Classes and static call back procedures

I understood this a while back, but now, with more knowledge of Windows, I don't understand:

When a callback procedure is a class member function, it must be declared static, so that there is a copy available at all times. But, in this code, there IS already a copy available.

Code:

---

class benny
{
public:
LRESULT CALLBACK WndProc(params);
BOOL Create(params);
};

LRESULT CALLBACK benny::WndProc(params)
{
blah
}

BOOL benny::Create(params)
{
WNDCLASSEX wcx;

wcx.lpfnWndProc=WndProc;

---

There IS already an instance of the window procedure, because for the Create function to run, someone must have created an instance of the class. Why then, can I not do away with static?

No...the windows subsystems doesnt know or care about if there's an instance of a class.....

static member functions exist before during & after an object is created, that's why they can be used as callbacks for windows

Non-static members can only be called when an object has been created - they need an extra param so to speak by way of the "this" pointer, so due to this, they cannot be used as a callback for a windows proc

Oh. What about if you create a class, then pass a pointer to a member function to another class. Will that work, if the member function is not static?

Quote:

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Originally posted by bennyandthejets
Oh. What about if you create a class, then pass a pointer to a member function to another class. Will that work, if the member function is not static?

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If it's a nonstatic member function - no...because for the windows subsystem to call that function correctly, it would need to have a "this" pointer....

I'm not sure I understand about the this pointer. Is it passed implicitly to every member function? Could you show an example?

Quote:

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Originally posted by bennyandthejets
I'm not sure I understand about the this pointer. Is it passed implicitly to every member function? Could you show an example?

---

That's right...it's passed implicitly....it represents the identity of a specific object loaded into memory....but I cant show an example really as it's all controlled by the compiler......but just imagine it as an extra param that identifies a specific object

So in the following code, it is the compiler that prevents me from doing it?

Code:

---

class benny
{
public:
void DoStuff();
(* void()) RetrieveFunc();
};

void benny::DoStuff()
{
}

(* void()) Benny::RetrieveFunc()
{
return DoStuff();
}

---

Note: I know the code is probably not done right, but you probably know what I mean.

about the this pointer, whenever you're in a member function, and you're directly accessing its members, you are using the this pointer, just a shorthand version of it.

Code:

---

class Foo
{
public:
    void Bar(int a)    { b = a; }  // 'b' is using this
    void Boo(int a)    { this->b = a; } // same thing as above

private:
    int b;
};

---

At least I'm pretty sure that's the idea.

This is an implicit pointer to the object making the function call.

Don't even remember this thread, it was a whole month ago. I understand now why I can't do what I wanted, because of calling conventions. I'm doing some assembly language programming now to get a better idea of what happens under the hood, that should clear everything up completely.

It is possible to explicitly specify a this pointer for a non static member function without using any assembler code, if thats what you are looking for.

There is a post about it here.