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std::partial_order

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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:

Contents

[edit] Expression-equivalent

Expression e is expression-equivalent to expression f, if e and f have the same effects, either are both potentially-throwing or are both not potentially-throwing (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 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.

[edit] Notes

[edit] Example

[edit] See also

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