C++20

C++20 is the name for the revision of the ISO/IEC standard for the C++ programming language following C++17.[1] The standard was technically finalized[2] by WG21 at the meeting in Prague in February 2020,[3] approved on 4th September 2020,[4][5] and published by ISO in December 2020.[6]

C++ language revisions

History

Changes applied to the C++20 working draft in July 2017 (Toronto) include:[7]

  • concepts (what made it into the standard is a cut-down version; also described as "Concepts Lite"[8])
  • designated initializers
  • [=, this] as a lambda capture
  • template parameter lists on lambdas
  • std::make_shared and std::allocate_shared for arrays

Changes applied to the C++20 working draft in the fall meeting in November 2017 (Albuquerque) include:[9][10]

  • three-way comparison using the "spaceship operator", operator <=>
  • initialization of an additional variable within a range-based for statement
  • lambdas in unevaluated contexts
  • default constructible and assignable stateless lambdas
  • allow pack expansions in lambda init-capture
  • string literals as template parameters
  • atomic smart pointers (such as std::atomic<shared_ptr<T>> and std::atomic<weak_ptr<T>>)
  • std::to_address to convert a pointer to a raw pointer

Changes applied to the C++20 working draft in March 2018 (Jacksonville) include:[11]

  • removing the need for typename in certain circumstances
  • new standard attributes [[no_unique_address]], [[likely]] and [[unlikely]]
  • calendar and time-zone additions to <chrono>
  • std::span, providing a view to a contiguous array (analogous to std::string_view but span can mutate the referenced sequence)
  • <version> header

Changes applied to the C++20 working draft in the summer meeting in June 2018 (Rapperswil) include:[12]

  • contracts (later deferred to a later standard)[13]
  • feature test macros
  • bit-casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals
  • conditional explicit, allowing the explicit modifier to be contingent on a boolean expression
  • constexpr virtual functions

Changes applied to the C++20 working draft in the fall meeting in November 2018 (San Diego) include:

  • ranges (The One Ranges Proposal)
  • concept terse syntax
  • constexpr union, try and catch, dynamic_cast, typeid and std::pointer_traits.
  • various constexpr library bits
  • immediate functions using the new consteval keyword
  • signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)
  • refinements of the contracts facility (access control in contract conditions)[14] (see list of features deferred to a later standard)
  • a revised memory model
  • Smart pointer creation with default initialization

Changes applied to the C++20 working draft in the winter meeting in February 2019 (Kona) include:[15][16]

  • coroutines
  • modules
  • various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)

Changes applied to the C++20 working draft in the summer meeting in July 2019 (Cologne) include:[17][18][19]

  • contracts were removed (see list of features deferred to a later standard)[20]
  • use of comma operator in subscript expressions has been deprecated[21]
  • constexpr additions (trivial default initialization,[22] unevaluated inline-assembly[23])
  • using scoped enums[24]
  • various changes to the spaceship operator[25][26]
  • DR: minor changes to modules[27]
  • constinit keyword
  • changes to concepts (removal of -> Type return-type-requirements[28])
  • (most of) volatile has been deprecated[29]
  • DR: [[nodiscard]] effects on constructors[30]
  • The new standard library concepts will not use PascalCase (rather standard_case, as the rest of the standard library)[31]
  • text formatting[32][33] (chrono integration,[34] corner case fixes[35])
  • bit operations[36]
  • constexpr INVOKE[37]
  • math constants[38]
  • consistency additions to atomics (std::atomic_ref<T>,[39] std::atomic<std::shared_ptr<T>>[40])
  • add the <=> operator to the standard library[41]
  • header units for the standard library[42]
  • synchronization facilities[43] (merged from: Efficient atomic waiting and semaphores,[44] latches and barriers,[45] Improving atomic_flag,[46] Don't Make C++ Unimplementable On Small CPUs[47])
  • std::source_location[48]
  • constexpr containers (std::string,[49] std::vector[50])
  • std::stop_token and joining thread (std::jthread)[51]

