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 conststd::remove_reference_t<T> and conststd::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_r357894054template<class T, class U = T>using compare_three_way_result_t = decltype(std::declval<conststd::remove_reference_t<T>&>()<=>std::declval<conststd::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(){ifconstexpr(std::is_same_v<Ord, std::strong_ordering>)std::cout<<"strong ordering\n";elseifconstexpr(std::is_same_v<Ord, std::weak_ordering>)std::cout<<"weak ordering\n";elseifconstexpr(std::is_same_v<Ord, std::partial_ordering>)std::cout<<"partial ordering\n";elsestd::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)