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    std::format_kind

    From cppreference.com
     
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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

    (C++23)
    		 specifies how a range should be formatted
    (enum) [edit]