# C++ named requirements: Callable

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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:

- if `f` is a pointer to member function of class `T`:

  - If std::is_base_of<T, std::remove_reference_t<decltype(t1)>>::value is true, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.*f)(t2, ..., tN)
  - otherwise, if std::remove_cvref_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.get().*f)(t2, ..., tN) (since C++17)
  - otherwise, if `t1` does not satisfy the previous items, then INVOKE(f, t1, t2, ..., tN) is equivalent to ((*t1).*f)(t2, ..., tN).

- otherwise, if N == 1 and `f` is a pointer to data member of class `T`:

  - If std::is_base_of<T, std::remove_reference_t<decltype(t1)>>::value is true, then INVOKE(f, t1) is equivalent to t1.*f
  - otherwise, if std::remove_cvref_t<decltype(t1)> is a specialization of std::reference_wrapper, then INVOKE(f, t1) is equivalent to t1.get().*f (since C++17)
  - otherwise, if `t1` does not satisfy the previous items, then INVOKE(f, t1) is equivalent to (*t1).*f

- otherwise, INVOKE(f, t1, t2, ..., tN) is equivalent to f(t1, t2, ..., tN) (that is, `f` is a FunctionObject)

### 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

is_invocable is_invocable_r is_nothrow_invocable is_nothrow_invocable_r (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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