std::compare_partial_order_fallback

inline namespace /* unspecified */ {
    inline constexpr /* unspecified */
        compare_partial_order_fallback = /* unspecified */;
}

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 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), decltype(t<u) and decltype(u<t) 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. See CustomizationPointObject for details.

Example

Defect reports

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

See also