Customization point object (since C++20)

Explanation

The name cpo 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 __cpo_fn

All instances of __cpo_fn are equal. The effects of invoking different instances of type __cpo_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, cpo

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

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

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

See also

• RangeAdaptorObject (RAO)
• RangeAdaptorClosureObject (RACO)