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std::partial_order(3) C++ Standard Libary std::partial_order(3) NAME std::partial_order - std::partial_order Synopsis Defined in header <compare> inline namespace /* unspecified */ { inline constexpr /* unspecified */ partial_order = /* unspeci- fied (since C++20) */; } Call signature template< class T, class U > requires /* see below */ constexpr std::partial_ordering partial_order( T&& t, U&& u ) noexcept(/* see below */); Compares two values using 3-way comparison and produces a result of type std::partial_ordering. Let t and u be expressions and T and U denote decltype((t)) and de- cltype((u)) respectively, std::partial_order(t, u) is expression-equivalent to: * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true: * std::partial_ordering(partial_order(t, u)), if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of std::partial_order, * otherwise, std::partial_ordering(std::com- pare_three_way()(t, u)), if it is well-formed, * otherwise, std::partial_ordering(std::weak_order(t, u)), if it is well-formed. * In all other cases, the expression is ill-formed, which can re- sult in substitution failure when it appears in the immediate context of a template instantiation. Customization point objects The name std::partial_order 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 __partial_order_fn. All instances of __partial_order_fn are equal. The effects of invok- ing different instances of type __partial_order_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 in- stance is not required to be invocable). Thus, std::partial_order 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::partial_order above, __partial_order_fn models * std::invocable<__partial_order_fn, Args...>, * std::invocable<const __partial_order_fn, Args...>, * std::invocable<__partial_order_fn&, Args...>, and * std::invocable<const __partial_order_fn&, Args...>. Otherwise, no function call operator of __partial_order_fn partici- pates in overload resolution. Example This section is incomplete Reason: no example See also the result type of 3-way comparison that supports all partial_ordering 6 operators, is not substitutable, and allows (C++20) incomparable values (class) strong_order performs 3-way comparison and pro- duces a result of (C++20) type std::strong_ordering (customization point object) weak_order performs 3-way comparison and pro- duces a result of (C++20) type std::weak_ordering (customization point object) performs 3-way comparison and pro- duces a result of compare_partial_order_fallback type std::partial_ordering, even if operator<=> is (C++20) unavailable (customization point object) Category: * Todo no example http://cppreference.com 2024.06.10 std::partial_order(3)
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