op erat ors, and, if select ed, will b e used t o assign an ob j ect .

§ 15.8.2

© ISO/IEC 2017 – All rights reserved

303

ISO/IEC 14882: 2017(E)

a class X only has a copy assignment operator with a parameter of type X& , an expression of type const X cannot be assigned to an obj ect of type X . [ Example: struct X { X() ; X& operator=(X&) ; }; const X cx; X x; void f() { x = cx; // error: X: : operator=(X&) cannot assign cx into x }

— end example ] — end note ] 2

If the class definition does not explicitly declare a copy assignment operator, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy assignment operator is defined as deleted; otherwise, it is defined as defaulted (11.4) . The latter case is deprecated if the class has a user-declared copy constructor or a user-declared destructor. The implicitly-declared copy assignment operator for a class X will have the form X& X: : operator=(const X&)

if (2. 1 )

— each direct base class B of X has a copy assignment operator whose parameter is of type const B& , const volatile B& or B , and

(2. 2)

— for all the non-static data members of X that are of a class type M (or array thereof) , each such class type has a copy assignment operator whose parameter is of type const M& , const volatile M& or M. 1 21 Otherwise, the implicitly-declared copy assignment operator will have the form X& X: : operator=(X&)

3

A user-declared move assignment operator X: : operator= is a non-static non-template member function of class X with exactly one parameter of type X&& , const X&& , volatile X&& , or const volatile X&& . [ Note: An overloaded assignment operator must be declared to have only one parameter; see 16.5.3. — end note ] [ Note: More than one form of move assignment operator may be declared for a class. — end note ]

4

If the definition of a class X does not explicitly declare a move assignment operator, one will be implicitly declared as defaulted if and only if

(4. 1 )

— X does not have a user-declared copy constructor,

(4. 2)

— X does not have a user-declared move constructor,

(4. 3)

— X does not have a user-declared copy assignment operator, and

(4. 4)

— X does not have a user-declared destructor. [ Example: The class definition struct S { int a; S& operator=(const S&) = default; };

will not have a default move assignment operator implicitly declared because the copy assignment operator has been user-declared. The move assignment operator may be explicitly defaulted. struct S { 121 )

This implies t hat t he reference paramet er of t he implicit ly- declared copy assignment op erat or cannot bind t o a

volatile

lvalue; see C . 1 . 9.

§ 15.8.2

© ISO/IEC 2017 – All rights reserved

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int a; S& operator=(const S&) = default; S& operator=(S&&) = default; };

— end example ] 5

The implicitly-declared move assignment operator for a class X will have the form X& X: : operator=(X&&) ;

6

The implicitly-declared copy/move assignment operator for class X has the return type X& ; it returns the obj ect for which the assignment operator is invoked, that is, the obj ect assigned to. An implicitly-declared copy/move assignment operator is an inline public member of its class.

7

A defaulted copy/move assignment operator for class X is defined as deleted if X has:

(7. 1 )

— a variant member with a non-trivial corresponding assignment operator and X is a union-like class, or

(7. 2)

— a non-static data member of const non-class type (or array thereof) , or

(7. 3)

— a non-static data member of reference type, or

(7. 4)

— a direct non-static data member of class type M (or array thereof) or a direct base class M that cannot be copied/moved because overload resolution (16.3) , as applied to find M’s corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator. A defaulted move assignment operator that is defined as deleted is ignored by overload resolution (16.3, 16.4) .

8

Because a copy/move assignment operator is implicitly declared for a class if not declared by the user, a base class copy/move assignment operator is always hidden by the corresponding assignment operator of a derived class (16.5.3) . A using-declaration (10.3.3) that brings in from a base class an assignment operator with a parameter type that could be that of a copy/move assignment operator for the derived class is not considered an explicit declaration of such an operator and does not suppress the implicit declaration of the derived class operator; the operator introduced by the using-declaration is hidden by the implicitly-declared operator in the derived class.

9

A copy/move assignment operator for class X is trivial if it is not user-provided and if:

(9. 1 )

— class X has no virtual functions (13.3) and no virtual base classes (13.1) , and

(9. 2)

— the assignment operator selected to copy/move each direct base class subobj ect is trivial, and

(9. 3)

— for each non-static data member of X that is of class type (or array thereof) , the assignment operator selected to copy/move that member is trivial; otherwise the copy/move assignment operator is

10

non-trivial.

A copy/move assignment operator for a class X that is defaulted and not defined as deleted is implicitly defined when it is odr-used (6.2) (e.g., when it is selected by overload resolution to assign to an obj ect of its class type) or when it is explicitly defaulted after its first declaration. The implicitly-defined copy/move assignment operator is constexpr if

(10. 1)

— X is a literal type, and

(10. 2)

— the assignment operator selected to copy/move each direct base class subobject is a constexpr function, and

(10. 3)

— for each non-static data member of X that is of class type (or array thereof) , the assignment operator selected to copy/move that member is a constexpr function.

11

Before the defaulted copy/move assignment operator for a class is implicitly defined, all non-user-provided copy/move assignment operators for its direct base classes and its non-static data members shall have been § 15.8.2

© ISO/IEC 2017 – All rights reserved

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implicitly defined. [ Note: An implicitly-declared copy/move assignment operator has an implied exception specification (18.4) . — end note ] 12

The implicitly-defined copy/move assignment operator for a non-union class X performs memberwise copy/move assignment of its subobj ects. The direct base classes of X are assigned first, in the order of their declaration in the base-specifier-list, and then the immediate non-static data members of X are assigned, in the order in which they were declared in the class definition. Let x be either the parameter of the function or, for the move operator, an xvalue referring to the parameter. Each subobj ect is assigned in the manner appropriate to its type:

(12. 1)

— if the subobject is of class type, as if by a call to operator= with the subobject as the object expression and the corresponding subobject of x as a single function argument (as if by explicit qualification; that is, ignoring any possible virtual overriding functions in more derived classes) ;

(12. 2)

— if the subobj ect is an array, each element is assigned, in the manner appropriate to the element type;

(12. 3)

— if the subobj ect is of scalar type, the built-in assignment operator is used. It is unspecified whether subobj ects representing virtual base classes are assigned more than once by the implicitly-defined copy/move assignment operator. [ Example: struct struct struct struct

V A B C

{ : : :

}; virtual V { }; virtual V { }; B, A { };

It is unspecified whether the virtual base class subobj ect V is assigned twice by the implicitly-defined copy/move assignment operator for C . — end example ] 13

The implicitly-defined copy assignment operator for a union X copies the obj ect representation (6.9) of X. 1 5. 8. 3

Copy/move elision

[class. copy. elision]

1

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class obj ect, even if the constructor selected for the copy/move operation and/or the destructor for the obj ect have side effects. In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same obj ect. If the first parameter of the selected constructor is an rvalue reference to the obj ect’s type, the destruction of that obj ect occurs when the target would have been destroyed; otherwise, the destruction occurs at the later of the times when the two obj ects would have been destroyed without the optimization. 122 This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies) :

(1. 1 )

— in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic obj ect (other than a function parameter or a variable introduced by the exception-declaration of a handler (18.3) ) with the same type (ignoring cv-qualification) as the function return type, the copy/move operation can be omitted by constructing the automatic obj ect directly into the function call’s return obj ect

(1. 2)

— in a throw-expression (8.17) , when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one) , the copy/move operation from the operand to the exception obj ect (18.1) can be omitted by constructing the automatic obj ect directly into the exception obj ect

(1. 3)

— when the exception-declaration of an exception handler (Clause 18) declares an obj ect of the same type (except for cv-qualification) as the exception obj ect (18.1) , the copy operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the obj ect declared by the 122)

B ecause only one ob j ect is dest royed inst ead of two, and one copy/ move const ruct or is not execut ed, t here is st ill one ob j ect

dest royed for each one const ruct ed.

§ 15.8.3

© ISO/IEC 2017 – All rights reserved

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exception-declaration Note: — end note .

[

an lvalue.

There cannot

b e a move from t he except ion ob j ect

C opy elision is required where an expression is evaluat ed in a cont ext and in const ant

[

Note: — end note

init ializ at ion (6 . 6 . 2 ) .

evaluat ed in anot her cont ext .

2

b ecause it

is always

]

[

C opy elision might

not

requiring a const ant

expression (8 . 2 0 )

b e p erformed if t he same expression is

]

Example: class Thing { public: Thing() ; ~ Thing() ; Thing(const Thing&) ; }; Thing f() { Thing t; return t; } Thing t2 = f() ; struct A { void *p; constexpr A() : p(this) {} }; constexpr A g() { A a; return a; } constexpr A a; constexpr A b = g() ;

// well-formed, a. p points to a // well-formed, b. p points to b

void g() { A c = g() ; }

// well-formed, c. p may point to c or to an ephemeral temporary

Here t he crit eria for elision can eliminat e t he copying of t he lo cal aut omat ic ob j ect for t he funct ion call

f() ,

which is t he global ob j ect

b e viewed as direct ly init ializ ing t he global ob j ect exit .

Adding a move const ruct or t o

aut omat ic ob j ect

3 (3. 1 )

t2

t hat

Thing

is elided.

—

If t he

expression

return

( 8 . 1 7)

try-block

it

If t he first

t

can

program

is t he move const ruct ion from t he lo cal

b e used inst ead of a copy op erat ion:

id-expression parameter-declaration-clause

is a ( p ossibly parent hesiz ed)

,

t hat

names of t he

or

is t he name of a non- volat ile aut omat ic ob j ect

whos e scop e do es not

( ot her t han

ext end b eyond t he end of t he innermost

( if t here is one) ,

overload resolut ion t o select

§ 1 5. 8. 3

but

ob j ect

]

lambda-expression throw-expression

a funct ion or cat ch- clause paramet er)

by an rvalue.

( 9. 6. 3)

int o t he result

ob j ect ’ s dest ruct ion will o ccur at

a move op erat ion might

st at ement

enclosing funct ion or

if t he op erand of a

enclosing

and t hat

t

t he const ruct ion of t he lo cal ob j ect

wit h aut omat ic st orage durat ion declared in t he b o dy or

innermost

—

in a

Effect ively,

has t he same effect ,

— end example

In t he following copy- init ializ at ion cont ext s,

an ob j ect

(3. 2)

to

t2 . t2 ,

t he const ruct or for t he copy is first

overload resolut ion fails or was not

© IS O / IEC

p erformed as if t he ob j ect

p erformed,

were designat ed

or if t he typ e of t he firs t

2 0 1 7 – All right s reserved

paramet er

307

ISO/IEC 14882: 2017(E)

of t he select ed const ruct or is not resolut ion is p erformed again, mus t

an rvalue reference t o t he ob j ect ’ s typ e ( p ossib ly cv- qualified) ,

considering t he ob j ect

as an lvalue.

b e p erformed regardless of whet her copy elis ion will o ccur.

elision is not

note Example:

p erformed,

and t he select ed const ruct or mus t

It

[

Note:

overload

This two- st age overload resolut ion

det ermines t he const ruct or t o b e called if

b e accessib le even if t he call is elided.

— end

]

4

[

class Thing { public: Thing() ; ~ Thing() ; Thing(Thing&&) ; private: Thing(const Thing&) ; }; Thing f(bool b) { Thing t; if (b) throw t; return t; } Thing t2 = f(false) ;

// OK: Thing(Thing&&) used (or elided) to throw t // OK: Thing(Thing&&) used (or elided) to return t

// OK: no extra copy/move performed, t2 constructed by call to f

struct Weird { Weird() ; Weird(Weird&) ; }; Weird g() { Weird w; return w; }

— end example

§ 1 5. 8. 3

// OK: first overload resolution fails, second overload resolution selects Weird(Weird&) ]

© IS O / IEC

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ISO/IEC 1 4882:201 7(E)

16

Overloading

[over]

1

When two or more different declarations are specified for a single name in the same scope, that name is said to be overloaded . By extension, two declarations in the same scope that declare the same name but with different types are called overloaded declarations . Only function and function template declarations can be overloaded; variable and type declarations cannot be overloaded.

2

When an overloaded function name is used in a call, which overloaded function declaration is being referenced is determined by comparing the types of the arguments at the point of use with the types of the parameters in the overloaded declarations that are visible at the point of use. This function selection process is called overload resolution and is defined in 16.3. [ Example: double abs(double) ; int abs(int) ; abs(1) ; abs(1. 0) ;

// calls abs(int) ; // calls abs(double) ;

— end example ] 1 6. 1

Overloadable declarations

[over. load]

1

Not all function declarations can be overloaded. Those that cannot be overloaded are specified here. A program is ill-formed if it contains two such non-overloadable declarations in the same scope. [ Note: This restriction applies to explicit declarations in a scope, and between such declarations and declarations made through a using-declaration (10.3.3) . It does not apply to sets of functions fabricated as a result of name lookup (e.g., because of using-directive s) or overload resolution (e.g., for operator functions) . — end note ]

2

Certain function declarations cannot be overloaded:

(2. 1 )

— Function declarations that differ only in the return type, the exception specification (18.4) , or both cannot be overloaded.

