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)