std::compare_partial_order_fallback

 

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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;
• 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.

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.

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 (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

See also

partial_order (C++20)

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