NAME

std::compare_partial_order_fallback - std::compare_partial_order_fallback

Synopsis

Defined in header <compare>
inline namespace /* unspecified */ {

inline constexpr /* unspecified */ (since C++20)
compare_partial_order_fallback = /* unspecified */;

}
Call signature
template< class T, class U >

requires /* see below */
constexpr std::partial_ordering

compare_partial_order_fallback(T&& t, U&& u) noexcept(/* see below
*/);

Performs three-way comparison on t and u and produces a result of type
std::partial_ordering, even if the operator <=> is unavailable.

Let t and u be expressions and T and U denote decltype((t)) and decltype((u))
respectively, std::compare_partial_order_fallback(t, u) is expression-equivalent to:

* If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:

* std::partial_order(t, u), if it is a well-formed expression;
* otherwise,

t == u ? std::partial_ordering::equivalent :
t < u ? std::partial_ordering::less :
u < t ? std::partial_ordering::greater :
std::partial_ordering::unordered

if t == u, t < u, and u < t are all well-formed and convertible
to bool, except that t and u are evaluated only once.

* In all other cases, std::compare_partial_order_fallback(t, u) 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::compare_partial_order_fallback 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
__compare_partial_order_fallback_fn.

All instances of __compare_partial_order_fallback_fn are equal. The effects of
invoking different instances of type __compare_partial_order_fallback_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::compare_partial_order_fallback
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::compare_partial_order_fallback above,
__compare_partial_order_fallback_fn models

* std::invocable<__compare_partial_order_fallback_fn, Args...>,
* std::invocable<const __compare_partial_order_fallback_fn, Args...>,
* std::invocable<__compare_partial_order_fallback_fn&, Args...>, and
* std::invocable<const __compare_partial_order_fallback_fn&, Args...>.

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

Example

This section is incomplete
Reason: no example

See also

partial_order performs 3-way comparison and produces a result of type
(C++20) std::partial_ordering
(customization point object)