C++17

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from the standard library, including std::auto_ptr, std::random_shuffle, and old function adaptors.[7][8] These were superseded in C++11 by improved facilities such as std::unique_ptr, std::shuffle, std::bind, and lambdas.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

• Most of Library Fundamentals TS I, including:[31][32]
  • std::string_view, a read-only non-owning reference to a character sequence or string-slice[33]
  • std::optional, for representing optional objects, a data type that may not always be returned by a given algorithm with support for non-return
  • std::any, for holding single values of any type
• std::uncaught_exceptions, as a replacement of std::uncaught_exception in exception handling[34][14]
• New insertion functions try_emplace and insert_or_assign for std::map and std::unordered_map key-value associative data structures[35][36]
• Uniform container access: std::size, std::empty and std::data[36][37]
• Definition of "contiguous iterators"[36][38]
• A file system library based on boost::filesystem[39]
• Parallel versions of STL algorithms[40]
• Additional mathematical special functions, including elliptic integrals and Bessel functions[41]
• std::variant, a tagged union container[42]
• std::byte, allowing char to be replaced for data types intending to model a byte of data as a byte rather than a character[43]
• Logical operator traits: std::conjunction, std::disjunction and std::negation[44]

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

• C++ compilers
• C11 (C standard revision)
• C18 (C standard revision)

References

1. "N4661 Editors' Report -- Programming Languages -- C++". 21 March 2017. 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. "C++17 is formally approved".
4. "ISO/IEC 14882:2017".
5. "N3981: Removing trigraphs??! (Richard Smith)". 2014-05-06.
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. "Updates to my trip report".
8. "N4190: Removing auto_ptr, random_shuffle(), And Old <functional> Stuff (Stephan T. Lavavej)".
9. "C++ Keywords: register".
10. "N3928: Extending static_assert, v2 (Walter E. Brown)" (PDF).
11. "N4051: Allow typename in a template template parameter (Richard Smith)".
12. "N3922: New Rules for auto deduction from braced-init-list (James Dennett)".
13. "N4230: Nested namespace definition (Robert Kawulak, Andrew Tomazos)".
14. "New core language papers adopted for C++17".
15. "N4266: Attributes for namespaces and enumerators (Richard Smith)".
16. "N4640: Working Draft, Standard for Programming Language C++" (PDF). pp. 193–195.
17. "N4267: Adding u8 character literals (Richard Smith)".
18. Thomas Köppe. "Hexadecimal floating literals for C++".
19. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §5.13.4.
20. "N4268: Allow constant evaluation for all non-type template arguments (Richard Smith)".
21. "N4295: Folding expressions (Andrew Sutton, Richard Smith)".
22. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §9.4.1.
23. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §11.5.
24. "Selection statements with initializer".
25. "Dynamic memory allocation for over-aligned data".
26. "Class template argument deduction".
27. "CppCon 2018: Timur Doumler "Class template argument deduction in C++17"".
28. "N4640: Working Draft, Standard for Programming Language C++" (PDF). pp. 431–433.
29. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §19.8.
30. "P0012R1: Make exception specifications be part of the type system, version 5".
31. "Adopt Library Fundamentals V1 TS Components for C++17 (R1)".
32. "Current Status".
33. "std::basic_string_view - cppreference.com". en.cppreference.com. Retrieved 2016-06-23.
34. "N4259: Wording for std::uncaught_exceptions (Herb Sutter)" (PDF).
35. "N4279: Improved insertion interface for unique-key maps (Thomas Köppe)".
36. "New standard library papers adopted for C++17".
37. "N4280: Non-member size() and more (Riccardo Marcangelo)" (PDF).
38. "N4284: Contiguous Iterators (Jens Maurer)".
39. "Filesystem Library Proposal (Beman Dawes)".
40. "The Parallelism TS Should be Standardized".
41. "Mathematical Special Functions for C++17, v5" (PDF).
42. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §23.7.
43. "A byte type definition" (PDF).
44. "N4659: Working Draft, Standard for Programming Language C++" (PDF). §23.15.8.
45. "C++ Standards Support in GCC - GNU Project - Free Software Foundation (FSF)". gcc.gnu.org.
46. "Clang - C++17, C++14, C++11 and C++98 Status". clang.llvm.org.
47. corob-msft. "Visual C++ Language Conformance". docs.microsoft.com.
48. "Announcing: MSVC Conforms to the C++ Standard".
49. "Chapter 1. Status". gcc.gnu.org.
50. "libc++ C++17 Status". llvm.org.
51. "Announcing: MSVC Conforms to the C++ Standard". devblogs.microsoft.com.