std::compare_strong_order_fallback

Defined in header <compare>
inline namespace /* unspecified */ { inline constexpr /* unspecified */         compare_strong_order_fallback = /* unspecified */ ; }		(since C++20)
Call signature
template < class T, class U > requires /* see below */ constexpr std::strong_ordering     compare_strong_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::strong_ordering, even if the operator <=>

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

• std::strong_order(t, u)
• 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

In all other cases, std::compare_strong_order_fallback(t, u) is ill-formed, which can result in substitution failure

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. For exposition purposes, the cv-unqualified version of its type is denoted as __compare_strong_order_fallback_fn

All instances of __compare_strong_order_fallback_fn are equal. The effects of invoking different instances of type __compare_strong_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_strong_order_fallback

Given a set of types Args..., if std::declval <Args> ( ) meet the requirements for arguments to std::compare_strong_order_fallback above, __compare_strong_order_fallback_fn models

• std::invocable <__compare_strong_order_fallback_fn, Args...>
• std::invocable < const __compare_strong_order_fallback_fn, Args...>
• std::invocable <__compare_strong_order_fallback_fn&, Args...>
• std::invocable < const __compare_strong_order_fallback_fn&, Args...> .
  

Otherwise, no function call operator of __compare_strong_order_fallback_fn participates in overload resolution.

Example

This section is incomplete Reason: no example

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

See also

strong_order (C++20)

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