Defined in header <compare> | ||
---|---|---|
inline namespace /* unspecified */ { inline constexpr /* unspecified */ compare_partial_order_fallback = /* unspecified */; } | (since C++20) | |
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 */); | (since C++20) |
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; t == u ? std::partial_ordering::equivalent :t < u ? std::partial_ordering::less :u < t ? std::partial_ordering::greater : std::partial_ordering::unordered
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.
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.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 2114 (P2167R3) | C++20 | the fallback mechanism only required return types to be convertible to bool | constraints strengthened |
LWG 3465 | C++20 | the fallback mechanism did not require u < t to be well-formed | required |
(C++20) | performs 3-way comparison and produces a result of type std::partial_ordering (customization point object) |
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