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

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)