std:: format_kind

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

#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