(2. 2)

— Member function declarations with the same name and the same parameter-type-list (11.3.5) cannot be overloaded if any of them is a static member function declaration (12.2.3) . Likewise, member function template declarations with the same name, the same parameter-type-list, and the same template parameter lists cannot be overloaded if any of them is a static member function template declaration. The types of the implicit obj ect parameters constructed for the member functions for the purpose of overload resolution (16.3.1) are not considered when comparing parameter-type-lists for enforcement of this rule. In contrast, if there is no static member function declaration among a set of member function declarations with the same name and the same parameter-type-list, then these member function declarations can be overloaded if they differ in the type of their implicit obj ect parameter. [ Example: The following illustrates this distinction: class X { static void f() ; void f() ; void f() const; void f() const volatile; void g() ; void g() const; void g() const volatile; };

§ 16.1

// ill-formed // ill-formed // ill-formed // OK: no static g // OK: no static g

© ISO/IEC 2017 – All rights reserved

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ISO/IEC 14882: 2017(E)

— end example ] (2. 3)

— Member function declarations with the same name and the same parameter-type-list (11.3.5) as well as member function template declarations with the same name, the same parameter-type-list, and the same template parameter lists cannot be overloaded if any of them, but not all, have a ref-qualifier (11.3.5) . [ Example: class Y { void h() void h() void h() void i() void i()

&; const &; &&; &; const;

// OK // OK, all declarations have a ref-qualifier // ill-formed, prior declaration of i // has a ref-qualifier

};

3

— end example ] [ Note: As specified in 11.3.5, function declarations that have equivalent parameter declarations declare the same function and therefore cannot be overloaded:

(3. 1 )

— Parameter declarations that differ only in the use of equivalent typedef “types” are equivalent. A typedef is not a separate type, but only a synonym for another type (10.1.3) . [ Example: typedef int Int; void void void void

f(int f(Int f(int f(Int

i) ; i) ; i) { /* . . . */ } i) { /* . . . */ }

// OK: redeclaration of f(int) // error: redefinition of f(int)

— end example ] Enumerations, on the other hand, are distinct types and can be used to distinguish overloaded function declarations. [ Example: enum E { a }; void f(int i) { /* . . . */ } void f(E i) { /* . . . */ }

— end example ] (3. 2)

— Parameter declarations that differ only in a pointer * versus an array [] are equivalent. That is, the array declaration is adjusted to become a pointer declaration (11.3.5) . Only the second and subsequent array dimensions are significant in parameter types (11.3.4) . [ Example: int int int int

f(char*) ; f(char[] ) ; f(char[7] ) ; f(char[9] ) ;

// same as f(char*) ; // same as f(char*) ; // same as f(char*) ;

int int int int

g(char(*) [10] ) ; g(char[5] [10] ) ; g(char[7] [10] ) ; g(char(*) [20] ) ;

// same as g(char(*) [10] ) ; // same as g(char(*) [10] ) ; // different from g(char(*) [10] ) ;

— end example ] (3. 3)

— Parameter declarations that differ only in that one is a function type and the other is a pointer to the same function type are equivalent. That is, the function type is adj usted to become a pointer to function type (11.3.5) . [ Example: § 16.1

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ISO/IEC 1 4882:201 7(E)

void void void void

h(int() ) ; h(int (*) () ) ; h(int x() ) { } h(int (*x) () ) { }

— end example (3. 4)

—

// redeclaration of h(int() ) // definition of h(int() ) // ill-formed: redefinition of h(int() )

]

Paramet er

declarat ions

equivalent .

That

is,

t he

t hat

differ

const

and

only

in

t he

volatile

det ermining which funct ion is b eing declared,

presence

or

absence

of

const

and/ or

volatile

are

typ e- sp ecifiers for each paramet er typ e are ignored when defined,

or called.

[

Example:

typedef const int cInt; int int int int

f f f f

(int) ; (const int) ; (int) { /* . . . */ } (cInt) { /* . . . */ }

— end example O nly t he are

const

ignored

sp ecificat ion part icular,

]

and

in

t his

are

for

(3. 5)

—

Two

volatile fashion;

significant

any

considered dist inct

volatile T

// redeclaration of f(int) // definition of f(int) // error: redefinition of f(int)

typ e

T

,

typ e- sp ecifiers at t he out ermost level of t he p aramet er typ e sp ecificat ion

const

and

“ p oint er

paramet er typ es,

volatile

and

can

be to

used

T

”,

to

typ e- sp ecifiers

dist inguis h

“ p oint er

to

as are “ reference t o

const T T ”,

buried

overloaded ”,

and

wit hin

funct ion “ p oint er

“ reference t o

a

paramet er

declarat ions. to

const T

”,

typ e

123

volatile T

”

In are

and “ reference t o

”.

paramet er

declarat ions

t hat

differ

only

in

t heir

default

argument s

are

equivalent .

[

Example:

C onsider t he following:

void void void void

f f f f

(int i, int j ) ; (int i, int j = 99) ; (int i = 88, int j ) ; () ;

void prog () { f (1, 2) ; f (1) ; f () ; }

— end example — end note

// OK: redeclaration of f(int, int) // OK: redeclaration of f(int, int) // OK: overloaded declaration of f

// OK: call f(int, int) // OK: call f(int, int) // error: f(int, int) or f() ?

]

]

1 6. 2 1

Declaration matching

[over. dcl]

Two funct ion declarat ions of t he same name refer t o t he same funct ion if t hey are in t he same scop e and have equivalent paramet er declarat ions (1 6 . 1 ) .

A funct ion memb er of a derived class is

as a funct ion memb er of t he same name in a base class.

[

Example:

not

in t he same scop e

struct B { int f(int) ; }; struct D : B { int f(const char*) ;

When a parameter type includes a function type, such as in the case of a parameter type that is a pointer to function, the and volatile type-specifiers at the outermost level of the parameter type specifications for the inner function type are also ignored. 123)

const

§ 1 6. 2

© IS O / IEC

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31 1

ISO/IEC 14882: 2017(E)

};

D: : f(const char*)

Here

hides

B: : f(int)

void h(D* pd) { pd->f(1) ;

// error: // D: : f(const char*) hides B: : f(int) // OK // OK, calls D: : f

pd->B: : f(1) ; pd->f(" Ben" ) ; }

— end example 2

rat her t han overloading it .

]

A lo cally declared funct ion is not

in t he same scop e as a funct ion in a cont aining scop e.

void f(const char*) ; void g() { extern void f(int) ; f(" asdf" ) ;

[

Example:

// error: f(int) hides f(const char*) // so there is no f(const char*) in this scope

}

void caller () { extern void callee(int, int) ; { extern void callee(int) ; // hides callee(int, int) callee(88, 99) ; // error: only callee(int) in scope } }

— end example 3

D ifferent

]

versions of an overloaded memb er funct ion can b e given different

access rules.

[

Example:

class buffer { private: char* p; int size; protected: buffer(int s, char* store) { size = s; p = store; } public: buffer(int s) { p = new char[size = s] ; } };

— end example 1 6. 3 1

]

Overload resolution

[over. match]

O verload resolut ion is a mechanism for select ing t he b est t o b e t he argument s of t he call and a set t he call.

The select ion crit eria for t he b es t

mat ch t he ob j ect The

paramet er- typ e- list

mat ches t he implicit

funct ion

rest rict ions,

note 2

of

s elect ed

by

of t he

ob j ect

t hat

candidat e

resolut ion

funct ion,

how well

of expressions t hat

( for

non- st at ic

not

guarant eed

to

be

appropriat e

of

how well t he argument s memb er

funct ions )

and cert ain ot her prop ert ies of t he candidat e funct ion. is

are

can b e called based on t he cont ext

funct ion are t he numb er of argument s ,

paramet er,

overload

funct ion t o call given a list

candidate functions

for

t he

cont ext .

such as t he acces sibility of t he funct ion, can make it s us e in t he calling cont ext ill- formed.

[

t he

Note:

O t her

— end

]

O verload resolut ion select s t he funct ion t o call in seven dist inct

cont ext s wit hin t he language:

(2. 1 )

—

invo cat ion of a funct ion named in t he funct ion call synt ax (1 6 . 3 . 1 . 1 . 1 ) ;

(2. 2)

—

invo cat ion of a funct ion call op erat or,

§ 1 6. 3

a p oint er- t o- funct ion convers ion funct ion,

© IS O / IEC

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a reference- t o- p oint er-

31 2

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to-function conversion function, or a reference-to-function conversion function on a class obj ect named in the function call syntax (16.3.1.1.2) ; (2. 3)

— invocation of the operator referenced in an expression (16.3.1.2) ;

(2. 4)

— invocation of a constructor for default- or direct-initialization (11.6) of a class obj ect (16.3.1.3) ;

(2. 5)

— invocation of a user-defined conversion for copy-initialization (11.6) of a class obj ect (16.3.1.4) ;

(2. 6)

— invocation of a conversion function for initialization of an object of a non-class type from an expression of class type (16.3.1.5) ; and

(2. 7)

— invocation of a conversion function for conversion to a glvalue or class prvalue to which a reference (11.6.3) will be directly bound (16.3.1.6) . Each of these contexts defines the set of candidate functions and the list of arguments in its own unique way. But, once the candidate functions and argument lists have been identified, the selection of the best function is the same in all cases:

(2. 8)

— First, a subset of the candidate functions (those that have the proper number of arguments and meet certain other conditions) is selected to form a set of viable functions (16.3.2) .

(2. 9)

— Then the best viable function is selected based on the implicit conversion sequences (16.3.3.1) needed to match each argument to the corresponding parameter of each viable function.

3

If a best viable function exists and is unique, overload resolution succeeds and produces it as the result. Otherwise overload resolution fails and the invocation is ill-formed. When overload resolution succeeds, and the best viable function is not accessible (Clause 14) in the context in which it is used, the program is ill-formed. 1 6. 3. 1

Candidate functions and argument lists

[over. match. funcs]

1

The subclauses of 16.3.1 describe the set of candidate functions and the argument list submitted to overload resolution in each context in which overload resolution is used. The source transformations and constructions defined in these subclauses are only for the purpose of describing the overload resolution process. An implementation is not required to use such transformations and constructions.

2

The set of candidate functions can contain both member and non-member functions to be resolved against the same argument list. So that argument and parameter lists are comparable within this heterogeneous set, a member function is considered to have an extra parameter, called the implicit object parameter , which represents the object for which the member function has been called. For the purposes of overload resolution, both static and non-static member functions have an implicit obj ect parameter, but constructors do not.

3

Similarly, when appropriate, the context can construct an argument list that contains an implied object argument to denote the object to be operated on. Since arguments and parameters are associated by position within their respective lists, the convention is that the implicit obj ect parameter, if present, is always the first parameter and the implied obj ect argument, if present, is always the first argument.

4 (4. 1 ) (4. 2)

For non-static member functions, the type of the implicit obj ect parameter is

cv X” for functions declared without a ref-qualifier or with the & ref-qualifier — “rvalue reference to cv X” for functions declared with the && ref-qualifier where X is the class of which the function is a member and cv is the cv-qualification on the member function declaration. [ Example: For a const member function of class X , the extra parameter is assumed to have type “reference to const X ”. — end example ] For conversion functions, the function is considered to be — “lvalue reference to

a member of the class of the implied obj ect argument for the purpose of defining the type of the implicit obj ect parameter. For non-conversion functions introduced by a using-declaration into a derived class, the function is considered to be a member of the derived class for the purpose of defining the type of the implicit obj ect parameter. For static member functions, the implicit obj ect parameter is considered to match any § 16.3.1

© ISO/IEC 2017 – All rights reserved

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ISO/IEC 14882: 2017(E)

obj ect (since if the function is selected, the obj ect is discarded) . [ Note: No actual type is established for the implicit obj ect parameter of a static member function, and no attempt will be made to determine a conversion sequence for that parameter (16.3.3) . — end note ] 5

During overload resolution, the implied obj ect argument is indistinguishable from other arguments. The implicit obj ect parameter, however, retains its identity since no user-defined conversions can be applied to achieve a type match with it. For non-static member functions declared without a ref-qualifier , an additional rule applies:

(5. 1 )

— even if the implicit object parameter is not const-qualified, an rvalue can be bound to the parameter as long as in all other respects the argument can be converted to the type of the implicit object parameter. [ Note: The fact that such an argument is an rvalue does not affect the ranking of implicit conversion sequences (16.3.3.2) . — end note ]

6

Because other than in list-initialization only one user-defined conversion is allowed in an implicit conversion sequence, special rules apply when selecting the best user-defined conversion (16.3.3, 16.3.3.1) . [ Example: class T { public: T() ; }; class C : T { public: C(int) ; }; T a = 1;

// ill-formed: T(C(1) ) not tried

— end example ] 7

In each case where a candidate is a function template, candidate function template specializations are generated using template argument deduction (17.8.3, 17.8.2) . Those candidates are then handled as candidate functions in the usual way. 1 24 A given name can refer to one or more function templates and also to a set of overloaded non-template functions. In such a case, the candidate functions generated from each function template are combined with the set of non-template candidate functions.

8

A defaulted move constructor or assignment operator (15.8) that is defined as deleted is excluded from the set of candidate functions in all contexts. 16. 3. 1. 1

1

Function call syntax

[over.match. call]

In a function call (8.2.2) postfix-expression ( expression-listopt )

if the postfix-expression denotes a set of overloaded functions and/or function templates, overload resolution is applied as specified in 16.3.1.1.1. If the postfix-expression denotes an obj ect of class type, overload resolution is applied as specified in 16.3.1.1.2. 2

If the postfix-expression denotes the address of a set of overloaded functions and/or function templates, overload resolution is applied using that set as described above. If the function selected by overload resolution is a non-static member function, the program is ill-formed. [ Note: The resolution of the address of an overload set in other contexts is described in 16.4. — end note ] 124)

The pro cess of argument deduct ion fully det ermines t he paramet er typ es of t he funct ion t emplat e sp ecializ at ions, i. e. ,

t he paramet ers of funct ion t emplat e sp ecializat ions cont ain no t emplat e paramet er typ es.

Therefore, except where sp ecified

ot herwise, funct ion t emplat e sp ecializat ions and non- t emplat e funct ions ( 1 1 . 3. 5 ) are t reat ed equivalent ly for t he remainder of overload resolut ion.

