Customization point object (since C++20)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Standard library

Customization point object (C++20)

Exposition-only entities

decay-copy (C++11)

synth-three-waysynth-three-way-result (C++20)(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)