std::format_kind - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| template< class R > constexpr /* unspecified */ format_kind = /* unspecified */; | (1) | (since C++23) |
| template< ranges::input_range R > requires std::same_as<R, std::remove_cvref_t<R>> constexpr range_format format_kind<R> = /* see description */; | (2) | (since C++23) |
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_reference_t<R>>, R> is true, std::format_kind<R> is std::range_format::disabled.
- Otherwise, if
R::key_typeis valid and denotes a type:- If
R::mapped_typeis valid and denotes a type, letUbe std::remove_cvref_t<ranges::range_reference_t<R>>.
- If
If either U is a specialization of std::pair or U is a specialization 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 the primary template of the format_kind variable template is ill-formed.
Given a cv-unqualified program-defined type T that models input_range, a program can specialize format_kind for T. Such specializations are usable in constant expressions, and have type const std::range_format.
[edit] Possible implementation
namespace detail { template< typename > constexpr bool is_pair_or_tuple_2 = false; template< typename T, typename U > constexpr bool is_pair_or_tuple_2<std::pair<T, U>> = true; template< typename T, typename U > constexpr bool is_pair_or_tuple_2<std::tuple<T, U>> = true; template < typename T > requires std::is_reference_v || std::is_const_v constexpr bool is_pair_or_tuple_2 = is_pair_or_tuple_2<std::remove_cvref_t>; } template< class R > constexpr range_format format_kind = [] { static_assert(false, "instantiating a primary template is not allowed"); return range_format::disabled; }(); template< ranges::input_range R > requires std::same_as<R, std::remove_cvref_t> constexpr range_format format_kind = [] { if constexpr (std::same_as<std::remove_cvref_t<std::ranges::range_reference_t>, R>) return range_format::disabled; else if constexpr (requires { typename R::key_type; }) { if constexpr (requires { typename R::mapped_type; } && detail::is_pair_or_tuple_2<std::ranges::range_reference_t>) return range_format::map; else return range_format::set; } else return range_format::sequence; }();
[edit] Example
#include #include #include #include #include struct A {}; static_assert(std::format_kind<std::vector> == std::range_format::sequence); static_assert(std::format_kind<std::map<int, int>> == std::range_format::map); static_assert(std::format_kind<std::set> == std::range_format::set); static_assert(std::format_kind<std::filesystem::path> == std::range_format::disabled); // ill-formed: // static_assert(std::format_kind == std::range_format::disabled); int main() {}