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std::compar...er_fallback(3) C++ Standard Libary std::compar...er_fallback(3) NAME std::compare_weak_order_fallback - std::compare_weak_order_fallback Synopsis Defined in header <compare> inline namespace /* unspecified */ { inline constexpr /* unspecified */ (since C++20) compare_weak_order_fallback = /* unspecified */; } Call signature template< class T, class U > requires /* see below */ constexpr std::weak_ordering (since C++20) compare_weak_order_fallback( T&& t, U&& u ) noexcept(/* see be- low */); Performs three-way comparison on subexpressions t and u and produces a result of type std::weak_ordering, even if the operator <=> is unavailable. If std::decay_t<T> and std::decay_t<U> are the same type, std::compare_weak_order_fallback(t, u) is expression-equivalent to: * std::weak_order(t, u), if it is a well-formed expression; other- wise, * t == u ? std::weak_ordering::equivalent : t < u ? std::weak_ordering::less : std::weak_ordering::greater, if the expressions t == u and t < u are both well-formed and each of decltype(t == u) and decltype(t < u) models boolean-testable, except that t and u are evaluated only once. In all other cases, std::compare_weak_order_fallback(t, u) is ill- formed, which can result in substitution failure when it appears in the immediate con- text of a template instantiation. Example // Run this code #include <compare> #include <iostream> // does not support <=> struct Rational_1 { int num; int den; // > 0 }; inline constexpr bool operator<(Rational_1 lhs, Rational_1 rhs) { return lhs.num * rhs.den < rhs.num * lhs.den; } inline constexpr bool operator==(Rational_1 lhs, Rational_1 rhs) { return lhs.num * rhs.den == rhs.num * lhs.den; } // supports <=> struct Rational_2 { int num; int den; // > 0 }; inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Ratio- nal_2 rhs) { return lhs.num * rhs.den <=> rhs.num * lhs.den; } inline constexpr bool operator==(Rational_2 lhs, Rational_2 rhs) { return lhs <=> rhs == 0; } void print(int id, std::weak_ordering value) { std::cout << id << ") "; if (value == 0) std::cout << "equal\n"; else if (value < 0) std::cout << "less\n"; else std::cout << "greater\n"; } int main() { Rational_1 a{1, 2}, b{3, 4}; // print(0, a <=> b); // does not work print(1, std::compare_weak_order_fallback(a, b)); // works, de- faults to < and == Rational_2 c{6, 5}, d{8, 7}; print(2, c <=> d); // works print(3, std::compare_weak_order_fallback(c, d)); // works Rational_2 e{2, 3}, f{4, 6}; print(4, e <=> f); // works print(5, std::compare_weak_order_fallback(e, f)); // works } Output: 1) less 2) greater 3) greater 4) equal 5) equal Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior LWG 2114 C++20 the fallback mechanism only required con- straints strengthened (P2167R3) return types to be convertible to bool See also weak_order performs 3-way comparison and produces a result of type (C++20) std::weak_ordering (customization point object) http://cppreference.com 2024.06.10 std::compar...er_fallback(3)
NAME | Synopsis | Example | Output: | See also
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