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 typeTorconst 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
fis a pointer to member function of classT:
- 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
t1does not satisfy the previous items, then INVOKE(f, t1, t2, ..., tN) is equivalent to ((*t1).*f)(t2, ..., tN).
- otherwise, if N == 1 and
fis a pointer to data member of classT:
- 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
t1does 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,
fis aFunctionObject)
[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 |