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std::weak_order(3) C++ Standard Libary std::weak_order(3) NAME std::weak_order - std::weak_order Synopsis Defined in header <compare> inline namespace /* unspecified */ { inline constexpr /* unspecified */ weak_order = /* unspecified */; (since C++20) } Call signature template< class T, class U > requires /* see below */ constexpr std::weak_ordering weak_order(T&& t, U&& u) noexcept(/* see below */); Compares two values using 3-way comparison and produces a result of type std::weak_ordering Let t and u be expressions and T and U denote decltype((t)) and de- cltype((u)) respectively, std::weak_order(t, u) is expression-equivalent to: * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true: * std::weak_ordering(weak_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::weak_order, * otherwise, if T is a floating-point type: * if std::numeric_limits<T>::is_iec559 is true, performs the weak ordering comparison of floating-point values (see be- low) and returns that result as a value of type std::weak_ordering, * otherwise, yields a value of type std::weak_ordering that is consistent with the ordering observed by T's compari- son operators, * otherwise, std::weak_ordering(std::compare_three_way()(t, u)), if it is well-formed, * otherwise, std::weak_ordering(std::strong_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. 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::weak_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 __weak_order_fn. All instances of __weak_order_fn are equal. The effects of invoking different instances of type __weak_order_fn on the same arguments are equiva- lent, 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::weak_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::weak_order above, __weak_order_fn models * std::invocable<__weak_order_fn, Args...>, * std::invocable<const __weak_order_fn, Args...>, * std::invocable<__weak_order_fn&, Args...>, and * std::invocable<const __weak_order_fn&, Args...>. Otherwise, no function call operator of __weak_order_fn participates in overload resolution. Notes Strict weak order of IEEE floating-point types Let x and y be values of same IEEE floating-point type, and weak_or- der_less(x, y) be the boolean result indicating if x precedes y in the strict weak or- der defined by the C++ standard. * If neither x nor y is NaN, then weak_order_less(x, y) == true if and only if x < y, i.e. all representations of equal floating-point value are equivalent; * If x is negative NaN and y is not negative NaN, then weak_or- der_less(x, y) == true; * If x is not positive NaN and y is positive NaN, then weak_or- der_less(x, y) == true; * If both x and y are NaNs with the same sign, then (weak_or- der_less(x, y) || weak_order_less(y, x)) == false, i.e. all NaNs with the same sign are equivalent. Example This section is incomplete Reason: no example See also weak_ordering the result type of 3-way comparison that supports all 6 (C++20) operators and is not substitutable (class) strong_order performs 3-way comparison and produces a result of type (C++20) std::strong_ordering (customization point object) partial_order performs 3-way comparison and produces a result of type (C++20) std::partial_ordering (customization point object) compare_weak_order_fallback performs 3-way comparison and produces a result of type (C++20) std::weak_ordering, even if operator<=> is unavailable (customization point object) http://cppreference.com 2022.07.31 std::weak_order(3)
NAME | Synopsis | Notes | Example | See also
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