std::compare_partial_order_fallback - cppreference.com (original) (raw)

| Defined in header | | | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | ------------- | | 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), 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.

[edit] Example

[edit] Defect reports

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

[edit] See also

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