C++17

Next standard

• C++20^[5][6][7]

Language

• Making the text message for static_assert optional^[12]

• Allow typename (as an alternative to class) in a template template parameter^[13]

• New rules for auto deduction from braced-init-list^[14][10]

• Nested namespace definitions, e.g., namespace X::Y { … } instead of namespace X { namespace Y { … }}^[10][15]

• Allowing attributes for namespaces and enumerators^[16][17]

• New standard attributes [[fallthrough]], [[maybe_unused]] and [[nodiscard]]^[18]

• UTF-8 (u8) character literals^[16][19] (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^[20][21]

• Constant evaluation for all non-type template arguments^[16][22]

• Fold expressions, for variadic templates^[16][23]

• A compile-time static if with the form if constexpr(expression)^[24]

• Structured binding declarations, allowing auto [a, b] = getTwoReturnValues();^[25]

• Initializers in if and switch statements^[26]

• 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^[27]

• Template deduction of constructors, allowing std::pair(5.0, false) instead of std::pair<double, bool>(5.0, false)

• 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]

• The keyword register is now reserved and unused (formerly deprecated)^[29]

• Value of __cplusplus changed to 201703L^[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 be 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][16]

• 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]