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Defined in header <compare> | ||
template<class T,class U= T> struct compare_three_way_result; | (since C++20) | |
Lett andu denote lvalue ofconststd::remove_reference_t<T> andconststd::remove_reference_t<U> respectively, if the expressiont<=> u is well-formed, provides the member typedeftype equal todecltype(t<=> u), otherwise there is no membertype.
If the program adds specializations forstd::compare_three_way_result, the behavior is undefined.
Contents |
| Name | Definition |
type | the result type ofoperator<=> on const-qualified lvalue ofT andU |
template<class T,class U= T> using compare_three_way_result_t= compare_three_way_result<T, U>::type; | (since C++20) | |
// 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>;}; |
#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 orderingpartial ordering
(C++20) | the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class)[edit] |
(C++20) | the result type of 3-way comparison that supports all 6 operators and is not substitutable (class)[edit] |
(C++20) | the result type of 3-way comparison that supports all 6 operators and is substitutable (class)[edit] |
