##### cppreference.com

##### Search

Create account

##### Namespaces

- Page
- Discussion

##### Variants

##### Views

- View
- Edit
- History

##### Actions

# C++ concepts: Callable

From cppreference.com
< cpp‎ | concept

A `Callable` type is a type, for which the INVOKE operation, as defined by std::function, std::bind, or std::thread::thread is applicable.

### [edit] Requirements

The type `T` satisfies `Callable` if

Given

- `f`, an object of type `T` or `const T`
- `ArgTypes`, suitable argument list
- `R`, suitable return type

The following expressions must be valid:

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

where INVOKE(f, t1, t2, ..., tn, R) is defined as 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` and `t1` is an object of class `T` or reference to an object of class `T` or derived from `T`, then INVOKE(f, t1, t2, ..., tn) is equivalent to (t1.*f)(t2, ..., tn)
- otherwise, if `f` is a pointer to member function and `t1` is not one of the types described above, 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` and `t1` is an object of class `T` or reference to an object of class `T` or derived from `T`, then INVOKE(f, t1) is equivalent to t1.*f
- otherwise, if N == 1 and `f` is a pointer to data member of class `T` and `t1` is not one of the types described above, 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`)

### [edit] 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.

### [edit] 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

Retrieved from "http://en.cppreference.com/mwiki/index.php?title=cpp/concept/Callable&oldid=71476"