§ 16.3.1.1

© ISO/IEC 2017 – All rights reserved

314

ISO/IEC 14882: 2017(E)

16. 3. 1. 1. 1 1

Call to named function

[over. call. func]

Of interest in 16.3.1.1.1 are only those function calls in which the postfix-expression ultimately contains a name that denotes one or more functions that might be called. Such a postfix-expression , perhaps nested arbitrarily deep in parentheses, has one of the following forms: postfix-expression: postfix-expression . id-expression postfix-expression -> id-expression primary-expression

These represent two syntactic subcategories of function calls: qualified function calls and unqualified function calls. 2

In qualified function calls, the name to be resolved is an id-expression and is preceded by an -> or . operator. Since the construct A->B is generally equivalent to (*A) . B , the rest of Clause 16 assumes, without loss of generality, that all member function calls have been normalized to the form that uses an obj ect and the . operator. Furthermore, Clause 16 assumes that the postfix-expression that is the left operand of the . operator has type “ cv T” where T denotes a class 125 . Under this assumption, the id-expression in the call is looked up as a member function of T following the rules for looking up names in classes (13.2) . The function declarations found by that lookup constitute the set of candidate functions. The argument list is the expression-list in the call augmented by the addition of the left operand of the . operator in the normalized member function call as the implied obj ect argument (16.3.1) .

3

In unqualified function calls, the name is not qualified by an -> or . operator and has the more general form of a primary-expression . The name is looked up in the context of the function call following the normal rules for name lookup in function calls (6.4) . The function declarations found by that lookup constitute the set of candidate functions. Because of the rules for name lookup, the set of candidate functions consists (1) entirely of non-member functions or (2) entirely of member functions of some class T. In case (1) , the argument list is the same as the expression-list in the call. In case (2) , the argument list is the expression-list in the call augmented by the addition of an implied obj ect argument as in a qualified function call. If the keyword this (12.2.2.1) is in scope and refers to class T, or a derived class of T, then the implied obj ect argument is (*this) . If the keyword this is not in scope or refers to another class, then a contrived obj ect of type T becomes the implied obj ect argument 126 . If the argument list is augmented by a contrived obj ect and overload resolution selects one of the non-static member functions of T, the call is ill-formed. 16. 3. 1. 1. 2

Call to obj ect of class type

[over. call.obj ect]

1

If the primary-expression E in the function call syntax evaluates to a class object of type “ cv T ”, then the set of candidate functions includes at least the function call operators of T . The function call operators of T are obtained by ordinary lookup of the name operator() in the context of (E) . operator() .

2

In addition, for each non-explicit conversion function declared in T of the form operator conversion-type-id ( ) cv-qualifier ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt ;

where cv-qualifier is the same cv-qualification as, or a greater cv-qualification than, cv , and where conversiontype-id denotes the type “pointer to function of ( P 1 P n ) returning R”, or the type “reference to pointer , . . . ,

to function of ( P 1 , . . . , P n ) returning R”, or the type “reference to function of ( P 1 , . . . , P n ) returning R”, a surrogate call function with the unique name call-function and having the form R call-function ( conversion-type-id F, P 1 a1 ,

. . .

, P n an ) { return F (a1 ,

. . .

, an ) ; }

is also considered as a candidate function. Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not 125) 126)

Not e t hat cv- qualifiers on t he typ e of ob j ect s are significant in overload resolut ion for b ot h glvalue and class prvalue ob j ect s. An implied ob j ect argument must b e cont rived t o corresp ond t o t he implicit ob j ect paramet er at t ribut ed t o memb er

funct ions during overload resolut ion.

It is not used in t he call t o t he select ed funct ion.

S ince t he memb er funct ions all have t he

same implicit ob j ect paramet er, t he cont rived ob j ect will not b e t he cause t o select or rej ect a funct ion.

§ 16.3.1.1.2

© ISO/IEC 2017 – All rights reserved

315

ISO/IEC 14882: 2017(E)

hidden wit hin

3

T

by anot her int ervening declarat ion

127

.

If such a surrogat e call funct ion is select ed by overload resolut ion,

E

b e called t o convert

t he corresp onding conversion funct ion will

t o t he appropriat e funct ion p oint er or reference,

wit h t he argument s of t he call.

If t he convers ion funct ion cannot

and t he funct ion will t hen b e invoked

b e called ( e. g. ,

b ecause of an ambiguity) ,

t he program is ill- formed.

4

The

argument

list

submit t ed

to

overload

resolut ion

funct ion call synt ax preceded by t he implied ob j ect t he funct ion call op erat ors, t he funct ion call op erat or. argument

consist s

argument

of t he

( E) .

[

argument

Note:

express ions

present

in

t he

When comparing t he call against

t he implied ob j ect argument is compared against t he implicit ob j ect paramet er of When comparing t he call agains t

is compared against

t he first

a surrogat e call funct ion,

paramet er of t he surrogat e call funct ion.

t he implied ob j ect

The convers ion funct ion

from which t he surrogat e call funct ion was derived will b e used in t he conversion s equence for t hat paramet er s ince it t hat

convert s t he implied ob j ect

first

— end note

paramet er.

]

argument [

Example:

t o t he appropriat e funct ion p oint er or reference required by

int f1(int) ; int f2(float) ; typedef int (*fp1) (int) ; typedef int (*fp2) (float) ; struct A { operator fp1() { return f1; } operator fp2() { return f2; } } a; int i = a(1) ; // calls f1 via pointer returned from conversion function

— end example 16. 3. 1 .2 1

If no

]

Operators in expressions

op erand

of an

op erat or

in

an

[over. match.oper]

expression

has

a

typ e

t hat

is

a

class

or

is ass umed t o b e a built - in op erat or and int erpret ed according t o C lause 8 . cannot

b e overloaded,

cannot

b e overloaded,

t hes e op erat ors but

are always built - in op erat ors

an [

enumerat ion,

Note:

t he

.

B ecaus e

,

op erat or

. *,

and

int erpret ed according t o C lause 8 .

:: ?:

t he rules in t his sub clause are used t o det ermine t he conversions t o b e applied t o

t he second and t hird op erands when t hey have class or enumerat ion typ e (8 . 1 6 ) .

— end note

]

[

Example:

struct String { String (const String&) ; String (const char*) ; operator const char* () ; }; String operator + (const String&, const String&) ; void f() { const char* p= " one" + " two" ; int I = 1 + 1;

// ill-formed because neither operand has class or enumeration type // always evaluates to 2 even if class or enumeration types exist // that would perform the operation.

}

— end example 2

]

If eit her op erand has a typ e t hat

is

a class

or an enumerat ion,

a user- defined op erat or

funct ion might

be

declared t hat implement s t his op erat or or a user- defined conversion can b e necessary t o convert t he op erand t o a typ e t hat is appropriat e for a built - in op erat or.

In t his case,

overload resolut ion is used t o det ermine which

op erat or funct ion or built - in op erat or is t o b e invoked t o implement

127)

t he op erat or.

Therefore,

t he op erat or

Not e t hat t his const ruct ion can yield candidat e call funct ions t hat cannot b e different iat ed one from t he ot her by overload

resolut ion b ecause t hey have ident ical declarat ions or differ only in t heir ret urn typ e.

The call will b e ambiguous if overload

resolut ion cannot select a mat ch t o t he call t hat is uniquely b et t er t han such undifferent iable funct ions.

§ 1 6. 3. 1 . 2

© IS O / IEC

2 0 1 7 – All right s reserved

31 6

ISO/IEC 14882: 2017(E)

notation is first transformed to the equivalent function-call notation as summarized in Table 12 (where @ denotes one of the operators covered in the specified subclause) . However, the operands are sequenced in the order prescribed for the built-in operator (Clause 8) . Table 12 — Relationship between operator and function call notation

3

Subclause

Expression

As member function

As non-member function

16.5.1 16.5.2 16.5.3 16.5.5 16.5.6 16.5.7

@a a@b a=b a[b] a-> a@

(a) . operator@ ( ) (a) . operator@ (b) (a) . operator= (b) (a) . operator[] (b) (a) . operator->( ) (a) . operator@ (0)

operator@(a) operator@(a, b)

operator@(a, 0)

For a unary operator @ with an operand of a type whose cv-unqualified version is T1 , and for a binary operator @ with a left operand of a type whose cv-unqualified version is T1 and a right operand of a type whose cv-unqualified version is T2 , three sets of candidate functions, designated member candidates , non-member

candidates and built-in candidates, are constructed as follows: (3. 1 )

— If T1 is a complete class type or a class currently being defined, the set of member candidates is the result of the qualified lookup of T1: : operator@ (16.3.1.1.1) ; otherwise, the set of member candidates is empty.

(3. 2)

— The set of non-member candidates is the result of the unqualified lookup of operator@ in the context of the expression according to the usual rules for name lookup in unqualified function calls (6.4.2) except that all member functions are ignored. However, if no operand has a class type, only those non-member functions in the lookup set that have a first parameter of type T1 or “reference to cv T1 ”, when T1 is an enumeration type, or (if there is a right operand) a second parameter of type T2 or “reference to cv T2 ”, when T2 is an enumeration type, are candidate functions.

(3. 3)

— For the operator , , the unary operator & , or the operator -> , the built-in candidates set is empty. For all other operators, the built-in candidates include all of the candidate operator functions defined in 16.6 that, compared to the given operator,

(3. 3. 1 )

— have the same operator name, and

(3. 3. 2)

— accept the same number of operands, and

(3. 3. 3)

— accept operand types to which the given operand or operands can be converted according to 16.3.3.1, and

(3. 3. 4)

— do not have the same parameter-type-list as any non-member candidate that is not a function template specialization.

4

For the built-in assignment operators, conversions of the left operand are restricted as follows:

(4. 1 )

— no temporaries are introduced to hold the left operand, and

(4. 2)

— no user-defined conversions are applied to the left operand to achieve a type match with the left-most parameter of a built-in candidate.

5

For all other operators, no such restrictions apply.

6

The set of candidate functions for overload resolution is the union of the member candidates, the non-member candidates, and the built-in candidates. The argument list contains all of the operands of the operator. The best function from the set of candidate functions is selected according to 16.3.2 and 16.3.3. 128 [ Example: 128)

If t he set of candidat e funct ions is empty, overload resolut ion is unsuccessful.

§ 16.3.1.2

© ISO/IEC 2017 – All rights reserved

317

ISO/IEC 14882: 2017(E)

struct A { operator int() ; }; A operator+(const A&, const A&) ; void m() { A a, b; a + b; // operator+(a, b) chosen over int(a) + int(b) }

— end example 7

If a

built - in

]

candidat e

is

select ed

by

overload

resolut ion,

t he

op erands

of class

typ es of t he corresp onding paramet ers of t he select ed op erat ion funct ion, conversion sequence of a user- defined convers ion sequence (1 6 . 3 . 3 . 1 . 2 )

except

is not

typ e

are

t hat

applied.

convert ed

to

t he

t he second st andard Then t he op erat or is

t reat ed as t he corresp onding built - in op erat or and int erpret ed according t o C lause 8 .

[

Example:

struct X { operator double() ; }; struct Y { operator int*() ; };

// error: pointer arithmetic requires integral operand // error: pointer arithmetic requires integral operand

int *a = Y() + 100. 0; int *b = Y() + X() ;

— end example 8

The s econd op erand of op erat or when t he ret urned,

9

]

operator->

->

is ignored in select ing an

funct ion is called.

When

wit h t he original second op erand.

If t he op erat or is t he op erat or

,

,

129

operator->

t he unary op erat or

&

,

operator->

ret urns,

funct ion,

t he op erat or

or t he op erat or

->

,

and is not

->

an argument

is ap plied t o t he value

and t here are no viable funct ions,

t hen t he op erat or is ass umed t o b e t he built - in op erat or and int erpret ed according t o C lause 8 .

10

[

Note:

The lo okup rules for op erat ors in expressions are different

names in a funct ion call,

t han t he lo okup rules for op erat or funct ion

as shown in t he following example:

struct A { }; void operator + (A, A) ; struct B { void operator + (B) ; void f () ; }; A a; void B: : f() { operator+ (a, a) ; a + a; }

— end note 129)

]

If t he value ret urned by t he

funct ion.

// error: global operator hidden by member // OK: calls global operator+

operator-> funct ion has class typ e, operator-> funct ion ret urns

The pro cess rep eat s unt il an

§ 1 6. 3. 1 . 2

© IS O / IEC

t his may result in select ing and calling anot her

operator->

a value of non- class typ e.

2 0 1 7 – All right s reserved

31 8

ISO/IEC 14882: 2017(E)

16. 3. 1. 3 1

Initialization by constructor

[over.match. ctor]

When ob j ect s of class typ e are direct - init ializ ed (1 1 . 6 ) , a

derived

class

typ e

(1 1 . 6) ,

or

default - init ializ ed

direct - init ializ at ion or default - init ializ at ion t hat funct ions

are

candidat e

all

expression-list 16. 3. 1. 4 1

t he

funct ions

const ruct ors are

all

t he

of t he

assignment-expression

or

class

convert ing

is not

of t he

overload

ob j ect

const ruct ors

initializer

of t he

conversion O verload

can

be

invoked

resolut ion

is

p erformed for indirect

b eing

(1 5 . 3 . 1 )

to

used

t he

convert

to

init ializ ed.

of t hat

For

class.

an

s elect

t he

init ializ er

expression

user- defined

to

conversion

wit h

T

a class typ e,

copy- init ializ at ion,

The

argument

— end note

T

—

The convert ing const ruct ors (1 5 . 3 . 1 )

—

When t he typ e of t he init ializ er express ion is a class typ e “ and it s base classes are considered.

t he to

explicit

of

typ e

of t he

ob j ect

invoked.

