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std::compare_three_way_result

Defined in header <compare> 
template< class T, class U = T >
struct compare_three_way_result;
 (since C++20)

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.

The behavior of a program that adds specializations for std::compare_three_way_result is undefined.

Member types

Name Definition
type the result type of operator<=> on const-qualified lvalue of T and U

Helper types

template< class T, class U = T >
using compare_three_way_result_t = typename compare_three_way_result<T, U>::type;
 (since C++20)

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>>();
}

Output:

strong ordering
partial ordering

See also

(C++20)
the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values
(class)
(C++20)
the result type of 3-way comparison that supports all 6 operators and is not substitutable
(class)
(C++20)
the result type of 3-way comparison that supports all 6 operators and is substitutable
(class)

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