Visual C++ and MFC Programming 2nd Edition

Visual C++ and MFC Fundamentals Chapter 15: Fundamental Controls

14.4.1..Retrieving Control Information.........................................................

To get information about a control, you can call the GetWindowLong() function. Its

syntax is:

LONG GetWindowLong(HWND hWnd, int nIndex );

After this function executes, it returns a LONG value. If you prefer to get a pointer and if

you are working on a 64-bit environ, use the GetWindowLongPtr() function instead. Its

syntax is:

LONG_PTR GetWindowLongPtr(HWND hWnd, int nIndex);

The first argument, hWnd, is a handle to the control whose information you are seeking.

The nIndex argument specifies the type of information you are looking for. Its possible

values are:

nIndex Value

GetWindowLong() GetWindowLongPtr()

Description

GWL_EXSTYLE This is used to get information about the extended

style(s) used on the control

GWL_STYLE This is used to get information about the style(s)

used on the control

GWL_WNDPROC GWLP_WNDPROC Remember that a window procedure is a function

pointer. If such a procedure was used to handle the

message(s) for the hWnd control, use this constant

to get the address of that procedure

GWL_HINSTANCE GWLP_HINSTANCE This gives access to the handle to the current

application

GWL_HWNDPARENT GWLP_HWNDPARENT This constant can be used to get a handle to the

parent window. For example, you can use it the

get a handle to a dialog box that is hosting the

hWnd control.

GWL_ID GWLP_ID This gives you the ID of the hWnd control

GWL_USERDATA GWLP_USERDATA This gives you a 32-bit value used by the current

application

If the hWnd argument represents a dialog box, nIndex can use the following values:

nIndex Value

GetWindowLong GetWindowLongPtr

Description

DWL_DLGPROC DWLP_DLGPROC This provides the address of, or a pointer, to the

procedure of the dialog box

DWL_MSGRESULT DWLP_MSGRESULT This provides the return value of the dialog box’

procedure

DWL_USER DWLP_USER This provides additional information about the

application

After calling this function and specifying the type of information you need, it returns a

(constant) value you can use as you see fit. Here are two methods of getting a handle to

the instance of the application. The second example uses the GetWindowLong()

function:

BOOL CDialog1Dlg::OnInitDialog()

{

CDialog::OnInitDialog();