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 (since C++20)

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

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

If std::decay_t<T> and std::decay_t<U>> are the same type,
std::compare_partial_order_fallback(t, u) is expression-equivalent to:

* 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 the expressions t == u, t < u and u < t are all well-formed and each of
decltype(t == u) and decltype(t < u) models boolean-testable, 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.

Example¶

This section is incomplete
Reason: no example

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 2114 C++20 the fallback mechanism only required constraints strengthened
(P2167R3) return types to be convertible to bool
LWG 3465 C++20 the fallback mechanism did not require required
u < t to be well-formed

See also¶

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

Category:¶

* Todo no example