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

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

• std::strong_order(t, u), if it is a well-formed expression;
• otherwise, t == u ? std::strong_ordering::equal :
t < u ? std::strong_ordering::less :
         std::strong_ordering::greater

if the expressions t == u and t < u are both 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_strong_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_strong_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