Changes applied during the NB comment resolution in the fall meeting in November 2019 (Belfast) include:[52][53][54]

  • Class Types in Non-Type Template Parameters (NTTP): The restriction of no user-defined operator== allowed has been removed as the meaning of template argument equality has been divorced from operator==.[55] This allows also for array members in class-type NTTP.
  • Floating-point types,[56] pointers and references and unions and union-like classes (class types containing anonymous unions) are now allowed as NTTP.
  • Function identity now also includes trailing requires-clauses (P1971)
  • Constrained non-template functions have been removed
  • <compare> is now available in freestanding implementations[57]
  • std::spans typedef was changed from index_type to size_type to be consistent with the rest of the standard library[58]
  • Concept traits have been renamed to follow the renaming of the concepts as a result from the Cologne meeting
  • Several fixes and additions to ranges (P1456R1: Move-only views,[59] P1391R4: Range constructor for std::string_view (constructor from iterator-pair of characters),[60] P1394R4: Range constructor for std::span<ref>,[61] P1870R1: forwarding-range<T> is too subtle[62])
  • Initialization for std::atomic<T> has been changed to make it work with default and list initialization,[63] std::latch and std::barrier can now report the maximum number of threads that the implementation supports through the new member function max()
  • std::weak_equality and std::strong_equality have been removed as they are not used anymore
  • Algorithms in <numeric> have been made constexpr
  • Missing feature-test macros for new or changed features of C++20 have been added[64]

Features

C++20 adds more new major features than C++14 or C++17.[65] Changes that have been accepted into or have been discussed for inclusion into C++20 include:[66]

Language
  • concepts,[67] with terse syntax[68]
  • modules[69]
  • designated initializers[70] (based on the C99 feature, and common G++ extension)
  • [=, this] as a lambda capture[71]
  • template parameter lists on lambdas[72]
  • three-way comparison using the "spaceship operator", operator <=>
  • initialization of an additional variable within a range-based for statement[73]
  • lambdas in unevaluated contexts[74][75]
  • default constructible and assignable stateless lambdas[74][76]
  • allow pack expansions in lambda init-capture[74][77]
  • string literals as template parameters[74][78]
  • removing the need for typename in certain circumstances[79]
  • new standard attributes [[no_unique_address]],[80] [[likely]] and [[unlikely]][81]
  • conditional explicit, allowing the explicit modifier to be contingent on a boolean expression[82]
  • expanded constexpr: virtual functions,[83] union,[84] try and catch,[85] dynamic_cast and typeid,[86] std::pointer_traits[87]
  • immediate functions using the new consteval keyword[88]
  • signed integers are now defined to be represented using two's complement (signed integer overflow remains undefined behavior)[89]
  • a revised memory model[90]
  • various improvements to structured bindings (interaction with lambda captures, static and thread_local storage duration)[91][92]
  • coroutines[93]
  • using on scoped enums[94]
  • constinit keyword[95]

Library
  • ranges (The One Ranges Proposal)[96]
  • std::make_shared and std::allocate_shared for arrays[97]
  • atomic smart pointers (such as std::atomic<shared_ptr<T>> and std::atomic<weak_ptr<T>>)[98]
  • std::to_address to convert a pointer to a raw pointer[99]
  • calendar and time-zone additions to <chrono>[100]
  • std::span, providing a view to a contiguous array (analogous to std::string_view but span can mutate the referenced sequence)[101]
  • std::erase and std::erase_if, simplifying element erasure for most standard containers[102]
  • <version> header[103]
  • std::bit_cast<> for type casting of object representations, with less verbosity than memcpy() and more ability to exploit compiler internals[104]
  • feature test macros[105]
  • various constexpr library bits[106]
  • smart pointer creation with default initialization[107]
  • std::map::contains method[108]
  • bit operations, such as leading/trailing zero/one count,[109] and log2 operations[110][111][112]
  • std::bind_front[113]

New and changed keywords

Many new keywords added (and the new "spaceship operator", operator <=>), such as concept, constinit,[95] consteval, co_await, co_return, co_yield, requires (plus changed meaning for export), and char8_t.[114] And explicit can take an expression since C++20.[115] Most of the uses of the volatile keyword have been deprecated.[29]

In addition to keywords, there are identifiers with special meaning, including new import and module.

