C++ named requirements: Callable

A Callable type is a type for which the INVOKE and INVOKE<R> operations (used by, e.g., std::function, std::bind, and std::thread::thread) are applicable.

INVOKE can be performed explicitly using the library function std::invoke.	(since C++17)
INVOKE<R> can be performed explicitly using the library function std::invoke_r.	(since C++23)

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

Notes

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):

function (C++11)	wraps callable object of any copy constructible type with specified function call signature (class template)
move_only_function (C++23)	wraps callable object of any type with specified function call signature (class template)
bind (C++11)	binds one or more arguments to a function object (function template)
bind_frontbind_back (C++20)(C++23)	bind a variable number of arguments, in order, to a function object (function template)
reference_wrapper (C++11)	CopyConstructible and CopyAssignable reference wrapper (class template)
result_ofinvoke_result (C++11)(removed in C++20)(C++17)	deduces the result type of invoking a callable object with a set of arguments (class template)
thread (C++11)	manages a separate thread (class)
jthread (C++20)	std::thread with support for auto-joining and cancellation (class)
call_once (C++11)	invokes a function only once even if called from multiple threads (function template)
async (C++11)	runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result (function template)
packaged_task (C++11)	packages a function to store its return value for asynchronous retrieval (class template)

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)
invocableregular_invocable (C++20)	specifies that a callable type can be invoked with a given set of argument types (concept)
invokeinvoke_r (C++17)(C++23)	invokes any Callable object with given arguments and possibility to specify return type (since C++23) (function template)