[

Note:

b eing The

init ializ ed. conversion

]

Assuming t hat

single

argument

in

funct ions

t hat

X

of typ e

ret urn

“

cv1 T

”

is t he typ e of t he

cv S

” , t he non- explicit conversion funct ions of

When init ializ ing a t emp orary t o b e b ound t o t he first paramet er

t he

cont ext

whose cv- unqualified version is t he same typ e as

of reference,

t he

are candidat e funct ions .

“ reference

to

cv

- qualified

of direct - init ializ at ion

conversion funct ions are als o considered.

C onversion

t he

is

[over. match. copy]

be

of a const ruct or where t he paramet er is of typ e “ reference t o p ossibly a

list

t he candidat e funct ions are s elect ed as follows:

(1. 2)

wit h

For

t he candidat e

.

(1. 1 )

called

const ruct or.

binding t o a reference t o a p ossibly cv- qualified class typ e is det ermined in t erms of

b eing init ializ ed,

is

select s

of copy- init ializ at ion,

as part of a copy- init ializ at ion of an ob j ect of class typ e, a user- defined

a corres p onding non- reference copy- init ializ at ion.

S

resolut ion

in t he cont ext

Copy-initialization of class by user-defined conversion

Under t he condit ions sp ecified in 1 1 . 6 ,

ob j ect

copy- init ializ ed from an expres sion of t he same or

(1 1 . 6) ,

Those t hat

T X

are not

of an

T

” and t he const ruct or

ob j ect

hidden wit hin

S

of typ e

“

cv2 T

”,

and yield a typ e

or is a derived class t hereof are candidat e funct ions .

”

ret urn

lvalues

and are t herefore considered t o yield

X

or

xvalues ,

dep ending

on

t he

typ e

for t his pro cess of select ing candidat e

funct ions.

2

In b ot h cases ,

t he argument

will b e compared against

16. 3. 1. 5

”

can

be

invoked

to

class typ e,

The

convert

conversion

a st andard explicit

as part

an

conversion

to

of

S

and

hidden wit hin sequence

funct ions

T

typ e

funct ions t hat

expression

ret urn

lvalues

argument

list

comp ared against

§ 1 6. 3. 1 . 5

typ e

of non- class typ e,

to

t he

typ e

and “

cv S

”

is

t he typ e of t he init ializ er

of t he

a conversion ob j ect

b eing

Assuming t hat expression,

“

cv1

S

wit h

wit h

a

it s

S

base

(1 6 . 3 . 3 . 1 . 1 )

t hat

are

classes

and yield typ e

not

qualificat ion

are

are

T

considered.

or a typ e t hat

candidat e

hidden

S

wit hin

conversion

funct ions.

(7. 5 )

and

are

Those

For

yield

also

non- explicit

a

conversion

T

can b e convert ed t o typ e direct - init ializ at ion,

T

typ e

candidat e

or

a

typ e

t hat

funct ions.

or

has

X

xvalues ,

dep ending

on

t he

C onversion funct ions t hat

typ e

of reference,

of typ e

“

can

”

and

be

C onversion typ e

ret urn “ reference t o

cv2 X

via

t hose

ret urn a cv- qualified typ e are considered t o yield t he cv- unqualified version of t hat

considered t o yield

The

of class

t he conversion funct ion t o b e invoked.

for t his pro ces s of select ing candidat e funct ions.

2

paramet er of

[over. match.conv]

of an init ializ at ion of an ob j ect

init ializ er

b eing init ializ ed,

funct ions

are not

conversion

convert ed

”

This argument

ob j ect

t he candidat e funct ions are select ed as follows:

funct ions t hat

X

Note:

]

O verload resolut ion is used t o select

is t he typ e of t he ob j ect

—

[

t he implicit

Initialization by conversion function

init ializ ed.

T

which is t he init ializ er expression.

paramet er of t he const ruct ors and against

Under t he condit ions sp ecified in 1 1 . 6 , funct ion

(1. 1 )

has one argument ,

— end note

t he conversion funct ions.

1

list

t he first

are

cv2

t herefore

for t his pro cess of select ing candidat e funct ions.

one

argument ,

t he implicit

ob j ect

which

is

t he

init ializ er

expression.

paramet er of t he conversion funct ions.

© IS O / IEC

2 0 1 7 – All right s reserved

[

Note: — end note

This

argument

will

be

]

31 9

ISO/IEC 14882: 2017(E)

16. 3. 1. 6

Initialization by conversion function for direct reference binding

[over. match.ref]

1

Under the conditions specified in 11.6.3, a reference can be bound directly to a glvalue or class prvalue that is the result of applying a conversion function to an initializer expression. Overload resolution is used to select the conversion function to be invoked. Assuming that “reference to cv1 T” is the type of the reference being initialized, and “ cv S ” is the type of the initializer expression, with S a class type, the candidate functions are selected as follows:

(1. 1 )

— The conversion functions of S and its base classes are considered. Those non-explicit conversion functions that are not hidden within S and yield type “lvalue reference to cv2 T2 ” (when initializing an lvalue reference or an rvalue reference to function) or “ cv2 T2 ” or “rvalue reference to cv2 T2 ” (when initializing an rvalue reference or an lvalue reference to function) , where “ cv1 T ” is reference-compatible (11.6.3) with “ cv2 T2 ”, are candidate functions. For direct-initialization, those explicit conversion functions that are not hidden within S and yield type “lvalue reference to cv2 T2 ” or “ cv2 T2 ” or “rvalue reference to cv2 T2 ”, respectively, where T2 is the same type as T or can be converted to type T with a qualification conversion (7.5) , are also candidate functions.

2

The argument list has one argument, which is the initializer expression. [ Note: This argument will be compared against the implicit obj ect parameter of the conversion functions. — end note ] 16. 3. 1. 7

Initialization by list-initialization

[over.match. list]

1

When objects of non-aggregate class type T are list-initialized such that 11.6.4 specifies that overload resolution is performed according to the rules in this section, overload resolution selects the constructor in two phases:

(1. 1 )

— Initially, the candidate functions are the initializer-list constructors (11.6.4) of the class T and the argument list consists of the initializer list as a single argument.

(1. 2)

— If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class T and the argument list consists of the elements of the initializer list. If the initializer list has no elements and T has a default constructor, the first phase is omitted. In copy-listinitialization, if an explicit constructor is chosen, the initialization is ill-formed. [ Note: This differs from other situations (16.3.1.3, 16.3.1.4) , where only converting constructors are considered for copy-initialization. This restriction only applies if this initialization is part of the final result of overload resolution. — end note ] 1 6. 3. 1 .8

1 (1. 1 )

Class template argument deduction

[over. match. class.deduct]

A set of functions and function templates is formed comprising: — For each constructor of the primary class template designated by the defined, a function template with the following properties:

template-name, if the template is

(1. 1. 1 )

— The template parameters are the template parameters of the class template followed by the template parameters (including default template arguments) of the constructor, if any.

(1. 1. 2)

— The types of the function parameters are those of the constructor.

(1. 1. 3)

— The return type is the class template specialization designated by the template-name and template arguments corresponding to the template parameters obtained from the class template.

(1. 2)

— If the primary class template C is not defined or does not declare any constructors, an additional function template derived as above from a hypothetical constructor C() .

(1. 3)

— An additional function template derived as above from a hypothetical constructor C(C) , called the deduction candidate.

(1. 4)

— For each

(1. 4. 1 )

copy

deduction-guide, a function or function template with the following properties: — The template parameters, if any, and function parameters are those of the deduction-guide .

§ 16.3.1.8

© ISO/IEC 2017 – All rights reserved

320

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— The return type is the

(1. 4. 2)

simple-template-id of the deduction-guide.

2

Initialization and overload resolution are performed as described in 11.6 and 16.3.1.3, 16.3.1.4, or 16.3.1.7 (as appropriate for the type of initialization performed) for an obj ect of a hypothetical class type, where the selected functions and function templates are considered to be the constructors of that class type for the purpose of forming an overload set, and the initializer is provided by the context in which class template argument deduction was performed. Each such notional constructor is considered to be explicit if the function or function template was generated from a constructor or deduction-guide that was declared explicit . All such notional constructors are considered to be public members of the hypothetical class type.

3

[ Example: template struct A { explicit A(const T&, . . . ) noexcept; A(T&&, . . . ) ; }; int i; A a1 = { i, i };

A a2{i, i}; A a3{0, i}; A a4 = {0, i};

// #1 // #2

// error: explicit constructor #1 selected in copy-list-initialization during deduction, // cannot deduce from non-forwarding rvalue reference in #2 // OK, #1 deduces to A and also initializes // OK, #2 deduces to A and also initializes // OK, #2 deduces to A and also initializes

template A(const T&, const T&) -> A; template explicit A(T&&, T&&) -> A; A A A A

a5 = {0, 1}; a6{0, 1}; a7 = {0, i}; a8{0, i};

// #3 // #4

// error: explicit deduction guide #4 selected in copy-list-initialization during deduction // OK, #4 deduces to A and #2 initializes // error: #3 deduces to A , #1 and #2 declare same constructor // error: #3 deduces to A , #1 and #2 declare same constructor

template struct B { template using TA = T; template B(U, TA) ; }; B b{(int*) 0, (char*) 0};

// OK, deduces B

— end example ] 1 6. 3. 2

Viable functions

[over. match. viable]

1

From the set of candidate functions constructed for a given context (16.3.1) , a set of viable functions is chosen, from which the best function will be selected by comparing argument conversion sequences for the best fit (16.3.3) . The selection of viable functions considers relationships between arguments and function parameters other than the ranking of conversion sequences.

2

First, to be a viable function, a candidate function shall have enough parameters to agree in number with the arguments in the list.

(2. 1 ) (2. 2)

m arguments in the list, all candidate functions having exactly m parameters are viable. — A candidate function having fewer than m parameters is viable only if it has an ellipsis in its parameter

— If there are

list (11.3.5) . For the purposes of overload resolution, any argument for which there is no corresponding parameter is considered to “match the ellipsis” (16.3.3.1.3) .

(2. 3)

— A candidate function having more than § 16.3.2

m parameters is viable only if the (m+1)-st parameter has a

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default argument (11.3.6) . 130 For the purposes of overload resolution, the parameter list is truncated on the right, so that there are exactly m parameters. 3

Second, for F to be a viable function, there shall exist for each argument an implicit conversion sequence (16.3.3.1) that converts that argument to the corresponding parameter of F. If the parameter has reference type, the implicit conversion sequence includes the operation of binding the reference, and the fact that an lvalue reference to non- const cannot be bound to an rvalue and that an rvalue reference cannot be bound to an lvalue can affect the viability of the function (see 16.3.3.1.4) .

1 6. 3. 3 1

Best viable function

[over. match. best]

Define ICS i ( F ) as follows:

(1. 1 )

— If F is a static member function, ICS 1 ( F ) is defined such that ICS 1 ( F ) is neither better nor worse than ICS 1 ( G ) for any function G, and, symmetrically, ICS 1 ( G ) is neither better nor worse than ICS 1 ( F ) ; 1 31 otherwise,

(1 . 2)

— let ICS i ( F ) denote the implicit conversion sequence that converts the i-th argument in the list to the type of the i -th parameter of viable function F . 16.3.3.1 defines the implicit conversion sequences and 16.3.3.2 defines what it means for one implicit conversion sequence to be a better conversion sequence or worse conversion sequence than another. Given these definitions, a viable function F1 is defined to be a better function than another viable function F2 if for all arguments i, ICS i ( F1 ) is not a worse conversion sequence than ICS i( F2 ) , and then

(1. 3)

— for some argument j, ICS j ( F1 ) is a better conversion sequence than ICS j ( F2 ) , or, if not that,

(1. 4)

— the context is an initialization by user-defined conversion (see 11.6, 16.3.1.5, and 16.3.1.6) and the standard conversion sequence from the return type of F1 to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the return type of F2 to the destination type [ Example: struct A { A() ; operator int() ; operator double() ; } a; int i = a; // a. operator int() followed by no conversion is better than // a. operator double() followed by a conversion to int float x = a; // ambiguous: both possibilities require conversions, // and neither is better than the other

— end example ] or, if not that, (1. 5)

— the context is an initialization by conversion function for direct reference binding (16.3.1.6) of a reference to function type, the return type of F1 is the same kind of reference (i.e. lvalue or rvalue) as the reference being initialized, and the return type of F2 is not [ Example: template struct A { operator T&() ; // #1 operator T&&() ; // #2 }; typedef int Fn() ; A a; Fn& lf = a; // calls #1 Fn&& rf = a; // calls #2 130) 131 )

According t o 1 1 . 3. 6, paramet ers following t he

(m+1)

- st paramet er must also have default argument s.

If a funct ion is a st at ic memb er funct ion, t his definit ion means t hat t he first argument , t he implied ob j ect argument , has no

effect in t he det erminat ion of whet her t he funct ion is b et t er or worse t han any ot her funct ion.