New attributes in C++20: [[likely]], [[unlikely]], and [[no_unique_address]][116]

Published as Technical Specifications

Deferred to a later standard
  • Contracts – a new study group (SG21) has been formed to work on a new proposal[121]
  • Reflection[122][123]
  • Metaclasses[124]
  • Executors[125]
  • Networking extensions,[126][127] including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols (blocked by executors)
  • Properties[128]
  • Extended futures[129]

Removed features and deprecation

C++20 also removed some features.[130]

  • The C-derived headers <ccomplex>, <ciso646>, <cstdalign>, <cstdbool> and <ctgmath> were removed, as they serve no purpose in C++. (The corresponding <*.h> headers remain, for compatibility with C.)
  • The use of throw() as an exception specification was removed.
  • Some previously deprecated library features were removed, including std::uncaught_exception, std::raw_storage_iterator, std::is_literal_type, std::is_literal_type_v, std::result_of and std::result_of_t.

Deprecated features include:

  • Use of comma operator in subscript expressions has been deprecated[131]
  • (most of) volatile has been deprecated[29]

Compiler support

Developers of major compilers have prepared for the final C++20 standard by implementing experimental support for various features from the C++20 drafts.

  • Clang has partial C++20 support that can be enabled with the option -std=c++20 (version 10 and later) or -std=c++2a (version 9 and earlier).[132]
  • EDG eccp started implementing C++20 features in version 5.0 and as of version 6.1 supports most C++20 core language features.[133]
  • GCC added partial, experimental C++20 support in 2017[134] in version 8 through the option -std=c++2a. Like Clang, GCC replaced this option with -std=c++20 in version 10. It also has an option to enable GNU extensions in addition to the experimental C++20 support, -std=gnu++20.[135]
  • Microsoft Visual Studio supports some C++20 features through its std:c++latest option. An option /std:c++20 to enable C++20 mode is expected as soon as the compiler is C++20 feature complete.[136]

