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>> is 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(std::compare_three_way()(t, u)), if it is wellformed,
 otherwise, std::partial_ordering(std::weak_order(t, u)), if it is wellformed,
 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
 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::partial_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 __partial_order_fn
.
All instances of __partial_order_fn
are equal. The effects of invoking different instances of type __partial_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::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_fn
models
 std::invocable<__partial_order_fn, Args...>,
 std::invocable<const __partial_order_fn, Args...>,
 std::invocable<__partial_order_fn&, Args...>, and
 std::invocable<const __partial_order_fn&, Args...>.
Otherwise, no function call operator of __partial_order_fn
participates in overload resolution.
Notes
Example
This section is incomplete Reason: no example 
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) 