C++ concepts: Callable

 

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A Callable type is a type, for which the INVOKE operation, as defined by std::function, std::bind, or std::thread::thread is applicable.

Requirements

The type T satisfies Callable if

Given

• f, an object of type T or const T
• ArgTypes, suitable argument list
• 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 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 and t1 is an object of class T or reference to an object of class T or derived from T, then INVOKE(f, t1, t2, ..., tn) is equivalent to (t1.*f)(t2, ..., tn)
• otherwise, if f is a pointer to member function and t1 is not one of the types described above, 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 and t1 is an object of class T or reference to an object of class T or derived from T, then INVOKE(f, t1) is equivalent to t1.*f
• otherwise, if N == 1 and f is a pointer to data member of class T and t1 is not one of the types described above, 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