problem with SetWindowLong(hwnd,0,0) and GetWindowLong(hwnd,0)

[Stern_Pea]

Hi everybody :

I saw this two functions on a book

and then looked up in MSDN.
like this:
LONG SetWindowLong(
HWND hWnd,
int nIndex,
LONG dwNewLong
);

but I still don't understand the funtion of :

SetWindowLong(hwnd,0,0) ;//on/off flag ?

SetWindowLong (hwnd, 0, 1 ^ GetWindowLong (hwnd, 0));

GetWindowLong (hwnd, 0));

why using the these paramaters and when to use them.

only use a buffer to set flag? user define it or windows define it?

Thank you

///////////////////////////////////////////

attach part of the code

LRESULT CALLBACK ChildWndProc (HWND hwnd, UINT message,
WPARAM wParam, LPARAM lParam)
{
HDC hdc ;
PAINTSTRUCT ps ;
RECT rect ;

switch (message)
{
case WM_CREATE :
SetWindowLong (hwnd, 0, 0) ; // on/off flag
return 0 ;

case WM_KEYDOWN:
// Send most key presses to the parent window

if (wParam != VK_RETURN && wParam != VK_SPACE)
{
SendMessage (GetParent (hwnd), message, wParam, lParam) ;
return 0 ;
}
// For Return and Space, fall through to toggle the square

case WM_LBUTTONDOWN :
SetWindowLong (hwnd, 0, 1 ^ GetWindowLong (hwnd, 0)) ;
SetFocus (hwnd) ;
InvalidateRect (hwnd, NULL, FALSE) ;
return 0 ;

// For focus messages, invalidate the window for repaint

case WM_SETFOCUS:
idFocus = GetWindowLong (hwnd, GWL_ID) ;

// Fall through

case WM_KILLFOCUS:
InvalidateRect (hwnd, NULL, TRUE) ;
return 0 ;

case WM_PAINT :
hdc = BeginPaint (hwnd, &ps) ;

GetClientRect (hwnd, &rect) ;
Rectangle (hdc, 0, 0, rect.right, rect.bottom) ;

// Draw the "x" mark

if (GetWindowLong (hwnd, 0))
{
MoveToEx (hdc, 0, 0, NULL) ;
LineTo (hdc, rect.right, rect.bottom) ;
MoveToEx (hdc, 0, rect.bottom, NULL) ;
LineTo (hdc, rect.right, 0) ;
}
// Draw the "focus" rectangle

if (hwnd == GetFocus ())
{
rect.left += rect.right / 10 ;
rect.right -= rect.left ;
rect.top += rect.bottom / 10 ;
rect.bottom -= rect.top ;

SelectObject (hdc, GetStockObject (NULL_BRUSH)) ;
SelectObject (hdc, CreatePen (PS_DASH, 0, 0)) ;
Rectangle (hdc, rect.left, rect.top, rect.right, rect.bottom) ;
DeleteObject (SelectObject (hdc, GetStockObject (BLACK_PEN))) ;
}

EndPaint (hwnd, &ps) ;
return 0 ;
}
return DefWindowProc (hwnd, message, wParam, lParam) ;
}