std::compare_three_way_result

 

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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.

If the program adds specializations for std::compare_three_way_result, the behavior 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 = 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

Run this code

#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

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