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std::compar...der_fallback(3) C++ Standard Libarystd::compar...der_fallback(3) NAME std::compare_partial_order_fallback - std::compare_partial_order_fall- back Synopsis Defined in header <compare> inline namespace /* unspecified */ { inline constexpr /* unspecified */ (since C++20) compare_partial_order_fallback = /* unspecified */; } Call signature template< class T, class U > requires /* see below */ constexpr std::partial_ordering compare_partial_order_fallback(T&& t, U&& u) noexcept(/* see below */); Performs three-way comparison on t and u and produces a result of type std::partial_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_partial_order_fallback(t, u) is expres- sion-equivalent to: * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true: * std::partial_order(t, u), if it is a well-formed expres- sion; * otherwise, t == u ? std::partial_ordering::equivalent : t < u ? std::partial_ordering::less : u < t ? std::partial_ordering::greater : std::partial_ordering::unordered if t == u, t < u, and u < t are all well-formed and convertible to bool, except that t and u are evaluated only once. * In all other cases, std::compare_partial_order_fallback(t, u) is ill-formed, which can result in substitution failure when it appears in the immediate 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_partial_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_partial_order_fallback_fn. All instances of __compare_partial_order_fallback_fn are equal. The effects of invoking different instances of type __compare_partial_order_fall- back_fn on the same arguments are equivalent, regardless of whether the expression de- noting 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::compare_par- tial_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_partial_order_fallback above, __compare_partial_order_fallback_fn models * std::invocable<__compare_partial_order_fallback_fn, Args...>, * std::invocable<const __compare_partial_order_fallback_fn, Args...>, * std::invocable<__compare_partial_order_fallback_fn&, Args...>, and * std::invocable<const __compare_partial_order_fallback_fn&, Args...>. Otherwise, no function call operator of __compare_partial_or- der_fallback_fn participates in overload resolution. Example This section is incomplete Reason: no example See also partial_order performs 3-way comparison and produces a result of type (C++20) std::partial_ordering (customization point object) http://cppreference.com 2022.07.31 std::compar...der_fallback(3)
NAME | Synopsis | Example | See also
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