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:

• 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_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r (C++17)

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