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>> == 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,
 other the expression is illformed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
 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
 Otherwise, std::weak_order(t, u) is illformed.
Contents 
[edit] Expressionequivalent
Expression e is expressionequivalent to expression f, if e and f have the same effects, either are both potentiallythrowing or are both not potentiallythrowing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.
[edit] 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 (denoted, for exposition purposes, as weak_order_ftor
). All instances of weak_order_ftor
are equal. 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_ftor
will satisfy std::invocable<const weak_order_ftor&, Args...>. Otherwise, no function call operator of weak_order_ftor
participates in overload resolution.
[edit] Notes
[edit] 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.
[edit] Example
This section is incomplete Reason: no example 
[edit] 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) 