Thread: illegal references in static member functions

my class has a static member window procedure. from this procedure i cant refer to any data members of that class without the compiler telling me:

illegal reference to data member 'cBen::value' in a static member function

ie:

Code:

class cBen
{
public:
    static LRESULT CALLBACK WndProc(...);
    int value;
};

LRESULT CALLBACK cBen::WndProc(...)
{
    value=5;
    return DefWindowProc(...);
}

what can i do?

davros, i cant get your method to work, and anyway, it seems a bit annoying to have to declare everything static if i want it to be usable. heres what happens:

Code:

class cBen
{
public:
    static int value;
    static LRESULT CALLBACK proc(...);
};

LRESULT CALLBACK cBen::proc(...)
{
    value=5;
}

compiler error:
cBen.obj : error LNK2001: unresolved external symbol "public: static int cBen::value" (?value@cBen@@2HA)

i have no idea what THAT means.

ken, i cant work how to use SetWindowLongPtr to set the class pointer. there doesn't seem to be a pre-defined offset for that information. how do i do it?

and also davros, reading on MSDN i found that declaring a data member of a class static means that one copy of the data member is shared by all instances of the class, and that does not sound like what i want.

i think the problem can be resolved way back when i asked about window procedures within classes. according to MSDN, declaring a member function to be static means that it can only access static members of that class. that was not the goal that i was trying to achieve. all i wanted was to be able to set the wndproc of a WNDCLASSEX structure to a member function window procedure, and for some reason, static made that work. but is there any other way to do it?

You'll have to do this to get rid of the undefined external symbol (that static variable value):

Code:

class cBen {
   public:
      static int value;
      static LRESULT CALLBACK proc(...);
};

static int cBen::value;


// then the use of value becomes valid

That is, redeclare the static variable outside the class definition. This is a consequence of the way compilers used to register static class variables. Some saw static variables declared within the class definition valid, while others only recognized static variables defined outside the declaration. As a result, the standard requires the programmer to declare the variable both ways.

Search Google or the boards for a more in-depth explanation.

nup, that doesn't work. the redeclaration causes the following compiler error:

'value' : 'static ' storage-class specifier illegal on members

anyway, i really dont want all my data members to be static. i would rather find a way to use the window procedure without it being static. thanks anyway.

okay, i think i just figured something out. the reason the wndproc member function must be static is because in the Create function i try to pass a pointer to the function while initializing the WNDCLASSEX structure. So that means the pointer MUST point to a valid function, and therefore an instance of the class or at least that function must be in existence. static makes sure there is always an instance available.

but that's pretty crappy. why can't i pass a pointer that refers to the wndproc that is created with the instance that is created? i hope you catch my drift. i mean, say you have the following:

Code:

class cBen
{
LRESULT CALLBACK cBenProc();   
blah blah
};

cBen::Create()
{
    WNDCLASSEX wcx;

     wcx.lpfnWndProc=(WNDPROC)cBenProc;
}

cBen::cBenProc()
{
  blah blah
}

cBen instBen;

int WINAPI WinMain(...)
{
    instBen.Create();
}

when instBen is created, shouldn't an instance of cBenProc() be created to? therefore, isn't the pointer being passed a valid pointer? i am confused.

thanks ken, that works, but when i tried to make it a bit more efficient, i ran into some trouble.
observe: (by the way, i allocated an extra byte to the window memory)

Code:

LRESULT CALLBACK cBen::cBenProc(HWND hwnd,UINT msg,WPARAM wParam,LPARAM lParam)
{
	cBen *it;

	if (GetWindowLong(hwnd,0))
		it=(cBen *)GetWindowLong(hwnd,GWL_USERDATA);
	

	switch (msg)
	{
	case WM_NCCREATE:
		LPCREATESTRUCT lpc;

		lpc=(LPCREATESTRUCT)lParam;
		SetWindowLong(hwnd,GWL_USERDATA,(LONG)lpc->lpCreateParams);
		SetWindowLong(hwnd,0,1);
		return DefWindowProc(hwnd,msg,wParam,lParam);

	case WM_CLOSE:
		it->value=5;

		DestroyWindow(hwnd);
		return 0;

	case WM_DESTROY:
		return 0;

	default:
		return DefWindowProc(hwnd,msg,wParam,lParam);

	}
}

clearly, i'm trying to make it so i only ever have to retrieve a pointer once during the function. but the line:

Code:

it->value=5;

causes one of those annoying access violations.

Code:

First-chance exception in class.exe: 0xC0000005: Access Violation.

If im not mistaken, the pointer i retrieved points to somewhere i shouldn't be changing. however, it works fine when the pointer is retrieved within the WM_CLOSE message capture, and the code has not been modified, just moved. so what is going on?

damn i hate this static crap! if only the wndproc could avoid being static, then it would be easy. are you sure it has to be so difficult?