Access violation writing location 0x00000000

A class-based program of mine is giving some issues, in the form of unhandled exceptions. To summarize the program:

The main class, graphcalc, functions perfectly. It is created on the stack. It creates a seperate thread, which then creates a window. In the creation procedure for this window, an instance of the second class, drawbox, is created dynamically. It is not deleted anywhere.

The problem occurs when trying to access member data/functions from member functions (other than the constructor). For example:

Code:

LRESULT CALLBACK drawbox::WndProc(HWND hwnd , UINT msg,WPARAM wParam , LPARAM lParam)
{
	hMain=hwnd; //access violation occurs here
	switch (msg)
	{
	case WM_PAINT:
		break;
	default:
		break;
	}
	return CallWindowProc (wpPrevProc , hwnd , msg , wParam , lParam);
}

This function is non-static, and is a member of drawbox. hMain is likewise non-static and a member of drawbox.

As far as I know, this function SHOULD have the 'this' pointer, but it would appear it thinks it does, but actually doesn't.

Any ideas as to whats going here?

Also, 'this' is NULL in the function.

Can you post your "thunking" code that calls drawbox::WndProc?

Is this what you mean?

Code:

LRESULT CALLBACK drawbox::WndProcS(HWND hwnd,UINT msg,WPARAM wParam,LPARAM lParam)
{
	drawbox *lpDB;
	if (msg==WM_NCCREATE)
	{
		LPCREATESTRUCT lpcs=(LPCREATESTRUCT)lParam;
		lpDB=(drawbox *)lpcs->lpCreateParams;
		SetWindowLong(hwnd,GWL_USERDATA,(LONG)lpDB);
	}
	lpDB=(drawbox *)GetWindowLong(hwnd,GWL_USERDATA);
	return lpDB->WndProc(hwnd,msg,wParam,lParam);
}

Yep. WM_NCCREATE is not the first message that your window procedure receives, so you are calling a function on a NULL class pointer. Just check that lpDB is not NULL before using it.

Won't I miss out on messages then?

Code:

	if (!lpDB)
		return 0;
	return lpDB->StaticProc(hwnd,msg,wParam,lParam);

This code prevents the window from being seen. The first message through the loop is WM_PAINT. Any ideas?

Try:

Code:

if (!lpDB)
	return DefWindowProc(hwnd, msg, wParam, lParam);
else
	return lpDB->StaticProc(hwnd, msg, wParam, lParam);

>> Won't I miss out on messages then? <<

Some obscure messages come through before NCCREATE but nothing you would want to handle(I think). However, you must pass them on to windows to process as above.

>> The first message through the loop is WM_PAINT. <<

I didn't think WM_PAINT came through before WM_NCCREATE but I could be wrong.

EDIT: Of course. Missed that.

I just realized the problem. I'm subclassing the window, not superclassing it. Therefore I never receive a message that contains a pointer to the class instance (such as WM_CREATE or WM_NCCREATE). So, the whole time, I'm effectively calling the WndProc code without giving it a valid 'this' pointer. I'll superclass and see what happens. Thanks.

Works fine now.

Code:

if (!lpDB)
	return DefWindowProc(hwnd, msg, wParam, lParam);
else
	return lpDB->StaticProc(hwnd, msg, wParam, lParam);

Unfortunately it's not a default window, it's a static control. That would probably change things.