References
  1. "The Standard". isocpp.org. Retrieved 7 September 2020.
  2. Sutter, Herb (2019-10-01). "P1000R3: C++ IS schedule" (PDF). Retrieved 2020-02-13.
  3. Dusíková, Hana (2019-11-06). "N4817: 2020 Prague Meeting Invitation and Information" (PDF). Retrieved 2020-02-13.
  4. "Current Status". isocpp.org. Retrieved 7 September 2020.
  5. "C++20 Approved -- Herb Sutter". isocpp.org. Retrieved 8 September 2020.
  6. "ISO/IEC 14882:2020".
  7. Herb Sutter. "Trip report: Summer ISO C++ standards meeting (Toronto)".
  8. Andrew Sutton; Bjarne Stroustrup (2013-02-24). "Concepts Lite: Constraining Templates with Predicates". isocpp.org. isocpp.org. Retrieved 2021-01-13.
  9. Herb Sutter. "Trip report: Fall ISO C++ standards meeting (Albuquerque)".
  10. Smith, Richard; Perchik, Dawn; Köppe, Thomas. "N4714 Editors' Report -- Programming Languages -- C++". C++ standards drafts. GitHub. Retrieved 27 December 2018.
  11. Botond Ballo. "Trip Report: C++ Standards Meeting in Jacksonville, March 2018".
  12. Herb Sutter. "Trip report: Summer ISO C++ standards meeting (Rapperswil)".
  13. "Support for contract based programming in C++". www.open-std.org. Retrieved 2018-11-10.
  14. "P1289R0 - Access control in contract conditions" (PDF).
  15. "r/cpp - 2019-02 Kona ISO C++ Committee Trip Report (C++20 design is complete; Modules in C++20; Coroutines in C++20; Reflection TS v1 published; work begins on a C++ Ecosystem Technical Report)". reddit. Retrieved 2019-02-24.
  16. "Trip report: Winter ISO C++ standards meeting (Kona)". Sutter's Mill. 2019-02-23. Retrieved 2019-02-24.
  17. "r/cpp - 2019-07 Cologne ISO C++ Committee Trip Report — 🚀 The C++20 Eagle has Landed 🚀 (C++20 Committee Draft shipped; Contracts Moved From C++20 to a Study Group; 'std::format' in C++20; C++20 Synchronization Library)". reddit. Retrieved 2019-09-15.
  18. Botond Ballo. "Trip Report: C++ Standards Meeting in Cologne, July 2019".
  19. Sutter, Herb. "Trip report: Summer ISO C++ standards meeting (Cologne)".
  20. Josuttis, Nicolai. "P1823R0: Remove Contracts from C++20" (PDF).
  21. "P1161R2: Deprecate uses of the comma operator in subscripting expressions". www.open-std.org. Retrieved 2019-07-20.
  22. "Permitting trivial default initialization in constexpr contexts" (PDF).
  23. "P1668R1: Enabling Constexpr Intrinsics By Permitting Unevaluated inline-asm in Constexpr Functions". www.open-std.org. Retrieved 2019-07-20.
  24. "P1099R5: Using Enum". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  25. "P1186R3: When do you actually use <=>?". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  26. "P1630R1: Spaceship needs a tune-up". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  27. "P1766R1: Mitigating minor modules maladies". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  28. "P1452R2: On the non-uniform semantics of return-type-requirements". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  29. "P1152R4: Deprecating volatile". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  30. "P1771R1: nodiscard for constructors" (PDF). www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  31. "P1754R1: Rename concepts to standard_case for C++20, while we still can" (PDF). www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  32. "P0645R10: Text Formatting". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  33. "std::format in C++20". www.zverovich.net. 2019-07-23. Retrieved 2019-09-15.
  34. "P1361R2: Integration of chrono with text formatting" (PDF). www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  35. "P1652R1: Printf corner cases in std::format". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  36. "P0553R4: Bit operations". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  37. "P1965R2: constexpr INVOKE". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  38. "P0631R8: Math Constants" (PDF). www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  39. "P1643R1: Add wait/notify to atomic_ref<T>". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  40. "P1664R0: Add wait/notify to atomic<shared_ptr<T>>". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  41. "P1614R2: The Mothership has Landed - Adding <=> to the Library". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  42. "P1502R1: Standard library header units for C++20". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  43. "P1135R6: The C++20 Synchronization Library". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  44. "P0514R4: Efficient concurrent waiting for C++20" (PDF).
