C++ named requirements: Callable

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C++ named requirements

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)


The type T satisfies Callable if


  • 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 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, INVOKE(f, t1, t2, ..., tN) is equivalent to f(t1, t2, ..., tN) (that is, f is a FunctionObject)


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)


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

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