std::format_kind

 

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Defined in header <format>
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_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 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 a 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.

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<T> || std::is_const_v<T> constexpr bool is_pair_or_tuple_2<T> = is_pair_or_tuple_2<std::remove_cvref_t<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<R>> constexpr range_format format_kind<R> = [] { if constexpr (std::same_as<std::remove_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_tuple_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_format::sequence);
static_assert(std::format_kind<std::map<int>> == std::range_format::map);
static_assert(std::format_kind<std::set<int>> == std::range_format::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 (C++23)

specifies how a range should be formatted (enum)