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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 compare_weak_order_fallback(T&& t, U&& u) noexcept(/* see below */); Performs three-way comparison on t and u and produces a result of type std::weak_ordering, even if the operator <=> is unavailable. Let t and u be expressions and T and U denote decltype((t)) and de- cltype((u)) respectively, std::compare_weak_order_fallback(t, u) is expression- equivalent to: * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true: * std::weak_order(t, u), if it is a well-formed expression; * otherwise, t == u ? std::weak_ordering::equivalent : t < u ? std::weak_ordering::less : std::weak_ordering::greater if t == u and t < u are both well-formed and convertible to bool, 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 imme- diate context of a template instantiation. Expression-equivalent Expression e is expression-equivalent to expression f, if * e and f have the same effects, and * either both are constant subexpressions or else neither is a constant subexpression, and * either both are potentially-throwing or else neither is poten- tially-throwing (i.e. noexcept(e) == noexcept(f)). Customization point objects The name std::compare_weak_order_fallback denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __compare_weak_order_fallback_fn. All instances of __compare_weak_order_fallback_fn are equal. The ef- fects of invoking different instances of type __compare_weak_order_fallback_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, std::com- pare_weak_order_fallback can be copied freely and its copies can be used interchangeably. Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to std::compare_weak_order_fallback above, __compare_weak_order_fallback_fn models * std::invocable<__compare_weak_order_fallback_fn, Args...>, * std::invocable<const __compare_weak_order_fallback_fn, Args...>, * std::invocable<__compare_weak_order_fallback_fn&, Args...>, and * std::invocable<const __compare_weak_order_fallback_fn&, Args...>. Otherwise, no function call operator of __compare_weak_order_fall- back_fn participates in overload resolution. Example // Run this code #include <iostream> #include <compare> // 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 bool operator==(Rational_2 const&) const = default; }; inline constexpr std::weak_ordering operator<=>(Rational_2 lhs, Ratio- nal_2 rhs) { return lhs.num * rhs.den <=> rhs.num * lhs.den; } void print(std::weak_ordering value) { 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}; Rational_1 b{3, 4}; // print(a <=> b); // doesn't work print(std::compare_weak_order_fallback(a, b)); // works, defaults to < and == Rational_2 c{6, 5}; Rational_2 d{8, 7}; print(c <=> d); // works print(std::compare_weak_order_fallback(c, d)); // works } Output: less greater greater 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 2022.07.31 std::compar...er_fallback(3)
NAME | Synopsis | Example | Output: | See also
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