std:: compare_three_way_result

Let t and u denote lvalue of const std:: remove_reference_t < T > and const std:: remove_reference_t < U > respectively, if the expression t <=> u is well-formed, provides the member typedef type equal to decltype ( t <=> u ) , otherwise there is no member type .

If the program adds specializations for std::compare_three_way_result , the behavior is undefined.

Member types

Helper types

Possible implementation

// recommended by Casey Carter
// see also: https://github.com/microsoft/STL/pull/385#discussion_r357894054
template<class T, class U = T>
using compare_three_way_result_t = decltype(
    std::declval<const std::remove_reference_t<T>&>() <=>
    std::declval<const std::remove_reference_t<U>&>()
);
 
template<class T, class U = T>
struct compare_three_way_result {};
 
template<class T, class U>
    requires requires { typename compare_three_way_result_t<T, U>; }
struct compare_three_way_result<T, U>
{
    using type = compare_three_way_result_t<T, U>;
};

Example

#include <compare>
#include <iostream>
#include <type_traits>
 
template<class Ord>
void print_cmp_type()
{
    if constexpr (std::is_same_v<Ord, std::strong_ordering>)
        std::cout << "strong ordering\n";
    else if constexpr (std::is_same_v<Ord, std::weak_ordering>)
        std::cout << "weak ordering\n";
    else if constexpr (std::is_same_v<Ord, std::partial_ordering>)
        std::cout << "partial ordering\n";
    else
        std::cout << "illegal comparison result type\n";
}
 
int main()
{
    print_cmp_type<std::compare_three_way_result_t<int>>();
    print_cmp_type<std::compare_three_way_result_t<double>>();
}

strong ordering
partial ordering

See also