  45. "P0666R2: Revised Latches and Barriers for C++20" (PDF).
  46. "P0995R1: Improving atomic_flag".
  47. "P1285R0: Don't Make C++ Unimplementable On Small CPUs".
  48. "P1208R6: Adopt source_location from Library Fundamentals V3 for C++20" (PDF).
  49. "P0980R1: Making std::string constexpr" (PDF).
  50. "P1004R2: Making std::vector constexpr" (PDF).
  51. "P0660R10: Stop Token and Joining Thread" (PDF).
  52. "r/cpp - 2019-11 Belfast ISO C++ Committee Trip Report — Started Processing Feedback on the C++20 Committee Draft; ABI Review Group Formed". reddit. Retrieved 2019-11-09.
  53. Sutter, Herb (2019-11-09). "Trip report: Autumn ISO C++ standards meeting (Belfast)". Sutter's Mill. Retrieved 2019-11-09.
  54. botondballo (2019-11-15). "Trip Report: C++ Standards Meeting in Belfast, November 2019". There's Waldo!. Retrieved 2019-11-24.
  55. "P1907R0: Inconsistencies with non-type template parameters". www.open-std.org. 2019-10-07. Retrieved 2019-11-09.
  56. "P1714: NTTP are incomplete without float, double, and long double! (Revision 1)". www.open-std.org. 2019-07-19. Retrieved 2019-11-09.
  57. "P1855R=: Make <compare> freestanding". www.open-std.org. 2019-10-05. Retrieved 2019-11-09.
  58. "P1872R0: span should have size_type not index_type" (PDF). www.open-std.org. 2019-09-16. Retrieved 2019-11-09.
  59. "P1456: Move-only views" (PDF). www.open-std.org. 2019-01-25. Retrieved 2019-11-09.
  60. "P1391: Range constructor for std::string_view" (PDF). www.open-std.org. Retrieved 2019-11-09.
  61. "P1394: Range constructor for std::span" (PDF). www.open-std.org. 2019-08-02. Retrieved 2019-11-09.
  62. "P1870R1: forwarding-range<T> is too subtle". www.open-std.org. Retrieved 2019-11-09.
  63. "P0883: Fixing Atomic Initialization" (PDF). www.open-std.org. 2018-06-05. Retrieved 2019-11-09.
  64. "Missing feature-test macros 2018-2019". www.open-std.org. 2019-10-06. Retrieved 2019-11-09.
  65. "Why does the C++ standard ship every three years?".
  66. "P0592R0: To boldly suggest an overall plan for C++20".
  67. "P0606R0: Concepts Are Ready" (PDF).
  68. "P1141R1 - Yet another approach for constrained declarations".
  69. "N4720: Working Draft, Extensions to C++ for Modules" (PDF).
  70. Tim Shen; Richard Smith. "Designated Initialization Wording".
  71. Thomas Köppe. "Allow lambda capture [=, this]".
  72. "Familiar template syntax for generic lambdas".
  73. "Range-based for statements with initializer".
  74. "Trip Report: C++ Standards Meeting in Albuquerque, November 2017". There's Waldo!. 2017-11-20. Retrieved 2017-12-11.
  75. "Wording for lambdas in unevaluated contexts" (PDF).
  76. "Default constructible and assignable stateless lambdas" (PDF).
  77. "Pack expansion in lambda init-capture". www.open-std.org. Retrieved 2017-12-11.
  78. "String literals as non-type template parameters" (PDF).
  79. Nina Ranns; Daveed Vandevoorde. "Down with typename!".
  80. "Language support for empty objects".
  81. "Proposed wording for likely and unlikely attributes (Revision 5)".
  82. "explicit(bool)". www.open-std.org. Retrieved 2018-11-13.
  83. "Allowing Virtual Function Calls in Constant Expressions". www.open-std.org. Retrieved 2019-03-11.
  84. "P1330R0 - Changing the active member of a union inside constexpr".
  85. "P1002R0 - Try-catch blocks in constexpr functions" (PDF).
  86. "P1327R0 - Allowing dynamic_cast, polymorphic typeid in Constant Expressions".
  87. "P1006R1 - Constexpr in std::pointer_traits" (PDF).
  88. "P1073R2 - Immediate functions".
