C++ concepts: Callable

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

[edit] Requirements

The type T satisfies Callable if

Given

f, an object of type T or const T
ArgTypes, suitable list of argument types
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 static_cast<void>(INVOKE(f, t1, t2, ..., tN)) if R is possibly cv-qualified void, otherwise (since C++17) 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::decay_t<decltype(t1)>>::value is true, then INVOKE(f, t1, t2, ..., tN) is equivalent to (t1.*f)(t2, ..., tN)
otherwise, if std::decay_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::decay_t<decltype(t1)>>::value is true, then INVOKE(f, t1) is equivalent to t1.*f
otherwise, if std::decay_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)

[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

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