C++17

C++17

C++17

2017 edition of the C++ programming language standard

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

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.

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

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