std::partial_order
Defined in header <compare>


inline namespace /* unspecified */ { inline constexpr /* unspecified */ partial_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::partial_ordering
Let t
and u
be expressions and T
and U
denote decltype((t)) and decltype((u)) respectively, std::partial_order(t, u) is expressionequivalent to:
 If std::is_same_v<std::decay_t<T>, std::decay_t<U>> == true:
 std::partial_ordering(partial_order(t, u)), if it is a wellformed expression with overload resolution performed in a context that does not include a declaration of
std::partial_order
,  otherwise std::partial_ordering(t <=> u), if it is wellformed,
 otherwise std::partial_ordering(std::weak_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::partial_ordering(partial_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::partial_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::partial_order
denotes a customization point object, which is a function object of a literal semiregular class type (denoted, for exposition purposes, as partial_order_ftor
). All instances of partial_order_ftor
are equal. Thus, std::partial_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::partial_order
above, partial_order_ftor
will satisfy std::invocable<const partial_order_ftor&, Args...>. Otherwise, no function call operator of partial_order_ftor
participates in overload resolution.
[edit] Notes
[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, is not substitutable, and allows incomparable values (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::weak_ordering (customization point object) 
performs 3way comparison and produces a result of type std::partial_ordering , even if operator<=> is unavailable (customization point object) 