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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) are applicable.

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

pubExpressionRequirements
pub
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):

pubfunction(C++11)wraps callable object of any copy constructible type with specified function call signature
pubmove_only_function(C++23)wraps callable object of any type with specified function call signature
pubbind(C++11)binds one or more arguments to a function object
pubbind_front(C++20)
bind_back(C++23)
bind a variable number of arguments, in order, to a function object
pubreference_wrapper(C++11)CopyConstructible and CopyAssignable reference wrapper
pubresult_of(C++11) (removed in C++20)
invoke_result(C++17)
deduces the result type of invoking a callable object with a set of arguments
pubthread(C++11)manages a separate thread
pubjthread(C++20)std::thread with support for auto-joining and cancellation
pubcall_once(C++11)invokes a function only once even if called from multiple threads
pubasync(C++11)runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result
pubpackaged_task(C++11)packages a function to store its return value for asynchronous retrieval

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) are applicable.

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

pubExpressionRequirements
pub
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):

pubfunction(C++11)wraps callable object of any copy constructible type with specified function call signature
pubmove_only_function(C++23)wraps callable object of any type with specified function call signature
pubbind(C++11)binds one or more arguments to a function object
pubbind_front(C++20)
bind_back(C++23)
bind a variable number of arguments, in order, to a function object
pubreference_wrapper(C++11)CopyConstructible and CopyAssignable reference wrapper
pubresult_of(C++11) (removed in C++20)
invoke_result(C++17)
deduces the result type of invoking a callable object with a set of arguments
pubthread(C++11)manages a separate thread
pubjthread(C++20)std::thread with support for auto-joining and cancellation
pubcall_once(C++11)invokes a function only once even if called from multiple threads
pubasync(C++11)runs a function asynchronously (potentially in a new thread) and returns a std::future that will hold the result
pubpackaged_task(C++11)packages a function to store its return value for asynchronous retrieval