§ 16.3.3

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— end example (1. 6)

—

F1

is not

(1. 7)

—

F1

and

]

or,

if not

t hat ,

a funct ion t emplat e sp ecializ at ion and

F2

are funct ion t emplat e sp ecializ at ions,

t han t he t emplat e for

F2

deduction-guide

—

F1

is generat ed from a

(1. 9)

—

F1

is t he copy deduct ion candidat e (1 6 . 3 . 1 . 8 )

(1. 10)

—

F1

is

generat ed

from

Example:

a

is a funct ion t emplat e sp ecializ at ion,

and t he funct ion t emplat e for

F1

non- t emplat e

( 1 6. 3. 1 . 8)

and

and

const ruct or

F2

F2

is not ,

is not ,

and

F2

or,

is

or,

if not

if not

or,

if not

t hat ,

is more sp ecializ ed

according t o t he part ial ordering rules des crib ed in 1 7. 5 . 6 . 2 ,

(1. 8)

[

F2

or,

if not

t hat ,

t hat ,

t hat ,

generat ed

from

a

const ruct or

t emplat e.

template struct A { using value_type = T; A(value_type) ; // #1 A(const A&) ; // #2 A(T, T, int) ; // #3 template A(int, T, U) ; // #4 // #5 is the copy deduction candidate, A(A) }; A x(1, 2, 3) ;

// uses #3, generated from a non-template constructor

template A(T) -> A;

// #6, less specialized than #5

A a(42) ; A b = a;

// uses #6 to deduce A and #1 to initialize // uses #5 to deduce A and #2 to initialize

template A(A) -> A; // #7, as specialized as #5

// uses #7 to deduce A and #1 to initialize

A b2 = a;

— end example 2

]

If t here is exact ly one viable funct ion t hat one select ed by overload resolut ion;

void Fcn(const int*, void Fcn(int*, int) ;

is a b et t er funct ion t han all ot her viable funct ions,

ot herwise t he call is ill- formed.

1 32

[

Example:

t hen it

is t he

short) ;

int i; short s = 0; void f() { Fcn(&i, s) ;

→ const int* → // calls Fcn(int*, int) , because &i → int* is better than &i → const // and 1L → short and 1L → int are indistinguishable →

Fcn(&i, 1L) ;

132)

→

// is ambiguous because &i int* is better than &i // but s short is also better than s int

The algorit hm for select ing t he b est viable funct ion is linear in t he numb er of viable funct ions.

t o find a funct ion least as go o d as such t hat

F

W

t hat is not worse t han any opp onent it faced.

W, F

cannot b e t he b est funct ion b ecause at some p oint in t he t ournament

was not b et t er t han

G.

t he viable funct ions t o verify t hat

§ 1 6. 3. 3

Alt hough anot her funct ion

Hence,

W

W

int*

Run a simple t ournament

F t hat W did not face might b e at F encount ered anot her funct ion G

is eit her t he b est funct ion or t here is no b est funct ion.

S o, make a second pass over

is b et t er t han all ot her funct ions.

© IS O / IEC

2 0 1 7 – All right s reserved

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ISO/IEC 14882: 2017(E)

Fcn(&i, ’ c’ ) ; }

→

// calls Fcn(int*, int) , because &i int* is better than &i // and c int is better than c short

→

→

→ const

int*

— end example ] 3

If the best viable function resolves to a function for which multiple declarations were found, and if at least two of these declarations — or the declarations they refer to in the case of using-declaration s — specify a default argument that made the function viable, the program is ill-formed. [ Example: namespace A { extern " C" void f(int = 5) ; } namespace B { extern " C" void f(int = 5) ; } using A: : f; using B: : f; void use() { f(3) ; f() ; }

// OK, default argument was not used for viability // error: found default argument twice

— end example ] 16. 3. 3.1

Implicit conversion sequences

[over. best.ics]

1

An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called. The sequence of conversions is an implicit conversion as defined in Clause 7, which means it is governed by the rules for initialization of an obj ect or reference by a single expression (11.6, 11.6.3) .

2

Implicit conversion sequences are concerned only with the type, cv-qualification, and value category of the argument and how these are converted to match the corresponding properties of the parameter. Other properties, such as the lifetime, storage class, alignment, accessibility of the argument, whether the argument is a bit-field, and whether a function is deleted (11.4.3) , are ignored. So, although an implicit conversion sequence can be defined for a given argument-parameter pair, the conversion from the argument to the parameter might still be ill-formed in the final analysis.

3

A well-formed implicit conversion sequence is one of the following forms:

(3. 1 ) (3. 2) (3. 3) 4

standard conversion sequence (16.3.3.1.1) , — a user-defined conversion sequence (16.3.3.1.2) , or — an ellipsis conversion sequence (16.3.3.1.3) .

— a

However, if the target is

(4. 1 )

— the first parameter of a constructor or

(4. 2)

— the implicit obj ect parameter of a user-defined conversion function and the constructor or user-defined conversion function is a candidate by

(4. 3)

— 16.3.1.3, when the argument is the temporary in the second step of a class copy-initialization,

(4. 4)

— 16.3.1.4, 16.3.1.5, or 16.3.1.6 (in all cases) , or

(4. 5)

— the second phase of 16.3.1.7 when the initializer list has exactly one element that is itself an initializer list, and the target is the first parameter of a constructor of class X , and the conversion is to X or reference to cv X , § 16.3.3.1

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user-defined conversion sequences are not considered. [ Note: These rules prevent more than one user-defined conversion from being applied during overload resolution, thereby avoiding infinite recursion. — end note ] [ Example: struct Y { Y(int) ; }; struct A { operator int() ; }; Y y1 = A() ; // error: A: : operator int() is not a candidate struct X { }; struct B { operator X() ; }; B b; X x({b}) ; // error: B: : operator X() is not a candidate

— end example ] 5

For the case where the parameter type is a reference, see 16.3.3.1.4.

6

When the parameter type is not a reference, the implicit conversion sequence models a copy-initialization of the parameter from the argument expression. The implicit conversion sequence is the one required to convert the argument expression to a prvalue of the type of the parameter. [ Note: When the parameter has a class type, this is a conceptual conversion defined for the purposes of Clause 16; the actual initialization is defined in terms of constructors and is not a conversion. — end note ] Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion. [ Example: A parameter of type A can be initialized from an argument of type const A . The implicit conversion sequence for that case is the identity sequence; it contains no “conversion” from const A to A . — end example ] When the parameter has a class type and the argument expression has the same type, the implicit conversion sequence is an identity conversion. When the parameter has a class type and the argument expression has a derived class type, the implicit conversion sequence is a derived-to-base Conversion from the derived class to the base class. [ Note: There is no such standard conversion; this derived-to-base Conversion exists only in the description of implicit conversion sequences. — end note ] A derived-to-base Conversion has Conversion rank (16.3.3.1.1) .

7

In all contexts, when converting to the implicit obj ect parameter or when converting to the left operand of an assignment operation only standard conversion sequences are allowed.

8

If no conversions are required to match an argument to a parameter type, the implicit conversion sequence is the standard conversion sequence consisting of the identity conversion (16.3.3.1.1) .

9

If no sequence of conversions can be found to convert an argument to a parameter type, an implicit conversion sequence cannot be formed.

10

If several different sequences of conversions exist that each convert the argument to the parameter type, the implicit conversion sequence associated with the parameter is defined to be the unique conversion sequence designated the ambiguous conversion sequence . For the purpose of ranking implicit conversion sequences as described in 16.3.3.2, the ambiguous conversion sequence is treated as a user-defined conversion sequence that is indistinguishable from any other user-defined conversion sequence. [ Note: This rule prevents a function from becoming non-viable because of an ambiguous conversion sequence for one of its parameters. [ Example: class B; class A { class B { class C { void f(A) void f(C) B b; f(b) ;

A (B&) ; }; operator A () ; }; C (B&) ; }; { } { }

void f(B) { }

§ 16.3.3.1

→

// ill-formed: ambiguous because there is a conversion b C (via constructor) // and an (ambiguous) conversion b A (via constructor or conversion function)

→

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// OK, unambiguous

f ( b) ;

— end example ] — end note ] If a function that uses the ambiguous conversion sequence is selected as the best viable function, the call will be ill-formed because the conversion of one of the arguments in the call is ambiguous. 11

The three forms of implicit conversion sequences mentioned above are defined in the following subclauses. 16. 3. 3. 1. 1

Standard conversion sequences

[over. ics. scs]

1

Table 13 summarizes the conversions defined in Clause 7 and partitions them into four disj oint categories: Lvalue Transformation, Qualification Adj ustment, Promotion, and Conversion. [ Note: These categories are orthogonal with respect to value category, cv-qualification, and data representation: the Lvalue Transformations do not change the cv-qualification or data representation of the type; the Qualification Adjustments do not change the value category or data representation of the type; and the Promotions and Conversions do not change the value category or cv-qualification of the type. — end note ]

2

[ Note: As described in Clause 7, a standard conversion sequence is either the Identity conversion by itself (that is, no conversion) or consists of one to three conversions from the other four categories. If there are two or more conversions in the sequence, the conversions are applied in the canonical order: Lvalue Transformation, Promotion or Conversion , Qualification Adj ustment . — end note ]

3

Each conversion in Table 13 also has an associated rank (Exact Match, Promotion, or Conversion) . These are used to rank standard conversion sequences (16.3.3.2) . The rank of a conversion sequence is determined by considering the rank of each conversion in the sequence and the rank of any reference binding (16.3.3.1.4) . If any of those has Conversion rank, the sequence has Conversion rank; otherwise, if any of those has Promotion rank, the sequence has Promotion rank; otherwise, the sequence has Exact Match rank. Table 13 — Conversions Conversion

Category

No conversions required Lvalue-to-rvalue conversion Array-to-pointer conversion Function-to-pointer conversion Qualification conversions Function pointer conversion Integral promotions Floating-point promotion Integral conversions Floating-point conversions Floating-integral conversions Pointer conversions Pointer to member conversions Boolean conversions

16. 3. 3. 1. 2 1

Rank

Identity Lvalue Transformation

Exact Match

Qualification Adj ustment Promotion

Promotion

Conversion

Conversion

User-defined conversion sequences

Subclause

7.1 7.2 7.3 7.5 7.13 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.14

[over.ics.user]

A user-defined conversion sequence consists of an initial standard conversion sequence followed by a userdefined conversion (15.3) followed by a second standard conversion sequence. If the user-defined conversion is specified by a constructor (15.3.1) , the initial standard conversion sequence converts the source type to the type required by the argument of the constructor. If the user-defined conversion is specified by a conversion function (15.3.2) , the initial standard conversion sequence converts the source type to the implicit obj ect parameter of the conversion function. § 16.3.3.1.2

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2

The second standard conversion sequence converts the result of the user-defined conversion to the target type for the sequence. Since an implicit conversion sequence is an initialization, the special rules for initialization by user-defined conversion apply when selecting the best user-defined conversion for a user-defined conversion sequence (see 16.3.3 and 16.3.3.1) .

3

If the user-defined conversion is specified by a specialization of a conversion function template, the second standard conversion sequence shall have exact match rank.

4

A conversion of an expression of class type to the same class type is given Exact Match rank, and a conversion of an expression of class type to a base class of that type is given Conversion rank, in spite of the fact that a constructor (i.e., a user-defined conversion function) is called for those cases. 16. 3. 3. 1. 3

1

16. 3. 3. 1. 4 1

Ellipsis conversion sequences

[over.ics. ellipsis]

An ellipsis conversion sequence occurs when an argument in a function call is matched with the ellipsis parameter specification of the function called (see 8.2.2) . Reference binding

[over.ics. ref]

When a parameter of reference type binds directly (11.6.3) to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base Conversion (16.3.3.1) . [ Example: struct A {}; struct B : public A {} b; int f(A&) ; int f(B&) ; int i = f(b) ; // calls f(B&) , an exact match, rather than f(A&) , a conversion

— end example ] If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence (16.3.3.1.2) , with the second standard conversion sequence either an identity conversion or, if the conversion function returns an entity of a type that is a derived class of the parameter type, a derived-to-base Conversion. 2

When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert the argument expression to the referenced type according to 16.3.3.1. Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the referenced type with the argument expression. Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.

3

Except for an implicit obj ect parameter, for which see 16.3.1, a standard conversion sequence cannot be formed if it requires binding an lvalue reference other than a reference to a non-volatile const type to an rvalue or binding an rvalue reference to an lvalue other than a function lvalue. [ Note: This means, for example, that a candidate function cannot be a viable function if it has a non- const lvalue reference parameter (other than the implicit obj ect parameter) and the corresponding argument would require a temporary to be created to initialize the lvalue reference (see 11.6.3) . — end note ]

4

Other restrictions on binding a reference to a particular argument that are not based on the types of the reference and the argument do not affect the formation of a standard conversion sequence, however. [ Example: A function with an “lvalue reference to int ” parameter can be a viable candidate even if the corresponding argument is an int bit-field. The formation of implicit conversion sequences treats the int bit-field as an int lvalue and finds an exact match with the parameter. If the function is selected by overload resolution, the call will nonetheless be ill-formed because of the prohibition on binding a non- const lvalue reference to a bit-field (11.6.3) . — end example ]

§ 16.3.3.1.4

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16. 3. 3. 1. 5

List-initialization sequence

[over.ics.list]

1

When an argument is an initializer list (11.6.4) , it is not an expression and special rules apply for converting it to a parameter type.

2

If the parameter type is an aggregate class X and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence is the one required to convert the element to the parameter type.

3

Otherwise, if the parameter type is a character array133 and the initializer list has a single element that is an appropriately-typed string literal (11.6.2) , the implicit conversion sequence is the identity conversion.

4

Otherwise, if the parameter type is std: : initializer_list and all the elements of the initializer list can be implicitly converted to X , the implicit conversion sequence is the worst conversion necessary to convert an element of the list to X , or if the initializer list has no elements, the identity conversion. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor. [ Example: void f(std: : initializer_list) ; f( {} ) ; // OK: f(initializer_list) identity conversion f( {1, 2, 3} ) ; // OK: f(initializer_list) identity conversion f( {’ a’ , ’ b’ } ) ; // OK: f(initializer_list) integral promotion f( {1. 0} ) ; // error: narrowing struct A { A(std: : initializer_list) ; A(std: : initializer_list) ; A(std: : initializer_list) ; }; A a{ 1. 0, 2. 0 }; // OK, uses #1 void g(A) ; g({ " foo" , " bar" }) ;

// OK, uses #3

typedef int IA[3] ; void h(const IA&) ; h({ 1, 2, 3 }) ;

// OK: identity conversion

// #1 // #2 // #3

— end example ] 5

Otherwise, if the parameter type is “array of N X ”, if there exists an implicit conversion sequence for each element of the array from the corresponding element of the initializer list (or from {} if there is no such element) , the implicit conversion sequence is the worst such implicit conversion sequence.

