C++17

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

• Most of Library Fundamentals TS I, including:^[29][30]
  • std::string_view, a read-only non-owning reference to a character sequence or string-slice^[31]
  • 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^[32][10]
• New insertion functions try_emplace and insert_or_assign for std::map and std::unordered_map key-value associative data structures^[33][34]
• Uniform container access: std::size, std::empty and std::data^[34][35]
• Definition of "contiguous iterators"^[34][36]
• A file system library based on boost::filesystem^[37]
• Parallel versions of STL algorithms^[38]
• Additional mathematical special functions, including elliptic integrals and Bessel functions^[39]
• std::variant, a tagged union container^[40]
• std::byte, allowing char to be replaced for data types intending to model a byte of data as a byte rather than a character^[41]
• Logical operator traits: std::conjunction, std::disjunction and std::negation^[42]
• <memory_resource> header, for polymorphic memory resources^[43]

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

• C++ compilers
• C11 (C standard revision)
• C17 (C standard revision)