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:
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
, std::partial_ordering(std::compare_three_way()(t, u))
, if it is well-formed, std::partial_ordering(std::weak_order(t, u))
, if it is well-formed. 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.
(C++20) | the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class) |
(C++20) | performs 3-way comparison and produces a result of type std::strong_ordering (customization point object) |
(C++20) | performs 3-way comparison and produces a result of type std::weak_ordering (customization point object) |
(C++20) | 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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