6

Otherwise, if the parameter is a non-aggregate class X and overload resolution per 16.3.1.7 chooses a single best constructor C of X to perform the initialization of an obj ect of type X from the argument initializer list:

(6. 1 )

— If C is not an initializer-list constructor and the initializer list has a single element of type cv U, where U is X or a class derived from X, the implicit conversion sequence has Exact Match rank if U is X, or Conversion rank if U is derived from X.

(6. 2)

— Otherwise, the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion. If multiple constructors are viable but none is better than the others, the implicit conversion sequence is the ambiguous conversion sequence. User-defined conversions are allowed for conversion of the initializer list elements to the constructor parameter types except as noted in 16.3.3.1. [ Example: struct A { A(std: : initializer_list) ; 133)

S ince t here are no paramet ers of array typ e, t his will only o ccur as t he referenced typ e of a reference paramet er.

§ 16.3.3.1.5

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}; void f(A) ; f( {’ a’ , ’ b’ } ) ;

// OK: f(A(std: : initializer_list) ) user-defined conversion

struct B { B(int, double) ; }; void g(B) ; g( {’ a’ , ’ b’ } ) ; g( {1. 0, 1. 0} ) ;

// OK: g(B(int, double) ) user-defined conversion // error: narrowing

void f(B) ; f( {’ a’ , ’ b’ } ) ;

// error: ambiguous f(A) or f(B)

struct C { C(std: : string) ; }; void h(C) ; h({" foo" }) ;

// OK: h(C(std: : string(" foo" ) ) )

struct D { D(A, C) ; }; void i(D) ; i({ {1, 2}, {" bar" } }) ;

// OK: i(D(A(std: : initializer_list{1, 2}) , C(std: : string(" bar" ) ) ) )

— end example ] 7

Otherwise, if the parameter has an aggregate type which can be initialized from the initializer list according to the rules for aggregate initialization (11.6.1) , the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion. [ Example: struct A { int m1; double m2; }; void f(A) ; f( {’ a’ , ’ b’ } ) ; f( {1. 0} ) ;

// OK: f(A(int, double) ) user-defined conversion // error: narrowing

— end example ] 8

Otherwise, if the parameter is a reference, see 16.3.3.1.4. [ Note: The rules in this section will apply for initializing the underlying temporary for the reference. — end note ] [ Example: struct A { int m1; double m2; }; void f(const A&) ; f( {’ a’ , ’ b’ } ) ; f( {1. 0} ) ;

// OK: f(A(int, double) ) user-defined conversion // error: narrowing

void g(const double &) ; g({1}) ; // same conversion as int to double

— end example ] § 16.3.3.1.5

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9 (9. 1)

O t herwise,

—

if t he paramet er typ e is not

if t he init ializ er lis t has one element is t he one required t o convert

void f(int) ; f( {’ a’ } ) ; f( {1. 0} ) ;

— end example (9. 2)

—

if [

t he

init ializ er

list

has

no

1

element s,

implicit

and

better conversion

If conversion sequence S 1

.

—

(2. 2)

—

conversion

sequence

is

t he

ident ity

conversion.

no convers ion is p ossible.

t hen it

[over. ics.rank]

conversion sequences based on t he relat ionships

If an implicit

conversion sequence S 1

is also t he case t hat

S2

indistinguishable conversion sequences

better

is defined by t hese rules t o

worse conversion sequence

is a

is neit her b et t er t han nor wors e t han convers ion sequence S 2 ,

When comparing t he basic forms of implicit

(2. 1 )

conversion sequence

]

b e a b et t er conversion s equence t han S 2 ,

s aid t o b e

t he implicit

Example:

// OK: identity conversion

This sub clause defines a part ial ordering of implicit

S1 .

[

Ranking implicit conversion sequences

conversion sequence

2

t he

In all cases ot her t han t hose enumerat ed ab ove,

16. 3. 3. 2

t o t he paramet er typ e;

]

Example:

— end example

t hat is not it self an init ializ er list ,

t he element

// OK: same conversion as char to int // error: narrowing

void f(int) ; f( { } ) ;

10

a class:

S1

t han

and S 2 are

.

conversion sequences ( as defined in 1 6 . 3 . 3 . 1 )

a s t andard conversion sequence (1 6 . 3 . 3 . 1 . 1 ) is a b et t er conversion sequence t han a user- defined conversion sequence or an ellipsis conversion sequence,

and

a user- defined conversion sequence (1 6 . 3 . 3 . 1 . 2 ) is a b et t er conversion sequence t han an ellipsis conversion sequence ( 1 6 . 3 . 3 . 1 . 3 ) .

3

Two implicit

conversion sequences of t he same form are indist inguishable conversion sequences unless one of

t he following rules applies:

(3. 1 )

—

L ist - init ializ at ion sequence

(3. 1. 1 )

—

(3. 1. 2)

—

L1 L1

convert s t o

L1

is a b et t er conversion sequence t han list - init ializ at ion sequence

std: : initializer_list

convert s t o typ e “ array of

N1 T

”,

L2

for some

X

and

L2

do es not ,

convert s t o typ e “ array of

N2 T

even if one of t he ot her rules in t his p aragraph would ot herwise apply.

void f1(int) ; void f1(std: : initializer_list) ; void g1() { f1({42}) ; }

[

or,

” , and

if not

N1

L2

if

t hat ,

N2

is smaller t han

,

Example:

// #1 // #2 // chooses #2

void f2(std: : pair) ; // #3 void f2(std: : initializer_list) ; // #4 void g2() { f2({" foo" , " bar" }) ; } // chooses #4

— end example (3. 2)

—

]

S t andard conversion sequence

S1

is a b et t er conversion sequence t han st andard conversion sequence

S2

if

(3. 2. 1 )

—

S1

is

a

prop er

s ubsequence

defined by 1 6 . 3 . 3 . 1 . 1 ,

of

S2

( comparing

t he

conversion

excluding any L value Transformat ion;

sequences

considered t o b e a subsequence of any non- ident ity conversion sequence)

§ 1 6. 3. 3. 2

© IS O / IEC

in

t he

canonical

form

t he ident ity conversion sequence is

2 0 1 7 – All right s reserved

or,

if not

t hat ,

330

ISO/IEC 1 4882:201 7(E)

(3. 2. 2)

—

t he rank of

S1

is b et t er t han t he rank of

by t he rules in t he paragrap h b elow,

(3. 2. 3)

—

S1

and

S2

S2 ,

or,

or

S1

if not

are reference bindings ( 1 1 . 6 . 3 )

S2

binds an lvalue reference [

int i; int f1() ; int&& f2() ; int g(const int&) ; int g(const int&&) ; int j = g(i) ; int k = g(f1() ) ; int l = g(f2() ) ;

— end example —

S1 S2

and

S2

]

or,

ref-qualifier Example:

if not

// // // //

—

S1

and

S2

differ

resp ect ively,

int int int int

S1

and

S2

]

or,

and

S1

binds an lvalue reference t o a funct ion lvalue and

Example:

in

t hat ,

t heir

qualificat ion

Example:

conversion

if not

and

T1

yield

similar

typ es

is a prop er subset

T1

and

T2

( 7. 5 ) ,

of t he cv- qualificat ion

// calls f(const int*)

t hat ,

are reference bindings (1 1 . 6 . 3 ) ,

typ e except

p aramet er of a

// calls #1 if not

[

ob j ect

binds an rvalue reference t o

// #1 // #2

only

T2

S1

t hat ,

f(const volatile int *) ; f(const int *) ; i; j = f(&i) ;

— end example —

or,

and

A: : operator declaration template-parameter-list : template-parameter template-parameter-list , template-parameter

[ Note: The > token following the template-parameter-list of a template-declaration may be the product of replacing a >> token by two consecutive > tokens (17.2) . — end note ] The

declaration in a template-declaration shall

(1. 1 )

— declare or define a function, a class, or a variable, or

(1. 2)

— define a member function, a member class, a member enumeration, or a static data member of a class template or of a class nested within a class template, or

(1. 3)

— define a member template of a class or class template, or

(1. 4) (1. 5)

deduction-guide, or — be an alias-declaration . A template-declaration is a declaration . A template-declaration is also a definition if its declaration defines a — be a

function, a class, a variable, or a static data member. A declaration introduced by a template declaration of a variable is a variable template . A variable template at class scope is a static data member template .

[ Example: template constexpr T pi = T(3. 1415926535897932385L) ; template T circular_area(T r) { return pi * r * r; } struct matrix_constants { template using pauli = hermitian_matrix; template constexpr pauli sigma1 = { { 0, 1 }, { 1, 0 } }; template constexpr pauli sigma2 = { { 0, -1i }, { 1i, 0 } }; template constexpr pauli sigma3 = { { 1, 0 }, { 0, -1 } }; };

2

— end example ] A template-declaration can appear only as a namespace scope or class scope declaration. In a function template declaration, the last component of the declarator-id shall not be a template-id. [ Note: That last component may be an identifier , an operator-function-id , a conversion-function-id , or a literal-operator-id . In a class template declaration, if the class name is a simple-template-id , the declaration declares a class template partial specialization (17.5.5) . — end note ] Templates

© ISO/IEC 2017 – All rights reserved

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3

In a template-declaration , explicit specialization, or explicit instantiation the init-declarator-list in the declaration shall contain at most one declarator. When such a declaration is used to declare a class template, no declarator is permitted.

4

A template name has linkage (6.5) . Specializations (explicit or implicit) of a template that has internal linkage are distinct from all specializations in other translation units. A template, a template explicit specialization (17.7.3) , and a class template partial specialization shall not have C linkage. Use of a linkage specification other than " C" or " C++" with any of these constructs is conditionally-supported, with implementation-defined semantics. Template definitions shall obey the one-definition rule (6.2) . [ Note: Default arguments for function templates and for member functions of class templates are considered definitions for the purpose of template instantiation (17.5) and must also obey the one-definition rule. — end note ]

5

A class template shall not have the same name as any other template, class, function, variable, enumeration, enumerator, namespace, or type in the same scope (6.3) , except as specified in 17.5.5. Except that a function template can be overloaded either by non-template functions (11.3.5) with the same name or by other function templates with the same name (17.8.3) , a template name declared in namespace scope or in class scope shall be unique in that scope.

6

A

templated entity is

(6. 1 )

— a template,

(6. 2)

— an entity defined (6.1) or created (15.2) in a templated entity,

(6. 3)

— a member of a templated entity,

(6. 4)

— an enumerator for an enumeration that is a templated entity, or

(6. 5)

— the closure type of a

lambda-expression (8.1.5.1) appearing in the declaration of a templated entity.

[ Note: A local class, a local variable, or a friend function defined in a templated entity is a templated entity. — end note ] 7

A function template, member function of a class template, variable template, or static data member of a class template shall be defined in every translation unit in which it is implicitly instantiated (17.7.1) unless the corresponding specialization is explicitly instantiated (17.7.2) in some translation unit; no diagnostic is required. 1 7. 1

1

Template parameters

The syntax for

[temp. param]

template-parameters is:

template-parameter: type-parameter parameter-declaration type-parameter: type-parameter-key . . . opt identifieropt type-parameter-key identifieropt = type-id template < template-parameter-list > type-parameter-key . . . opt identifieropt template < template-parameter-list > type-parameter-key identifieropt = id-expression type-parameter-key : class typename

[ Note: The > token following the template-parameter-list of a a >> token by two consecutive > tokens (17.2) . — end note ] 2

type-parameter may be the product of replacing

There is no semantic difference between class and typename in a type-parameter-key . typename followed by an unqualified-id names a template type parameter. typename followed by a qualified-id denotes the type in § 17.1

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343

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a non-type 135 [ Example:

parameter-declaration. A template-parameter of the form class identifier is a type-parameter.

class T { /* . . . */ }; int i; template void f(T t) { T t1 = i; // template-parameters T and i : : T t2 = : : i; // global namespace members T and i }

Here, the template f has a type-parameter called T, rather than an unnamed non-type template-parameter of class T . — end example ] A storage class shall not be specified in a template-parameter declaration. Types shall not be defined in a template-parameter declaration. 3

A type-parameter whose identifier does not follow an ellipsis defines its identifier to be a typedef-name (if declared without template ) or template-name (if declared with template ) in the scope of the template declaration. [ Note: A template argument may be a class template or alias template. For example, template class myarray { /* . . . */ }; template class Map { C key; C value; };

4

— end note ] A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

(4. 1 )

— integral or enumeration type,

(4. 2)

— pointer to obj ect or pointer to function,

(4. 3)

— lvalue reference to obj ect or lvalue reference to function,

(4. 4)

— pointer to member,

(4. 5)

— std: : nullptr_t , or

(4. 6)

— a type that contains a placeholder type (10.1.7.4) .

5

[ Note: Other types are disallowed either explicitly below or implicitly by the rules governing the form of template-arguments (17.3) . — end note ] The top-level cv-qualifiers on the template-parameter are ignored when determining its type.

