std::compare_three_way_result

 

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three_way_comparablethree_way_comparable_with
strong_ordering weak_ordering partial_ordering strong_equality weak_equality common_comparison_category compare_three_way_result	strong_order weak_order partial_order compare_strong_order_fallback compare_weak_order_fallback compare_partial_order_fallback compare_three_way	is_eqis_neqis_ltis_lteqis_gtis_gteq
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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.

Attempting to specialize this template results in undefined behavior.

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>::type;		(since C++20)

Possible implementation

template<class T, class U = T> struct compare_three_way_result {};   template<class T, class U> requires requires(const std::remove_reference_t<T>& t, const std::remove_reference_t<U>& u) { t <=> u; } struct compare_three_way_result<T, U> { using type = decltype(std::declval<const std::remove_reference_t<T>&>() <=> std::declval<const std::remove_reference_t<U>&>()); };

Example

Run this code

#include <compare>
#include <type_traits>
#include <iostream>
 
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 if constexpr (std::is_same_v<Ord, std::strong_equality>)
        std::cout << "strong equality\n";
    else if constexpr (std::is_same_v<Ord, std::weak_equality>)
        std::cout << "weak equality\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>>();
    print_cmp_type<std::compare_three_way_result_t<int(*)()>>();
}

Output:

strong ordering
partial ordering
strong equality

See also

weak_equality (C++20)	the result type of 3-way comparison that supports only equality/inequality and is not substitutable (class)
strong_equality (C++20)	the result type of 3-way comparison that supports only equality/inequality and is substitutable (class)
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)