Thread: issues with accessing parent member functions by typecasting this pointer

First of all, these are the classes that I defined

Code:

class A
{
protected:
	void print() { cout << "A" << endl; }
};

class B : public A
{
public:
	void print() { static_cast<A*>(this)->print(); }
};

And in the main() function, I did the following

Code:

int main()
{
	B b;
	b.print();

	return 0;
}

But I get the following error - error C2248: 'A :: print' : cannot access protected member declared in class 'A'

But if I make the protected print() function of the class A static as shown below, print() function prints fine.

Code:

class A
{
protected:
	static void print() { cout << "A" << endl; }
};

Why does it work only when print() is declared static? I think it might have to do with the casting and all that, but I wasn't exactly sure. I would appreciate for any help. Thanks in advance.

I think it's because if you do it like that, it's basically the same as doing this:

Code:

A* a = new A;
a->print();

Although I'm not sure why the static version works.

Use this instead of the ugly cast:

Code:

void print() { A::print(); }

IMO, this is one of the downsides of how you access static methods in most object oriented languages. This code would work as well:

((A*) 5003)->print();

It doesn't matter what you put in place of the 5003. If you cast it to an A* and call a static method, its equivalent to A::print(); I think its rather unfortunate that you can even access static methods through an object... I mean it suggests that the method is somehow tied to the object, which it of course isn't as my example clearly shows.

Also, in the opposite direction, I think its unfortunate that you can call a base class non-static method using A::method(); This suggests that the method is somehow tied to the class, and not the object.

ARRRG.

Originally Posted by arpsmack

IMO, this is one of the downsides of how you access static methods in most object oriented languages. This code would work as well:

((A*) 5003)->print();

It doesn't matter what you put in place of the 5003. If you cast it to an A* and call a static method, its equivalent to A::print(); I think its rather unfortunate that you can even access static methods through an object... I mean it suggests that the method is somehow tied to the object, which it of course isn't as my example clearly shows.

Also, in the opposite direction, I think its unfortunate that you can call a base class non-static method using A::method(); This suggests that the method is somehow tied to the class, and not the object.

ARRRG.

Well whether the function is static or not, there's still just one of them in memory. You don't get a new copy of the function when you create a new instance of a class. The only difference between them is that the non-static function gets passed a this pointer behind the scenes. So I don't see a problem with either syntax.

Originally Posted by cpjust

Well whether the function is static or not, there's still just one of them in memory. You don't get a new copy of the function when you create a new instance of a class. The only difference between them is that the non-static function gets passed a this pointer behind the scenes.

I don't see how this relates to my argument at all, but I'm not shocked in the least that someone disagrees with me.

It's not like I feel very strongly about this. It just bothers me a little that I can look at code like this:

Code:

int main(void) {
   A myA;
   myA.DoStuff();
}

and wouldn't be able to tell if DoStuff() is a class method or not, even though it could have been easily been written:

Code:

int main(void) {
   A::DoStuff();
}

with absolutely no doubt at all.

I'm not sure why you'd need to know if the function is static or not, but I guess if you really want to know that (without looking at the header file) then you do have a point. A good IDE might be able to distinguish static vs non-static member functions for you, but I'm not aware of any that do that. (Maybe Eclipse?)

Originally Posted by cpjust

I'm not sure why you'd need to know if the function is static or not

Yeah I actually have no idea why I'd need to know. I spent a good couple minutes thinking about it, and I've got nothin'.