6

A non-type non-reference template-parameter is a prvalue. It shall not be assigned to or in any other way have its value changed. A non-type non-reference template-parameter cannot have its address taken. When a non-type non-reference template-parameter is used as an initializer for a reference, a temporary is always used. [ Example: template void f() { i++; // error: change of template-parameter value

135)

&x; &i;

// OK // error: address of non-reference template-parameter

int& ri = i; const int& cri = i;

// error: non-const reference bound to temporary // OK: const reference bound to temporary

template-parameters and t emplat e template-arguments are t reat ed as typ es for descript ive purp oses, non-type parameter and non-type argument are used t o refer t o non- typ e, non- t emplat e paramet ers and argument s.

S ince t emplat e

t erms

§ 17.1

© ISO/IEC 2017 – All rights reserved

t he

344

ISO/IEC 14882: 2017(E)

}

7

— end example ] A non-type template-parameter shall not be declared to have floating-point, class, or void type. [ Example: template class X; template class Y; template class Z;

8

// error // OK // OK

— end example ] A non-type template-parameter of type “array of T ” or of function type T is adj usted to be of type “pointer to T ”. [ Example: template struct R { /* . . . */ }; template struct S { /* . . . */ }; int p; R w; // OK S x; // OK due to parameter adjustment int v[5] ; R y; // OK due to implicit argument conversion S z; // OK due to both adjustment and conversion

9

— end example ] A default template-argument is a template-argument (17.3) specified after = in a template-parameter . A default template-argument may be specified for any kind of template-parameter (type, non-type, template) that is not a template parameter pack (17.5.3) . A default template-argument may be specified in a template declaration. A default template-argument shall not be specified in the template-parameter-lists of the definition of a member of a class template that appears outside of the member’s class. A default template-argument shall not be specified in a friend class template declaration. If a friend function template declaration specifies a default template-argument , that declaration shall be a definition and shall be the only declaration of the function template in the translation unit.

10

The set of default template-argument s available for use is obtained by merging the default arguments from all prior declarations of the template in the same way default function arguments are (11.3.6) . [ Example: template class A; template class A;

is equivalent to template class A;

11

— end example ] If a template-parameter of a class template, variable template, or alias template has a default templateargument, each subsequent template-parameter shall either have a default template-argument supplied or be a template parameter pack. If a template-parameter of a primary class template, primary variable template, or alias template is a template parameter pack, it shall be the last template-parameter . A template parameter pack of a function template shall not be followed by another template parameter unless that template parameter can be deduced from the parameter-type-list (11.3.5) of the function template or has a default argument (17.8.2) . A template parameter of a deduction guide template (17.9) that does not have a default argument shall be deducible from the parameter-type-list of the deduction guide template. [ Example: template class B;

// error

// U can be neither deduced from the parameter-type-list nor specified template void f() { } // error template void g() { } // error

§ 17.1

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ISO/IEC 14882: 2017(E)

12

— end example ] A template-parameter shall not be given default arguments by two different declarations in the same scope. [ Example: template class X; template class X { /* . . . */ };

// error

— end example ] 13

14

When parsing a default template-argument for a non-type template-parameter , the first non-nested > is taken as the end of the template-parameter-list rather than a greater-than operator. [ Example: template 4 > class X { /* . . . */ };

// syntax error

template 4) > class Y { /* . . . */ };

// OK

— end example ] A template-parameter of a template template-parameter is permitted to have a default template-argument . When such default arguments are specified, they apply to the template template-parameter in the scope of the template template-parameter . [ Example: template struct B {}; template struct A { inline void f() ; inline void g() ; }; template void A: : f() { T t; // error - TT has no default template argument } template void A: : g() { T t; // OK - T }

15

— end example ] If a template-parameter is a type-parameter with an ellipsis prior to its optional identifier or is a parameterdeclaration that declares a parameter pack (11.3.5) , then the template-parameter is a template parameter pack (17.5.3) . A template parameter pack that is a parameter-declaration whose type contains one or more unexpanded parameter packs is a pack expansion. Similarly, a template parameter pack that is a

type-parameter with a template-parameter-list containing one or more unexpanded parameter packs is a pack expansion. A template parameter pack that is a pack expansion shall not expand a parameter pack declared in the same template-parameter-list. [ Example: template class Tuple; template struct multi_array; template struct value_holder { template struct apply { };

// Types is a template type parameter pack // but not a pack expansion // Dims is a non-type template parameter pack // but not a pack expansion // Values is a non-type template parameter pack // and a pack expansion

}; template struct static_array; // error: Values expands template type parameter // pack T within the same template parameter list

— end example ]

§ 17.1

© ISO/IEC 2017 – All rights reserved

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ISO/IEC 1 4882:201 7(E)

1 7. 2 1

Names of template specializations

A template specialization (17.7) can be referred to by a

[temp. names]

template-id:

simple-template-id : template-name < template-argument-listopt > template-id : simple-template-id operator-function-id < template-argument-listopt > literal-operator-id < template-argument-listopt > template-name : identifier template-argument-list : template-argument . . . opt template-argument-list , template-argument . . . opt template-argument : constant-expression type-id id-expression

[ Note: The name lookup rules (6.4) are used to associate the use of a name with a template declaration; that is, to identify a name as a template-name . — end note ] 2

For a template-name to be explicitly qualified by the template arguments, the name must be known to refer to a template.

3

After name lookup (6.4) finds that a name is a template-name or that an operator-function-id or a literaloperator-id refers to a set of overloaded functions any member of which is a function template, if this is followed by a < , the < is always taken as the delimiter of a template-argument-list and never as the less-than operator. When parsing a template-argument-list , the first non-nested > 136 is taken as the ending delimiter rather than a greater-than operator. Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id. [ Note: The second > token produced by this replacement rule may terminate an enclosing template-id construct or it may be part of a different construct (e.g. a cast) . — end note ] [ Example: template class X { /* . . . */ };

// syntax error // OK

X< 1>2 > x1; X2) > x2;

template class Y { /* . . . */ }; Y x3; // OK, same as Y x3; Y1>> x4; // syntax error Y1) >> x5; // OK

— end example ] 4

The keyword template is said to appear at the top level in a qualified-id if it appears outside of a templateargument-list or decltype-specifier. In a qualified-id of a declarator-id or in a qualified-id formed by a class-head-name (Clause 12) or enum-head-name (10.2) , the keyword template shall not appear at the top level. In a qualified-id used as the name in a typename-specifier (17.6) , elaborated-type-specifier (10.1.7.3) , using-declaration (10.3.3) , or class-or-decltype (Clause 13) , an optional keyword template appearing at the top level is ignored. In these contexts, a < token is always assumed to introduce a template-argument-list . In all other contexts, when naming a template specialization of a member of an unknown specialization (17.6.2.1) , 136)

A

>

t hat encloses t he

template-arguments

§ 17.2

type-id

of a

of a subsequent

dynamic_cast , static_cast , reinterpret_cast

template-id,

or

const_cast ,

or which encloses t he

is considered nest ed for t he purp ose of t his descript ion.

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the member template name shall be prefixed by the keyword template . [ Example: struct X { template X* alloc() ; template static X* adj ust() ; }; template void f(T* p) { T* p1 = p->alloc() ; // ill-formed: < T* p2 = p->template alloc() ; // OK: < starts T: : adj ust() ; // ill-formed: < T: : template adj ust() ; // OK: < starts }

means less than template argument list means less than template argument list

— end example ] 5

A name prefixed by the keyword template shall be a template-id or the name shall refer to a class template or an alias template. [ Note: The keyword template may not be applied to non-template members of class templates. — end note ] [ Note: As is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the nested-name-specifier or the expression on the left of the -> or . is not dependent on a template-parameter , or the use does not appear in the scope of a template. — end note ] [ Example: template struct A { void f(int) ; template void f(U) ; }; template void f(T t) { A a; a. template f(t) ; a. template f(t) ; }

// OK: calls template // error: not a template-id

template struct B { template struct C { }; };

// OK: T: : template C names a class template: template struct D { }; D db; 6 7

— end example ] A simple-template-id that names a class template specialization is a class-name (Clause 12) . A template-id that names an alias template specialization is a type-name . 1 7. 3

1

Template arguments

[temp. arg]

There are three forms of template-argument, corresponding to the three forms of template-parameter : type, non-type and template. The type and form of each template-argument specified in a template-id shall match the type and form specified for the corresponding parameter declared by the template in its templateparameter-list. When the parameter declared by the template is a template parameter pack (17.5.3) , it will correspond to zero or more template-argument s. [ Example: template class Array { T* v; int sz; public:

§ 17.3

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explicit Array(int) ; T& operator[] (int) ; T& elem(int i) { return v[i] ; } }; Array v1(20) ; typedef std: : complex dcomplex; Array v2(30) ; Array v3(40) ;

// std: : complex is a standard library template

void bar() { v1[3] = 7; v2[3] = v3. elem(4) = dcomplex(7, 8) ; }

2

— end example ] In a template-argument , an ambiguity between a type-id and an expression is resolved to a type-id , regardless of the form of the corresponding template-parameter . 137 [ Example: template void f() ; template void f() ; void g() { f() ; }

3

// int() is a type-id: call the first f()

— end example ] The name of a template-argument shall be accessible at the point where it is used as a template-argument . [ Note: If the name of the template-argument is accessible at the point where it is used as a template-argument , there is no further access restriction in the resulting instantiation where the corresponding template-parameter name is used. — end note ] [ Example: template class X { static T t; }; class Y { private: struct S { /* . . . */ }; X x; // OK: S is accessible // X has a static member of type Y: : S // OK: even though Y: : S is private }; X y;

// error: S not accessible

— end example ] For a template-argument that is a class type or a class template, the template definition has no special access rights to the members of the template-argument . [ Example: template class A { typename T: : S s; }; template class B { private: 137)

There is no such ambiguity in a default

allowable forms of t he

§ 17.3

template-argument.

template-argument

b ecause t he form of t he

© ISO/IEC 2017 – All rights reserved

template-parameter

det ermines t he

349

ISO/IEC 14882: 2017(E)

struct S { /* . . . */ }; };

// ill-formed: A has no access to B: : S

A b;

— end example ] 4

When template argument packs or default template-argument s are used, a template-argument list can be empty. In that case the empty brackets shall still be used as the template-argument-list. [ Example: template class String; String* p; // OK: String String* q; // syntax error template class Tuple; Tuple* t; // OK: Elements is empty Tuple* u; // syntax error

— end example ] 5

An explicit destructor call (15.4) for an obj ect that has a type that is a class template specialization may explicitly specify the template-argument s. [ Example: template struct A { ~ A() ; }; void f(A* p, A* q) { p->A: : ~ A() ; q->A: : ~ A() ; }

6

// OK: destructor call // OK: destructor call

— end example ] If the use of a template-argument gives rise to an ill-formed construct in the instantiation of a template specialization, the program is ill-formed.

7

When the template in a template-id is an overloaded function template, both non-template functions in the overload set and function templates in the overload set for which the template-argument s do not match the template-parameters are ignored. If none of the function templates have matching template-parameters, the program is ill-formed.

8

When a simple-template-id does not name a function, a default template-argument is implicitly instantiated (17.7.1) when the value of that default argument is needed. [ Example: template struct S { }; S* p; // the type of p is S*

The default argument for U is instantiated to form the type S* . 9

A

template-argument followed by an ellipsis is a pack expansion (17.5.3) .

1 7. 3. 1 1 2

— end example ]

Template type arguments

[temp. arg. type]

template-argument for a template-parameter which is a type shall be a type-id. [ Example:

A

template class X { }; template void f(T t) { } struct { } unnamed_obj ; void f() { struct A { }; enum { e1 };

§ 17.3.1

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}

typedef struct { } B; B b; X x1; // OK X x2; // OK X x3; // OK f(e1) ; // OK f(unnamed_obj ) ; // OK f(b) ; // OK

— end example ] [ Note: A template type argument may be an incomplete type (6.9) . — end note ] 1 7. 3. 2

Template non-type arguments

[temp. arg. nontype]

1

If the type of a template-parameter contains a placeholder type (10.1.7.4, 17.1) , the deduced parameter type is determined from the type of the template-argument by placeholder type deduction (10.1.7.4.1) . If a deduced parameter type is not permitted for a template-parameter declaration (17.1) , the program is ill-formed.

2

A template-argument for a non-type template-parameter shall be a converted constant expression (8.20) of the type of the template-parameter . For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of) :

(2. 1 )

— a subobj ect (4.5) ,

(2. 2)

— a temporary obj ect (15.2) ,

(2. 3)

— a string literal (5.13.5) ,

(2. 4)

— the result of a typeid expression (8.2.8) , or

(2. 5)

— a predefined __func__ variable (11.4.1) . [ Note: If the template-argument represents a set of overloaded functions (or a pointer or member pointer to such) , the matching function is selected from the set (16.4) . — end note ]

3

[ Example: template struct X { /* . . . */ }; int ai[10] ; X xi; // array to pointer and qualification conversions struct Y { /* . . . */ }; template struct Z { /* . . . */ }; Y y; Z z; // no conversion, but note extra cv-qualification template struct W { /* . . . */ }; int b[5] ; W w; // no conversion void f(char) ; void f(int) ; template struct A { /* . . . */ }; A a;

// selects f(int)

template struct B { /* . . . */ }; B b1; // OK: template parameter type is int B b2; // OK: template parameter type is char B b3; // error: template parameter type cannot be double

§ 17.3.2

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4

— end example Note:

]

A st ring lit eral ( 5 . 1 3 . 5 )

[

is not

an accept able

template-argument

.

[

Example:

template class X { /* . . . */ };

// error: string literal as template-argument

X x1;

const char p[] = " Vivisectionist" ; X x2; // OK

5

— end example Note: Example:

[

]

— end note

]

The address of an array element

or non- st at ic dat a memb er is not

an accept able

template-argument

.

