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C++ named requirements: Callable

A Callable type is a type for which the INVOKE operation (used by, e.g., std::function, std::bind, and std::thread::thread) is applicable. This operation may be performed explicitly using the library function std::invoke. (since C++17).

Requirements

The type T satisfies Callable if.

Given.

  • f, an object of type T
  • ArgTypes, suitable list of argument types
  • R, suitable return type

The following expressions must be valid:

Expression Requirements
INVOKE<R>(f, std::declval<ArgTypes>()...) the expression is well-formed in unevaluated context

where INVOKE<R>(f, t1, t2, ..., tN) is defined as static_cast<void>(INVOKE(f, t1, t2, ..., tN)) if R is possibly cv-qualified void, otherwise INVOKE(f, t1, t2, ..., tN), implicitly converted to R.

where INVOKE(f, t1, t2, ..., tN) is defined as follows:

Notes

For pointers to member functions and pointers to data members, t1 may be a regular pointer or an object of class type that overloads operator*, such as std::unique_ptr or std::shared_ptr.

Pointers to data members are Callable, even though no function calls take place.

Standard library

In addition, the following standard library facilities accept any Callable type (not just FunctionObject).

std::function
std::bind
std::result_of
std::thread::thread
std::call_once
std::async
std::packaged_task
std::reference_wrapper

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 2420 C++11 when R is void, the result must be implicitly convertible to void (which is impossible) the result is explicitly converted to void when R is cv void

See also

(C++17)
checks if a type can be invoked (as if by std::invoke) with the given argument types
(class template)

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