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std::format_kind(3) C++ Standard Libary std::format_kind(3) NAME std::format_kind - std::format_kind Synopsis Defined in header <format> template< class R > inline constexpr /* unspecified */ format_kind = /* unspecified (1) (since C++23) */; template< ranges::input_range R > requires std::same_as<R, std::remove_cvref_t<R>> (2) (since C++23) inline constexpr range_format format_kind<R> = /* see description */; The variable template format_kind selects an appropriate std::range_format for a range R. std::format_kind<R> is defined as follows: * If std::same_as<std::remove_cvref_t<ranges::range_refer- ence_t<R>>, R> is true, std::format_kind<R> is std::range_format::disabled. * Otherwise, if R::key_type is valid and denotes a type: * If R::mapped_type is valid and denotes a type, let U be std::remove_cvref_t<ranges::range_reference_t<R>>. If either U is a specialization of std::pair or U is a specializa- tion of std::tuple and std::tuple_size_v<U> == 2, std::format_kind<R> is std::range_format::map. * * Otherwise, std::format_kind<R> is std::range_format::set. * Otherwise, std::format_kind<R> is std::range_format::sequence. A program that instantiates a primary template of the format_kind variable template is ill-formed. Given a cv-unqualified program-defined type T that models in- put_range, a program can spcialize format_kind for T. Such specializations are usable in con- stant expressions, and have type const std::range_format. Possible implementation namespace detail { template< typename > inline constexpr bool is_pair_or_tuple_2 = false; template< typename T, typename U > inline constexpr bool is_pair_or_tuple_2<std::pair<T, U>> = true; template< typename... Ts > inline constexpr bool is_pair_or_tuple_2<std::tuple<Ts...>> = sizeof...(Ts) == 2; template < typename T > requires std::is_reference_v<T> || std::is_const_v<T> inline constexpr bool is_pair_or_tuple_2<T> = is_pair_or_tuple_2<std::remove_cvref_t<T>>; } template< class R > inline constexpr range_format format_kind = [] { static_assert(false, "instantiating a primary template is not al- lowed"); return range_format::disabled; }(); template< ranges::input_range R > requires std::same_as<R, std::remove_cvref_t<R>> inline constexpr range_format format_kind<R> = [] { if constexpr (std::same_as<std::re- move_cvref_t<std::ranges::range_reference_t<R>>, R>) return range_format::disabled; else if constexpr (requires { typename R::key_type; }) { if constexpr (requires { typename R::mapped_type; } && detail::is_pair_or_tu- ple_2<std::ranges::range_reference_t<R>>) return range_format::map; else return range_format::set; } else return range_format::sequence; }(); Example // Run this code #include <filesystem> #include <format> #include <map> #include <set> #include <vector> struct A {}; static_assert(std::format_kind<std::vector<int>> == std::range_for- mat::sequence); static_assert(std::format_kind<std::map<int>> == std::range_for- mat::map); static_assert(std::format_kind<std::set<int>> == std::range_for- mat::set); static_assert(std::format_kind<std::filesystem::path> == std::range_format::disabled); // ill-formed: // static_assert(std::format_kind<A> == std::range_format::disabled); int main() {} See also range_format specifies how a range should be formatted (C++23) (enum) http://cppreference.com 2024.06.10 std::format_kind(3)
NAME | Synopsis | Possible implementation | Example | See also
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