I am reading C Primer 5th edition and there I came across the following statement:

As a result, unlike ordinary function pointers, a pointer to a member is not a callable object; these pointers do not support the function-call operator.

So my question is: Is the highlighted part correct according to the standard?

My current understanding and intuition are that functionally they behave in the same manner, so a pointer to a member function should also be a callable object(as its name suggest).

Note that I am not asking whether the book is correct in saying that member function pointers do not support the function call operator. Because I already know that that part of the statement is correct. What I am asking is whether the pointer to the member function is a callable object according to the standard.

CodePudding user response：

What i am asking is that whether pointer to member function are callable object according to the standard.

Yes, a pointer to a member function is a callable object according to the standard.

From func.def#4:

A callable object is an object of a callable type.

And from func.def#3:

A callable type is a function object type ([function.objects]) or a pointer to member.

Thus, the highlighted part of the quoted statement that says "a pointer to member is not a callable object" from C Primer is incorrect according the standard.

CodePudding user response：

As a result, unlike ordinary function pointers, a pointer to member is not a callable object; these pointers do not support the function-call operator.

So my question is, if the above quoted statement(the highlighted part in particular) correct according to the standard?

To my surprise, the standard actually says in [func.def]/3

1. The following definitions apply to this Clause:
2. ...
3. A callable type is a function object type ([function.objects]) or a pointer to member.
4. A callable object is an object of a callable type.
5. A call wrapper type is a type that holds a callable object and supports a call operation that forwards to that object.
6. A call wrapper is an object of a call wrapper type.
7. A target object is the callable object held by a call wrapper.

So in fact a pointer to member function is a callable object according to the standard ... in the context of describing which types can be the target of the function objects defined in <functional>.

There is no alternative definition of a "callable object" in the standard that would correspond to the more general sense implied by the book.

Summary:

1. The only standard definition of a callable object is when defining the types that are valid targets for the function objects in <functional>.

   A pointer to member function is a valid target, and is callable so long as the instance pointer is either bound or passed to the function call

2. The customary definition of a callable object as one providing a function call operator is not directly supported by the standard.

   This definition is arguably more useful though, because it tells me how I can use the object.