C++17

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C++17 is a revision of the ISO/IEC 14882 standard for the C++ programming language.

History

Before the C++ Standards Committee fixed a 3-year release cycle, C++17's release date was uncertain. In that time period, the C++17 revision was also called C++1z, following C++0x or C++1x for C++11 and C++1y for C++14. The C++17 specification reached the Draft International Standard (DIS) stage in March 2017.[1][2] This DIS was unanimously approved, with only editorial comments,[3] and the final standard was published in December 2017.[4] Few changes were made to the C++ Standard Template Library, although some algorithms in the <algorithm> header were given support for explicit parallelization and some syntactic enhancements were made.

Removed

This revision of C++ not only added new features but also removed a few.

  • Removal of trigraphs.[5][6]
  • Removal of some deprecated types and functions from the standard library, including std::auto_ptr, std::random_shuffle, and old function adaptors.[7][8] These were superseded in C++11 by improved facilities such as std::unique_ptr, std::shuffle, std::bind, and lambdas.
  • Removal of the (formerly deprecated) use of the keyword register as a storage class specifier.[9] This keyword is now reserved and unused.

New features

C++17 introduced many new features. The following lists may be incomplete.

Language

  • Making the text message for static_assert optional[10]
  • Allow typename (as an alternative to class) in a template template parameter[11]
  • New rules for auto deduction from braced-init-list[12][7]
  • Nested namespace definitions, e.g., namespace X::Y { … } instead of namespace X { namespace Y { … } }[7][13]
  • Allowing attributes for namespaces and enumerators[14][15]
  • New standard attributes fallthrough, maybe unused and nodiscard[16]
  • UTF-8 (u8) character literals[14][17] (UTF-8 string literals have existed since C++11; C++17 adds the corresponding character literals for consistency, though as they are restricted to a single byte they can only store ASCII)
  • Hexadecimal floating-point literals[18][19]
  • Constant evaluation for all non-type template arguments[14][20]
  • Fold expressions, for variadic templates[14][21]
  • A compile-time static if with the form if constexpr(expression)[22]
  • Structured binding declarations, allowing auto [a, b] = getTwoReturnValues();[23]
  • Initializers in if and switch statements[24]
  • copy-initialization and direct-initialization of objects of type T from prvalue expressions of type T (ignoring top-level cv-qualifiers) shall result in no copy or move constructors from the prvalue expression. See copy elision for more information.
  • Some extensions on over-aligned memory allocation[25]
  • Class template argument deduction (CTAD), introducing constructor deduction guides, eg. allowing std::pair(5.0, false) instead of requiring explicit constructor arguments types std::pair<double, bool>(5.0, false) or an additional helper template function std::make_pair(5.0, false).[26][27]
  • Inline variables, which allows the definition of variables in header files without violating the one definition rule. The rules are effectively the same as inline functions
  • __has_include, allowing the availability of a header to be checked by preprocessor directives[28]
  • Value of __cplusplus changed to 201703L[29]
  • Exception specifications were made part of the function type[30]

Library

Compiler support

  • GCC has had almost complete support for C++17 language features since version 7. Some library features are still not supported.[45]
  • Clang 5 and later implement all the features of C++17.[46]
  • Visual Studio 2017 15.7 (MSVC 19.14) supports almost all of C++17.[47][48]

Library support

  • libstdc++ since version 9.1 has complete support for c++17 (8.1 without Parallelism TS and referring to C99 instead of C11) [49]
  • libc++ as of version 9 has partial support for c++17, with the remainder "in progress" [50]
  • MSVC Standard Library since 19.15 complete support for c++17 except for "Elementary String Conversions" and referring to C99 instead of C11.[51]

