Problem accessing a class through a static function.

[Mlord]

Hello.
I am having problems accessing methods from a class inside a static function.
I basically have two classes. One called CWindow that creates a window(obviously ), which uses a callback function (also inside CWindow).
The other class is called CSkin which will be use the created window and draw things on it.

This is the thing. I want CSkin to call one of it's own methods (in this case PaintSkin(HDC hdc)) when the message WM_PAINT is sent to the window. But, I can't access anything from my CSkin class inside the callback function.
I can easily access them from other functions, but not in the callback function. Which I guess have something to do with it being static, it is however necessary or otherwise I receive a compile error.

My Window class:

class CWindow
{
public:
	CWindow();
	~CWindow();
        
        int CreateWindow( );
        ...
private:
        ...
	CSkin *skin;
	static LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
};

Constructor-code, where I create an instance of my CSkin class:

CWindow::CWindow()
{
        ...
	CWindow::skin = new CSkin;
        ...
}

CreateWindow()-code, WndProc needs to be static:

int CWindow::CreateWindow()
{
        ...
	wc.lpfnWndProc   = (WNDPROC)(WndProc);
        ...
}

Callback-code, this is where I want the instance of CSkin to call it's PaintSkin()-method. But "CWindow::skin->" doesn't give me anything at all, and I can't access it.

LRESULT CALLBACK CWindow::WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch(msg)
    {
		case WM_PAINT:
                        ...
			if(CWindow::skin != NULL)
				// CWindow::skin->PaintSkin(ps.hdc); // Desired result 
                        ...
    }
}

How shall I proceed?
(I am using VS.NET 2k5 and the Win32 API).

Thanks.

[Eli Gassert]

Please read up on what a static method is. You can't access member functions because a static method is stateless. It does not require an instance of a class to be called and can be called directly through scope resolution, for example CWindow::WndProc(hwnd, msg, wparam, lparam).

Anyway, the moral of the story: you can't call anything stateful -- member variables, member functions -- from a static method.

[MrViggy]

Make your variable ('CWindow::skin') static.

Viggy

[miteshpandey]

This is a typical problem. You need to pass the object's pointer as a parameter to you static function. You call the static function from one of the class's function and pass "this" pointer as a parameter. However this is not possible in your scenario because WndProc is a callback function and it is called by the system:

The following is a way to handle your problem: It supports for creation of MDI child window. You may not require MDI child window support and your application can be a simple one.

bool CWindow::CreateWindow(ExStyle,
  LPCTSTR lpszClass, LPCTSTR lpszName, DWORD dwStyle,
  int x, int y, int nWidth, int nHeight, HWND hParent,
  HMENU hMenu, HINSTANCE hInst)
{
if ( ! RegisterClass(lpszClass, hInst) )
    return false;

  // use MDICREATESTRUCT to pass this pointer, support MDI child window
  MDICREATESTRUCT mdic;
  memset(& mdic, 0, sizeof(mdic));
  mdic.lParam = (LPARAM) this;
  m_hWnd = CreateWindowEx(dwExStyle, lpszClass, lpszName,
    dwStyle, x, y, nWidth, nHeight,
    hParent, hMenu, hInst, & mdic);

  return m_hWnd!=NULL;
}

LRESULT CALLBACK CWindow::WndProc(HWND hWnd, UINT uMsg,
       WPARAM wParam, LPARAM lParam)
{
  CWindow * pWindow;

  if ( uMsg == WM_NCCREATE )
  {
    assert( ! IsBadReadPtr((void *) lParam,
      sizeof(CREATESTRUCT)) );
    MDICREATESTRUCT * pMDIC = (MDICREATESTRUCT *)
        ((LPCREATESTRUCT) lParam)->lpCreateParams;
    pWindow = (CWindow *) (pMDIC->lParam);

    assert( ! IsBadReadPtr(pWindow, sizeof(CWindow)) );
    SetWindowLong(hWnd, GWL_USERDATA, (LONG) pWindow);
  }
  else
    pWindow=(CWindow *)GetWindowLong(hWnd, GWL_USERDATA);

  if ( pWindow )
     ..........................
     ..........................
  else
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
}

The main two vital functions are SetWindowLong and GetWindowLong