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std::weak_order - cppreference.com
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 decltype((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 below) 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 comparison 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 result in substitution failure when it appears in the immediate context of a template instantiation.
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. See CustomizationPointObject for details.
[edit] Strict weak order of IEEE floating-point types
Let x and y be values of same IEEE floating-point type, and weak_order_less(x, y) be the boolean result indicating if x precedes y in the strict weak order 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_order_less(x, y) == true;
- If x is not positive NaN and y is positive NaN, then weak_order_less(x, y) == true;
- If both x and y are NaNs with the same sign, then (weak_order_less(x, y) || weak_order_less(y, x)) == false, i.e. all NaNs with the same sign are equivalent.
[edit] Example [edit] See also the result type of 3-way comparison that supports all 6 operators and is not substitutable
(class) [edit] performs 3-way comparison and produces a result of type std::strong_ordering
(customization point object)[edit] performs 3-way comparison and produces a result of type std::partial_ordering
(customization point object)[edit] performs 3-way comparison and produces a result of type std::weak_ordering, even if operator<=> is unavailable
(customization point object)[edit]
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