See also

References

  1. "N4661 Editors' Report -- Programming Languages -- C++". 21 March 2017. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4661.html. 
  2. "ISO/IEC DIS 14882: Programming Languages — C++". Archived from the original on 2017-03-25. https://web.archive.org/web/20170325025026/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4660.pdf. 
  3. Herb Sutter. "C++17 is formally approved". https://herbsutter.com/2017/09/06/c17-is-formally-approved/. 
  4. "ISO/IEC 14882:2017". https://www.iso.org/standard/68564.html. 
  5. "N3981: Removing trigraphs??! (Richard Smith)". 2014-05-06. http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2014/n3981.html. 
  6. IBM comment on preparing for a Trigraph-adverse future in C++17, IBM paper N4210, 2014-10-10. Authors: Michael Wong, Hubert Tong, Rajan Bhakta, Derek Inglis
  7. 7.0 7.1 7.2 "Updates to my trip report". http://isocpp.org/blog/2014/11/updates-to-my-trip-report. 
  8. "N4190: Removing auto_ptr, random_shuffle(), And Old <functional> Stuff (Stephan T. Lavavej)". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4190.htm. 
  9. "C++ Keywords: register". https://en.cppreference.com/w/cpp/keyword/register. 
  10. "N3928: Extending static_assert, v2 (Walter E. Brown)". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3928.pdf. 
  11. "N4051: Allow typename in a template template parameter (Richard Smith)". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4051.html. 
  12. "N3922: New Rules for auto deduction from braced-init-list (James Dennett)". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3922.html. 
  13. "N4230: Nested namespace definition (Robert Kawulak, Andrew Tomazos)". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4230.html. 
  14. 14.0 14.1 14.2 14.3 14.4 "New core language papers adopted for C++17". https://isocpp.org/blog/2014/11/new-papers-adopted-for-cpp17. 
  15. "N4266: Attributes for namespaces and enumerators (Richard Smith)". http://isocpp.org/files/papers/n4266.html. 
  16. "N4640: Working Draft, Standard for Programming Language C++". pp. 193–195. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4640.pdf. 
  17. "N4267: Adding u8 character literals (Richard Smith)". http://isocpp.org/files/papers/n4267.html. 
  18. Thomas Köppe. "Hexadecimal floating literals for C++". http://wg21.link/p0245r1. 
  19. "N4659: Working Draft, Standard for Programming Language C++". §5.13.4. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf. 
  20. "N4268: Allow constant evaluation for all non-type template arguments (Richard Smith)". http://isocpp.org/files/papers/n4268.html. 
  21. "N4295: Folding expressions (Andrew Sutton, Richard Smith)". http://isocpp.org/files/papers/n4295.html. 
  22. "N4659: Working Draft, Standard for Programming Language C++". §9.4.1. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf. 
  23. "N4659: Working Draft, Standard for Programming Language C++". §11.5. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf. 
  24. "Selection statements with initializer". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r1.html. 
  25. "Dynamic memory allocation for over-aligned data". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0035r4.html. 
  26. "Class template argument deduction". https://en.cppreference.com/w/cpp/language/class_template_argument_deduction. 
  27. "CppCon 2018: Timur Doumler “Class template argument deduction in C++17”". https://www.youtube.com/watch?v=UDs90b0yjjQ. 
  28. "N4640: Working Draft, Standard for Programming Language C++". pp. 431–433. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4640.pdf. 
  29. "N4659: Working Draft, Standard for Programming Language C++". §19.8. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf. 
  30. "P0012R1: Make exception specifications be part of the type system, version 5". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0012r1.html. 
  31. "Adopt Library Fundamentals V1 TS Components for C++17 (R1)". https://isocpp.org/files/papers/p0220r1.html. 
  32. "Current Status". https://isocpp.org/std/status. 
  33. "std::basic_string_view - cppreference.com". http://en.cppreference.com/w/cpp/string/basic_string_view. 
  34. "N4259: Wording for std::uncaught_exceptions (Herb Sutter)". http://isocpp.org/files/papers/n4259.pdf. 
  35. "N4279: Improved insertion interface for unique-key maps (Thomas Köppe)". https://isocpp.org/files/papers/n4279.html. 
  36. 36.0 36.1 36.2 "New standard library papers adopted for C++17". https://isocpp.org/blog/2014/11/new-standard-library-papers-adopted-for-cpp17. 
  37. "N4280: Non-member size() and more (Riccardo Marcangelo)". https://isocpp.org/files/papers/n4280.pdf. 
  38. "N4284: Contiguous Iterators (Jens Maurer)". https://isocpp.org/files/papers/n4284.html. 
  39. "Filesystem Library Proposal (Beman Dawes)". http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3505.html. 
  40. "The Parallelism TS Should be Standardized". https://isocpp.org/files/papers/P0024R2.html. 
  41. "Mathematical Special Functions for C++17, v5". https://isocpp.org/files/papers/P0226R1.pdf. 
  42. "N4659: Working Draft, Standard for Programming Language C++". §23.7. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf. 
  43. "A byte type definition". http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0298r3.pdf. 
  44. "N4659: Working Draft, Standard for Programming Language C++". §23.15.8. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf. 
  45. "C++ Standards Support in GCC - GNU Project - Free Software Foundation (FSF)". https://gcc.gnu.org/projects/cxx-status.html. 
  46. "Clang - C++17, C++14, C++11 and C++98 Status". https://clang.llvm.org/cxx_status.html. 
  47. corob-msft. "Visual C++ Language Conformance". https://docs.microsoft.com/en-us/cpp/visual-cpp-language-conformance. 
  48. "Announcing: MSVC Conforms to the C++ Standard". https://blogs.msdn.microsoft.com/vcblog/2018/05/07/announcing-msvc-conforms-to-the-c-standard/. 
  49. "Chapter 1. Status". https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html. 
  50. "libc++ C++17 Status". http://libcxx.llvm.org/cxx1z_status.html. 
  51. "Announcing: MSVC Conforms to the C++ Standard". https://devblogs.microsoft.com/cppblog/announcing-msvc-conforms-to-the-c-standard/. 






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