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_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
spcialize format_kind for T. Such specializations are usable in constant
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 allowed");
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::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 specifies how a range should be formatted
(C++23) (enum)