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 decltype((u)) respectively, std::weak_order(t, u)
is expression-equivalent to:
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
, T
is a floating-point type: 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
, std::weak_ordering
that is consistent with the ordering observed by T
's comparison operators, std::weak_ordering(std::compare_three_way()(t, u))
, if it is well-formed, std::weak_ordering(std::strong_order(t, u))
, if it is well-formed. 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 equivalent, 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.
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.
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; x
is negative NaN and y
is not negative NaN, then weak_order_less(x, y) == true
; x
is not positive NaN and y
is positive NaN, then weak_order_less(x, y) == true
; 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.
(C++20) | the result type of 3-way comparison that supports all 6 operators and is not substitutable (class) |
(C++20) | performs 3-way comparison and produces a result of type std::strong_ordering (customization point object) |
(C++20) | performs 3-way comparison and produces a result of type std::partial_ordering (customization point object) |
(C++20) | performs 3-way comparison and produces a result of type std::weak_ordering , even if operator<=> is unavailable (customization point object) |
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