NAME¶

std::partial_order - std::partial_order

Synopsis¶

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.

Expression-equivalent

Expression e is expression-equivalent 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 potentially-throwing or else neither is potentially-throwing
(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
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.

Notes¶

Example¶

This section is incomplete
Reason: no example

See also¶

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