  89. "P1236R0: Alternative Wording for P0907R4 Signed Integers are Two's Complement".
  90. "P0668R4: Revising the C++ memory model".
  91. "P1091R1: Extending structured bindings to be more like variable declarations". www.open-std.org. Retrieved 2019-02-24.
  92. "P1091R2: Extending structured bindings to be more like variable declarations". www.open-std.org. Retrieved 2019-02-24.
  93. "N4649: Working Draft, Technical Specification on C++ Extensions for Coroutines" (PDF).
  94. "P1099R5: Using Enum". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  95. "P1143R2: Adding the constinit keyword". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  96. "P0896R3" (PDF).
  97. "Extending make_shared to Support Arrays".
  98. Meredith, Alisdair; Sutter, Herb. "Revising atomic_shared_ptr for C++20". JTC1/SC22/WG21 - The C++ Standards Committee - ISOCPP. ISO. Retrieved 27 December 2018.
  99. "Utility to convert a pointer to a raw pointer".
  100. Howard E. Hinnant; Tomasz Kamiński. "Extending <chrono> to Calendars and Time Zones".
  101. Neil MacIntosh; Stephan T. Lavavej. "span: bounds-safe views for sequences of objects".
  102. Alisdair Meredith; Stephan T. Lavavej (2018-10-04). "Adopt Consistent Container Erasure from Library Fundamentals 2 for C++20".
  103. Alan Talbot. "<version>".
  104. "Bit-casting object representations". www.open-std.org. Retrieved 2018-11-10.
  105. "Integrating feature-test macros into the C++ WD". www.open-std.org. Retrieved 2018-11-10.
  106. "P1032R1 - Misc constexpr bits".
  107. "Smart pointer creation with default initialization".
  108. "std::map::contains - cppreference.com". en.cppreference.com. Retrieved 2020-05-12.
  109. "P0553R4: Bit operations". www.open-std.org. 2019-07-20. Retrieved 2019-07-20.
  110. "P0556R3: Integral power-of-2 operations". www.open-std.org. 2018-06-06. Retrieved 2020-08-08.
  111. "P1355R2 Exposing a narrow contract for ceil2". www.open-std.org. 2019-06-11. Retrieved 2020-08-08.
  112. "On the names of low-level bit manipulation functions" (PDF). www.open-std.org. 2020-02-11. Retrieved 2020-08-08.
  113. Tomasz Kamiński (2017-11-09). "Simplified partial function application".
  114. "C++ keywords - cppreference.com". en.cppreference.com. Retrieved 2019-08-04.
  115. "explicit specifier - cppreference.com". en.cppreference.com. Retrieved 2019-08-04.
  116. "attribute specifier sequence(since C++11) - cppreference.com". en.cppreference.com. Retrieved 2019-08-04.
  117. "C++ Extensions for Parallelism Version 2".
  118. "Task Blocks".
  119. "C++ Extensions for Networking".
  120. "C++ Extensions for Reflection".
  121. Sutter, Herb (2019-07-20). "Trip report: Summer ISO C++ standards meeting (Cologne)". Sutter's Mill. Retrieved 2019-07-21.
  122. "Reflections on the reflection proposals - Meeting C++". meetingcpp.com. Retrieved 2017-06-30.
  123. "Static reflection". www.open-std.org. Retrieved 2018-11-10.
  124. Herb Sutter. "Metaclasses" (PDF).
  125. "A Unified Executors Proposal for C++". www.open-std.org. Retrieved 2019-02-24.
  126. "N4771: Working Draft, C++ Extensions for Networking" (PDF).
  127. "ISO/IEC TS 19216:2018 Programming Languages -- C++ Extensions for Networking".
  128. "A General Property Customization Mechanism". www.open-std.org. Retrieved 2019-02-24.
  129. "A Unified Futures Proposal for C++".
  130. "Working Draft, Standard for Programming Language C++ (Annex C, §C.1)" (PDF).
  131. "P1161R2: Deprecate uses of the comma operator in subscripting expressions". www.open-std.org. Retrieved 2019-07-20.
  132. https://clang.llvm.org/cxx_status.html
  133. https://en.cppreference.com/w/cpp/compiler_support
  134. https://gcc.gnu.org/legacy-ml/gcc-patches/2017-07/msg01234.html
  135. https://gcc.gnu.org/projects/cxx-status.html
  136. https://github.com/MicrosoftDocs/cpp-docs/blob/master/docs/overview/cpp-conformance-improvements.md
  • JTC1/SC22/WG21 – the ISO/IEC C++ Standard Working Group (a.k.a. the C++ Standards Committee)
  • Ranges (range-v3) github repository, by Eric Niebler
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.