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 compare_three_way_result is undefined.
| Name | Definition |
|---|---|
type | the result type of operator<=> on const-qualified lvalue of T and U |
template< class T, class U = T > using compare_three_way_result_t = typename compare_three_way_result<T, U>::type; | (since C++20) |
// 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>;
}; |
#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
|
(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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