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 */ constexpr std::partial_ordering     partial_order( T&& t, U&& u ) noexcept ( /* see below */ ) ;

Compares two values using 3-way 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 expression-equivalent

• 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 well-formed 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) )
  • otherwise, std::partial_ordering ( std::weak_order (t, u) )
• 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.

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 cv-unqualified 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 const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, std::partial_order

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...>
• std::invocable < const __partial_order_fn&, Args...> .
  

Otherwise, no function call operator of __partial_order_fn participates in overload resolution.

Example

This section is incomplete Reason: no example

See also

partial_ordering (C++20)	the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class)
strong_order (C++20)	performs 3-way comparison and produces a result of type std::strong_ordering (customization point object)
weak_order (C++20)	performs 3-way comparison and produces a result of type std::weak_ordering (customization point object)
compare_partial_order_fallback (C++20)	performs 3-way comparison and produces a result of type std::partial_ordering, even if operator<=> is unavailable (customization point object)