std::weak_order
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 */ 

Compares two values using 3way 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 expressionequivalent 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 wellformed expression with overload resolution performed in a context that does not include a declaration of
std::weak_order
,  otherwise, if
T
is a floatingpoint type: if std::numeric_limits<T>::is_iec559 is true, performs the weak ordering comparison of floatingpoint 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 wellformed,
 otherwise, std::weak_ordering(std::strong_order(t, u)), if it is wellformed.
 std::weak_ordering(weak_order(t, u)), if it is a wellformed expression with overload resolution performed in a context that does not include a declaration of
 In all other cases, the expression is illformed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
Expressionequivalent
Expression e is expressionequivalent 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 potentiallythrowing or else neither is potentiallythrowing (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 cvunqualified 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 constqualified or not (however, a volatilequalified 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 floatingpoint types
Let x
and y
be values of same IEEE floatingpoint 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
nory
is NaN, then weak_order_less(x, y) == true if and only if x < y, i.e. all representations of equal floatingpoint value are equivalent;  If
x
is negative NaN andy
is not negative NaN, then weak_order_less(x, y) == true;  If
x
is not positive NaN andy
is positive NaN, then weak_order_less(x, y) == true;  If both
x
andy
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
(C++20) 
the result type of 3way comparison that supports all 6 operators and is not substitutable (class) 
(C++20) 
performs 3way comparison and produces a result of type std::strong_ordering (customization point object) 
(C++20) 
performs 3way comparison and produces a result of type std::partial_ordering (customization point object) 
(C++20) 
performs 3way comparison and produces a result of type std::weak_ordering , even if operator<=> is unavailable (customization point object) 