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 can be copied freely and its copies can be used interchangeably.

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

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

See also

• RangeAdaptorObject (RAO)
• RangeAdaptorClosureObject (RACO)