C++17

C++17 is a version of the ISO/IEC 14882 standard for the C++ programming language. C++17 replaced the prior version of the C++ standard, called C++14, and was later replaced by C++20.

Not to be confused with C17 (C standard revision).
C++ language revisions

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.

New features

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

Language
  • Making the text message for static_assert optional[5]
  • Allow typename (as an alternative to class) in a template template parameter[6]
  • New rules for auto deduction from braced-init-list[7][8]
  • Nested namespace definitions, e.g., namespace X::Y { … } instead of namespace X { namespace Y { … } }[8][9]
  • Allowing attributes for namespaces and enumerators[10][11]
  • New standard attributes [[fallthrough]], [[maybe_unused]] and [[nodiscard]][12]
  • UTF-8 (u8) character literals[10][13] (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 "Basic Latin" and C0 control codes, i.e. ASCII)
  • Hexadecimal floating-point literals[14][15]
  • Use of auto as the type for a non-type template parameter[16]
  • Constant evaluation for all non-type template arguments[10][17]
  • Fold expressions, for variadic templates[10][18]
  • A compile-time static if with the form if constexpr(expression)[19]
  • Structured binding declarations, allowing auto [a, b] = getTwoReturnValues();[20]
  • Initializers in if and switch statements[21]
  • 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[22]
  • Class template argument deduction (CTAD), introducing constructor deduction guides, e.g. 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).[23][24]
  • 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[25]
  • Value of __cplusplus changed to 201703L[26]
  • Exception specifications were made part of the function type[27]
  • Lambda expressions can capture "*this" by value[28]

Library

Removed features

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

  • Trigraphs were removed.[44][45]
  • Some deprecated types and functions were removed from the standard library, including std::auto_ptr, std::random_shuffle, and old function adaptors.[8][46] These were superseded in C++11 by improved facilities such as std::unique_ptr, std::shuffle, std::bind, and lambdas.
  • The (formerly deprecated) use of the keyword register as a storage class specifier was removed.[47] This keyword is still reserved but now unused.

Compiler support
  • GCC has had complete support for C++17 language features since version 8.[48]
  • Clang 5 and later supports all C++17 language features.[49]
  • Visual Studio 2017 15.8 (MSVC 19.15) and later supports all C++17 language features.[50][51]

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) [52]
  • libc++ as of version 9 has partial support for C++17, with the remainder "in progress" [53]
  • Visual Studio 2017 15.8 (MSVC 19.15) Standard Library and later supports all C++17 library features except for "Elementary String Conversions" and referring to C99 instead of C11. "Elementary String Conversions" is added in Visual Studio 2019 16.4[54]