[

template class X { }; int a[10] ; struct S { int m; static int s; } s;

// error: address of array element // error: address of non-static member // OK: address of static member // OK: address of static member

X x3; X x4; X x5; X x6;

6

— end example Note:

[

]

— end note

]

A t emp orary ob j ect is not an accept able

has reference typ e.

[

Example:

template-argument

when t he corresp onding

template-parameter

template struct B { /* . . . */ }; B b2;

// error: temporary would be required for template argument

int c = 1; B b1;

// OK

— end example 1 7. 3. 3 1

A

]

— end note

]

Template template arguments

template-argument

t emp lat e, expressed as

[temp. arg. template]

template-parameter id-expression template-argument

for a t emplat e .

shall b e t he name of a class t emplat e or an alias

When t he

names a clas s t emplat e, only primary class

t emplat es are considered when mat ching t he t emplat e t emplat e argument part ial sp ecializ at ions are not

wit h t he corresp onding paramet er;

considered even if t heir paramet er list s mat ch t hat

of t he t emplat e t emplat e

paramet er.

2

Any part ial sp ecializ at ions (1 7. 5 . 5 ) as so ciat ed wit h t he primary class t emplat e or primary variable t emplat e are considered when a s p ecializ at ion b ased on t he t emplat e is not visible at t he p oint of inst ant iat ion, ill- formed,

no diagnos t ic required.

[

template-parameter

is inst ant iat ed.

If a sp ecializ at ion

and it would have b een select ed had it b een visible,

Example:

t he program is

template class A { // primary template int x; }; template class A { // partial specialization long x; };

§ 1 7. 3 . 3

© IS O / IEC

2 0 1 7 – All right s reserved

352

ISO/IEC 14882: 2017(E)

template class C { V y; V z; }; C c; // V within C uses the primary template, so c. y. x has type int // V within C uses the partial specialization, so c. z. x has type long

— end example template-argument template-argument A P ]

3

A

mat ches

. If

a

t emplat e

template-parameter P

cont ains a paramet er pack, t hen

A

when

als o mat ches

P P

is

at

least

if each of

A

as

of

.

mat ches,

recursively.

and for t emplat e When

t he

Two t emplat e paramet ers

mat ch if t hey are of t he same kind ( typ e, non- typ e, t emplat e) , for non- typ e ( 1 7. 5 . 6 . 1 ) ,

as

template-parameter-list P template-parameter template-parameter template-parameter P template-parameter-list

mat ches t he corresp onding t emplat e paramet er in t he

equivalent

sp ecializ ed

’ s t emplat e paramet ers

s,

’s

s, t heir typ es are

each of t heir corresp onding

cont ains

a t emplat e

paramet er

pack (1 7. 5 . 3 ) ,

s

t he

t emp lat e paramet er pack will mat ch z ero or more t emplat e paramet ers or t emplat e paramet er packs in t he

template-parameter-list

of

A

wit h t he

same

typ e

and

form as

t he

t emplat e

paramet er

pack in

P

( ignoring

whet her t hos e t emplat e paramet ers are t emplat e paramet er packs) .

[

Example: template class A { /* . . . */ }; template class B { /* . . . */ }; template class C { /* . . . */ }; template class D { /* . . . */ }; template class X { /* . . . */ }; template class Y { /* . . . */ }; template class Z { /* . . . */ }; X X X Y Y Y Z

// OK // OK // OK // OK // OK // OK // OK

xa; xb; xc; ya; yb; yc; zd;

— end example Example:

]

[

template struct eval; template struct eval { }; template template template template template

struct A; struct B; struct C; struct D; struct E;

eval eA; eval eB; eval eC; eval eD; eval eE;

— end example § 1 7. 3 . 3

// OK: matches // OK: matches // error: C does // error: D does // error: E does

partial specialization of partial specialization of not match TT in partial not match TT in partial not match TT in partial

eval eval specialization specialization specialization

]

© IS O / IEC

2 0 1 7 – All right s reserved

353

ISO/IEC 1 4882:201 7(E)

4

A template template-parameter P is at least as specialized as a template template-argument A if, given the following rewrite to two function templates, the function template corresponding to P is at least as specialized as the function template corresponding to A according to the partial ordering rules for function templates (17.5.6.2) . Given an invented class template X with the template parameter list of A (including default arguments) :

(4. 1 )

— Each of the two function templates has the same template parameters, respectively, as P or A.

(4. 2)

— Each function template has a single function parameter whose type is a specialization of X with template arguments corresponding to the template parameters from the respective function template where, for each template parameter PP in the template parameter list of the function template, a corresponding template argument AA is formed. If PP declares a parameter pack, then AA is the pack expansion PP. . . (17.5.3) ; otherwise, AA is the id-expression PP . If the rewrite produces an invalid type, then P is not at least as specialized as A. 1 7. 4

1 (1. 1 ) (1. 2)

Type equivalence

[temp. type]

template-ids refer to the same class, function, or variable if — their template-name s, operator-function-id s, or literal-operator-ids refer to the same template and — their corresponding type template-argument s are the same type and

Two

(1. 3)

— their corresponding non-type template arguments of integral or enumeration type have identical values and

(1. 4)

— their corresponding non-type template-argument s of pointer type refer to the same obj ect or function or are both the null pointer value and

(1. 5)

— their corresponding non-type template-argument s of pointer-to-member type refer to the same class member or are both the null member pointer value and

(1. 6)

— their corresponding non-type and

template-arguments of reference type refer to the same object or function

(1. 7)

— their corresponding template

template-arguments refer to the same template.

[ Example: template class buffer { /* . . . */ }; buffer x; buffer y;

declares x and y to be of the same type, and template class list { /* . . . */ }; list x1; list x2; list x3; list x4;

declares x2 and x3 to be of the same type. Their type differs from the types of x1 and x4 . template struct X { }; template struct Y { }; template using Z = Y; X y; X z;

declares y and z to be of the same type. 2

— end example ]

If an expression e is type-dependent (17.6.2.2) , decltype( e ) denotes a unique dependent type. Two such

decltype-specifiers refer to the same type only if their expressions are equivalent (17.5.6.1) . [ Note: However, § 17.4

© ISO/IEC 2017 – All rights reserved

354

ISO/IEC 14882: 2017(E)

such a type may be aliased, e.g., by a 1 7. 5 1

typedef-name. — end note ]

Template declarations

[temp. decls]

A template-id , that is, the template-name followed by a template-argument-list shall not be specified in the declaration of a primary template declaration. [ Example: template class A { }; template void sort(T1 data[I] ) ;

2

// error // error

— end example ] [ Note: However, this syntax is allowed in class template partial specializations (17.5.5) . — end note ] For purposes of name lookup and instantiation, default arguments and noexcept-specifier s of function templates and default arguments and noexcept-specifier s of member functions of class templates are considered definitions; each default argument or noexcept-specifier is a separate definition which is unrelated to the function template definition or to any other default arguments or noexcept-specifier s. For the purpose of instantiation, the substatements of a constexpr if statement (9.4.1) are considered definitions.

3

Because an alias-declaration cannot declare a an alias template. 1 7. 5. 1

1 2

template-id, it is not possible to partially or explicitly specialize

Class templates

[temp. class]

class template defines the layout and operations for an unbounded set of related types. [ Example: A single class template List might provide an unbounded set of class definitions: one class A

List for every type T, each describing a linked list of elements of type T. Similarly, a class template Array describing a contiguous, dynamic array might be defined like this: template class Array { T* v; int sz; public: explicit Array(int) ; T& operator[] (int) ; T& elem(int i) { return v[i] ; } };

The prefix template specifies that a template is being declared and that a type-name T may be used in the declaration. In other words, Array is a parameterized type with T as its parameter. — end example ] 3

When a member function, a member class, a member enumeration, a static data member or a member template of a class template is defined outside of the class template definition, the member definition is defined as a template definition in which the template-parameter s are those of the class template. The names of the template parameters used in the definition of the member may be different from the template parameter names used in the class template definition. The template argument list following the class template name in the member definition shall name the parameters in the same order as the one used in the template parameter list of the member. Each template parameter pack shall be expanded with an ellipsis in the template argument list. [ Example: template struct A { void f1() ; void f2() ; }; template void A: : f1() { } template void A: : f2() { }

§ 17.5.1

// OK // error

© ISO/IEC 2017 – All rights reserved

355

ISO/IEC 14882: 2017(E)

template struct B { void f3() ; void f4() ; };

// OK // error

template void B: : f3() { } template void B: : f4() { }

— end example 4

In a redeclarat ion, t he

class-key

17. 5. 1. 1 1

]

part ial sp ecializ at ion,

explicit

sp ecializ at ion or explicit

inst ant iat ion of a class t emp lat e,

shall agree in kind wit h t he original class t emplat e declarat ion (1 0 . 1 . 7. 3 ) .

Member functions of class templates

[temp.mem. func]

A memb er funct ion of a class t emplat e may b e defined out side of t he class t emplat e definit ion in which it declared.

[

Example:

is

template class Array { T* v; int sz; public: explicit Array(int) ; T& operator[] (int) ; T& elem(int i) { return v[i] ; } }; declares t hree funct ion t emplat es.

The sub script

funct ion might

b e defined like t his:

template T& Array: : operator[] (int i) { if (i class A { }; // error

— end example

]

(8. 2)

—

The sp ecializ at ion shall b e more s p ecializ ed t han t he primary t emplat e (1 7. 5 . 5 . 2 ) .

(8. 3)

—

The t emplat e paramet er list

(8. 4)

—

An

argument

sion ( 1 7. 5 . 3 ) ,

1 7. 5.5.1 1

When

a

shall it

class

t emplat e

whet her

s p ecializ at ions.

(1. 2)

—

is

t he

an

unexp anded

argument

paramet er

pack.

t emplat e argument

If an

in t he t emplat e argument

used

in

a

inst ant iat ion

cont ext is

to

t hat

be

requires

generat ed

an

using

is

a

p ack

138

expan-

list .

[temp. class. spec. match]

inst ant iat ion t he

argument

values.

primary

of t he

class,

t emplat e

or

it

is

one

necessary

of t he

to

part ial

This is done by mat ching t he t emplat e argument s of t he class t emplat e sp ecializ at ion wit h

t he t emplat e argument

—

cont ain

shall b e t he last

cont ain default

Matching of class template partial specializations

det ermine

(1. 1 )

not

of a sp ecializ at ion shall not

list s of t he part ial sp ecializ at ions.

If exact ly one mat ching s p ecializ at ion is found,

t he inst ant iat ion is generat ed from t hat

sp ecializ at ion.

If more t han one mat ching sp ecializ at ion is found, t he part ial order rules (1 7. 5 . 5 . 2 ) are used t o det ermine whet her one of t he sp ecializ at ions is more sp ecializ ed t han t he ot hers. more sp ecializ ed t han all of t he ot her mat ching sp ecializ at ions,

If none of t he sp ecializ at ions is

t hen t he use of t he class t emplat e is

ambiguous and t he program is ill- formed.

(1. 3) 2

—

A

If no mat ches are found,

part ial

sp ecializ at ion

t he inst ant iat ion is generat ed from t he primary t emplat e.

mat ches

a

given

act ual

t emplat e

argument

list

part ial sp ecializ at ion can b e deduced from t he act ual t emplat e argument

template class A { }; template class A { }; template class A { }; 138)

if t he list

t emplat e

(1 7. 8 . 2 ) .

[

argument s

Example:

of t he

// #1 // #2 // #3

There is no way in which t hey could b e used.

§ 1 7. 5 . 5 . 1

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2 0 1 7 – All right s reserved

365

ISO/IEC 14882: 2017(E)

// #4 // #5

template class A { }; template class A { }; A A A A A

— end example 3

If t he

// uses #1 // uses #2, T is int , I is 1 // uses #4, T is char // uses #5, T1 is int , T2 is char , I is 1 // ambiguous: matches #3 and #5

a1; a2; a3; a4; a5;

]

t emplat e

argument s

template-parameter-list

of a

and t he

part ial

sp ecializ at ion

template-id

,

cannot

be

deduced

t he program is ill- formed.

template struct A {}; template struct A {};

// error

template struct A {};

// OK

[

b ecause

Example:

of t he

s t ruct ure

of it s

template struct B {}; template struct B {}; // OK

— end example 4

]

In a typ e name t hat

refers t o a class t emp lat e sp ecializ at ion,

mat ch t he t emplat e paramet er list

of t he primary t emplat e.

( e. g. ,

A )

t he argument

list

shall

The t emplat e argument s of a sp ecializ at ion are

deduced from t he argument s of t he primary t emplat e.

17. 5. 5. 2 1

For

two

Partial ordering of class template specializations class

t emp lat e

part ial

sp ecializ at ions,

following rewrit e t o two funct ion t emplat es,

t he

first

t he first

is

[temp.class.order]

more specialized

t han

t he

second

if,

given

t he

funct ion t emplat e is more sp ecializ ed t han t he second

according t o t he ordering rules for funct ion t emplat es (1 7. 5 . 6 . 2 ) :

(1. 1 )

—

Each of t he two

funct ion t emplat es

has

t he

same t emplat e

paramet ers

as

t he

corresp onding part ial

sp ecializ at ion.

(1. 2)

—

Each funct ion t emplat e has a single funct ion paramet er whose typ e is a class t emplat e sp ecializ at ion where t he t emplat e argument s are t he corresp onding t emplat e paramet ers from t he funct ion t emplat e for

each

t emplat e

argument

in

t he

template-argument-list

of t he

simple-template-id

of t he

part ial

sp ecializ at ion.

2

[

Example: template class X { }; template class X { }; template class X { };

// #1 // #2

template void f(X) ; template void f(X) ;

// A // B

template class Y { }; template class Y