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 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.

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 potentially-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 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.

Notes

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.

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)