std::format_kind

Defined in header <format>

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>> inline constexpr range_format format_kind<R> = /* see description */;`	(2)	(since C++23)

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>>
inline 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.

format_kind<R> is defined as follows:

If std::same_as<std::remove_cvref_t<ranges::range_reference_t<R>>, R> is true, format_kind<R> is 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, format_kind<R> is range_format::map.
- Otherwise, format_kind<R> is range_format::set.
Otherwise, format_kind<R> is range_format::sequence.

A program that instantiates a primary template of the format_kind variable template is ill-formed.

User-provided specialization of format_kind is allowed as long as:

R is cv-unqualified program-defined type,
R satisfies input_range,
its specialization shall be usable in constant expressions, and
format_kind<R> has type const 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

#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() {}

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std::format_kind

Possible implementation

Example

See also