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

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
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

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

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