See also

References
  1. "N4661 Editors' Report -- Programming Languages -- C++". 21 March 2017. Archived from the original on 2020-02-04. Retrieved 2017-03-21.
  2. "ISO/IEC DIS 14882: Programming Languages — C++" (PDF). Archived from the original (PDF) on 2017-03-25.
  3. Herb Sutter (6 September 2017). "C++17 is formally approved". Archived from the original on 29 January 2020. Retrieved 12 September 2017.
  4. "ISO/IEC 14882:2017". Archived from the original on 2013-01-29. Retrieved 2017-12-03.
  5. "N3928: Extending static_assert, v2 (Walter E. Brown)" (PDF). Archived (PDF) from the original on 2015-08-11. Retrieved 2015-04-03.
  6. "N4051: Allow typename in a template template parameter (Richard Smith)". Archived from the original on 2015-08-11. Retrieved 2015-04-03.
  7. "N3922: New Rules for auto deduction from braced-init-list (James Dennett)". Archived from the original on 2015-08-10. Retrieved 2014-11-28.
  8. "Updates to my trip report". Archived from the original on 2015-03-19. Retrieved 2014-11-28.
  9. "N4230: Nested namespace definition (Robert Kawulak, Andrew Tomazos)". Archived from the original on 2015-08-03. Retrieved 2014-11-28.
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  12. "N4640: Working Draft, Standard for Programming Language C++" (PDF). pp. 193–195. Archived (PDF) from the original on 2017-03-12. Retrieved 2017-03-09.
  13. "N4267: Adding u8 character literals (Richard Smith)". Archived from the original on 2015-10-28. Retrieved 2014-11-15.
  14. Thomas Köppe. "Hexadecimal floating literals for C++". Archived from the original on 2022-09-21. Retrieved 2017-07-16.
  15. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §5.13.4. Archived (PDF) from the original on 2017-12-07. Retrieved 2017-03-24.
  16. James Touton; Mike Spertus (2016-06-23). "Declaring non-type template parameters with auto". Archived from the original on 2017-09-16. Retrieved 2020-11-08.
  17. "N4268: Allow constant evaluation for all non-type template arguments (Richard Smith)". Archived from the original on 2016-03-12. Retrieved 2014-11-15.
  18. "N4295: Folding expressions (Andrew Sutton, Richard Smith)". Archived from the original on 2015-04-04. Retrieved 2014-11-15.
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  20. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §11.5. Archived (PDF) from the original on 2017-12-07. Retrieved 2017-03-24.
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  22. "Dynamic memory allocation for over-aligned data". Archived from the original on 2017-09-08. Retrieved 2017-03-13.
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  25. "N4640: Working Draft, Standard for Programming Language C++" (PDF). pp. 431–433. Archived (PDF) from the original on 2017-03-12. Retrieved 2017-03-09.
  26. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §19.8. Archived (PDF) from the original on 2017-12-07. Retrieved 2017-03-24.
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  28. "P0018R3: Lambda Capture of *this by Value as [=,*this], version 3". Archived from the original on 2017-08-22. Retrieved 2021-06-21.
  29. "Adopt Library Fundamentals V1 TS Components for C++17 (R1)". Archived from the original on 2016-04-05. Retrieved 2016-03-23.
  30. "Current Status". Archived from the original on 2020-09-08.
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  33. "N4279: Improved insertion interface for unique-key maps (Thomas Köppe)". Archived from the original on 2015-04-27. Retrieved 2014-11-15.
  34. "New standard library papers adopted for C++17". Archived from the original on 2014-11-29. Retrieved 2014-11-15.
  35. "N4280: Non-member size() and more (Riccardo Marcangelo)" (PDF). Archived (PDF) from the original on 2015-03-09. Retrieved 2014-11-15.
  36. "N4284: Contiguous Iterators (Jens Maurer)". Archived from the original on 2014-11-29. Retrieved 2014-11-15.
  37. "Filesystem Library Proposal (Beman Dawes)". Archived from the original on 2016-07-20. Retrieved 2015-12-12.
  38. "The Parallelism TS Should be Standardized". Archived from the original on 2016-04-05. Retrieved 2016-03-23.
  39. "Mathematical Special Functions for C++17, v5" (PDF). Archived (PDF) from the original on 2016-04-05. Retrieved 2016-03-23.
  40. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §23.7. Archived (PDF) from the original on 2017-12-07. Retrieved 2017-03-24.
  41. "A byte type definition" (PDF). Archived (PDF) from the original on 2017-03-25. Retrieved 2017-03-25.
  42. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §23.15.8. Archived (PDF) from the original on 2017-12-07. Retrieved 2017-03-24.
  43. "PMR (Polymorphic Memory Resources) fully described -- Nico Josuttis". Archived from the original on 2021-08-28. Retrieved 2020-11-16.
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  45. IBM comment on preparing for a Trigraph-adverse future in C++17 Archived 2018-09-11 at the Wayback Machine, IBM paper N4210, 2014-10-10. Authors: Michael Wong, Hubert Tong, Rajan Bhakta, Derek Inglis
  46. "N4190: Removing auto_ptr, random_shuffle(), And Old <functional> Stuff (Stephan T. Lavavej)". Archived from the original on 2017-10-20. Retrieved 2014-11-28.
  47. "C++ Keywords: register". Archived from the original on 2018-09-03. Retrieved 2018-09-03.
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