What does the GetWindowLong(hwnd, 0) mean? - c++

Question

LRESULT WINAPI TextViewWndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
TextView* ptv = (TextView*)GetWindowLong(hwnd, 0);
switch (msg)
{
case WM_NCCREATE:
if ((ptv = new TextView(hwnd)) == nullptr)
return FALSE;
SetWindowLong(hwnd, 0, (LONG)ptv);
return TRUE;
}
Code from: http://www.catch22.net/tuts/neatpad/neatpad-overview
The TextView here is Owner-Drawn Controls.
My question here is:
GetWindowLong(hwnd, 0)
SetWindowLong(hwnd, 0, (LONG)ptv);
what does that mean? I have searched MSDN, it says:
The following values are also available when the hWnd parameter identifies a dialog box.
...
DWL_MSGRESULT
0
Retrieves the return value of a message processed in the dialog box procedure.
If the function succeeds, the return value is the requested value.
If the function fails, the return value is zero. To get extended error information, call GetLastError.
If SetWindowLong has not been called previously, GetWindowLong returns zero for values in the extra window or class memory.

Answer 1

Per the GetWindowLong() documentation:
nIndex
Type: int
The zero-based offset to the value to be retrieved. Valid values are in the range zero through the number of bytes of extra window memory, minus four; for example, if you specified 12 or more bytes of extra memory, a value of 8 would be an index to the third 32-bit integer.
...
The following values are also available when the hWnd parameter identifies a dialog box.
DWL_MSGRESULT
0
Retrieves the return value of a message processed in the dialog box procedure.
The code in question is for a window that wraps a TextView object. The window is NOT a dialog box, and there is an InitTextView() function beint called that registers a WNDCLASSEX whose cbWndExtra field is set to sizeof(TextView*) when calling RegisterClassEx():
BOOL InitTextView()
{
WNDCLASSEX wcx;
//Window class for the main application parent window
wcx.cbSize = sizeof(wcx);
wcx.style = 0;
wcx.lpfnWndProc = TextViewWndProc; // <--
wcx.cbClsExtra = 0;
wcx.cbWndExtra = sizeof(TextView *); // <--
wcx.hInstance = GetModuleHandle(0);
wcx.hIcon = 0;
wcx.hCursor = LoadCursor (NULL, IDC_IBEAM);
wcx.hbrBackground = (HBRUSH)0; //NO FLICKERING FOR US!!
wcx.lpszMenuName = 0;
wcx.lpszClassName = TEXTVIEW_CLASS; // <--
wcx.hIconSm = 0;
return RegisterClassEx(&wcx) ? TRUE : FALSE;
}
And then there is a CreateTextView() function that creates a new window, and thus a new TextView object, using that registered class:
HWND CreateTextView(HWND hwndParent)
{
return CreateWindowEx(WS_EX_CLIENTEDGE,
TEXTVIEW_CLASS, _T(""), // <--
WS_VSCROLL |WS_HSCROLL | WS_CHILD | WS_VISIBLE,
0, 0, 0, 0,
hwndParent,
0,
GetModuleHandle(0),
0);
}
In the window's WM_NCCREATE handler, a new TextView object is created, and a pointer to that object is stored directly in the HWND itself, at offset 0 of the window's extra memory. Other message handlers for the same window can then retrieve that TextView* pointer and use the object as needed. In this case, WM_NCDESTROY and WM_PAINT specifically:
LRESULT WINAPI TextViewWndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
TextView *ptv = (TextView *)GetWindowLong(hwnd, 0);
switch(msg)
{
// First message received by any window - make a new TextView object
// and store pointer to it in our extra-window-bytes
case WM_NCCREATE:
if((ptv = new TextView(hwnd)) == 0)
return FALSE;
SetWindowLong(hwnd, 0, (LONG)ptv);
return TRUE;
// Last message received by any window - delete the TextView object
case WM_NCDESTROY:
delete ptv;
return 0;
// Draw contents of TextView whenever window needs updating
case WM_PAINT:
return ptv->OnPaint();
default:
break;
}
return DefWindowProc(hwnd, msg, wParam, lParam);
}