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:

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

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


See also

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

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