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std::strong_order(3) C++ Standard Libary std::strong_order(3) NAME std::strong_order - std::strong_order Synopsis Defined in header <compare> inline namespace /* unspecified */ { inline constexpr /* unspecified */ strong_order = /* unspecified */; (since C++20) } Call signature template< class T, class U > requires /* see below */ constexpr std::strong_ordering strong_order( T&& t, U&& u ) noexcept(/* see below */); Compares two values using 3-way comparison and produces a result of type std::strong_ordering Let t and u be expressions and T and U denote decltype((t)) and de- cltype((u)) respectively, std::strong_order(t, u) is expression-equivalent to: * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true: * std::strong_ordering(strong_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::strong_order, * otherwise, if T is a floating-point type: * if std::numeric_limits<T>::is_iec559 is true, performs the ISO/IEC/IEEE 60559 totalOrder comparison of floating- point values and returns that result as a value of type std::strong_or- dering (note: this comparison can distinguish between the positive and negative zero and between the NaNs with different representations), * otherwise, yields a value of type std::strong_ordering that is consistent with the ordering observed by T's compari- son operators, * otherwise, std::strong_ordering(std::compare_three_way()(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::strong_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 __strong_order_fn. All instances of __strong_order_fn are equal. The effects of invok- ing different instances of type __strong_order_fn on the same arguments are equiv- alent, 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::strong_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::strong_order above, __strong_order_fn models * std::invocable<__strong_order_fn, Args...>, * std::invocable<const __strong_order_fn, Args...>, * std::invocable<__strong_order_fn&, Args...>, and * std::invocable<const __strong_order_fn&, Args...>. Otherwise, no function call operator of __strong_order_fn partici- pates in overload resolution. Notes Strict total order of IEEE floating-point types Let x and y be values of same IEEE floating-point type, and to- tal_order_less(x, y) be the boolean result indicating if x precedes y in the strict total order defined by totalOrder in ISO/IEC/IEEE 60559. (total_order_less(x, y) || total_order_less(y, x)) == false if and only if x and y have the same bit pattern. * if neither x nor y is NaN: * if x < y, then total_order_less(x, y) == true; * if x > y, then total_order_less(x, y) == false; * if x == y, * if x is negative zero and y is positive zero, to- tal_order_less(x, y) == true, * if x is not zero and x's exponent field is less than y's, then total_order_less(x, y) == (x > 0) (only meaningful for decimal floating-point number); * if either x or y is NaN: * if x is negative NaN and y is not negative NaN, then to- tal_order_less(x, y) == true, * if x is not positive NaN and y is positive NaN, then to- tal_order_less(x, y) == true, * if both x and y are NaNs with the same sign and x's man- tissa field is less than y's, then total_order_less(x, y) == !std::signbit(x). Example This section is incomplete Reason: no example See also strong_ordering the result type of 3-way comparison that supports all (C++20) 6 operators and is substitutable (class) weak_order performs 3-way comparison and produces a result of (C++20) type std::weak_ordering (customization point object) partial_order performs 3-way comparison and produces a result of (C++20) type std::partial_ordering (customization point object) performs 3-way comparison and produces a result of compare_strong_order_fallback type std::strong_ordering, even if op- erator<=> is (C++20) unavailable (customization point object) http://cppreference.com 2022.07.31 std::strong_order(3)
NAME | Synopsis | Notes | Example | See also
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