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:

• If std::is_same_v<std::decay_t<T>, std::decay_t<U>> == 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,
  • other the expression is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.
• Otherwise, std::partial_order(t, u) is ill-formed.

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.

Customization point objects

The name std::partial_order denotes a customization point object, which is a function object of a literal semiregular class type (denoted, for exposition purposes, as partial_order_ftor). All instances of partial_order_ftor are equal. 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_ftor will satisfy std::invocable<const partial_order_ftor&, Args...>. Otherwise, no function call operator of partial_order_ftor participates in overload resolution.

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)