Continuing my discussion of How can I make a callback function a member of my C++ class?

Common special cases for wanting to use a member function as a callback function are the window procedure and its cousin the dialog procedure. The question, then, is where to put the reference data.

Let’s start with window procedures. The Create­Window function and its close friend Create­Window­Ex let you pass your reference data as the final parameter, prototyped as LPVOID lpParam. As noted in the documentation, that value is passed back to the window procedure by the WM_NC­CREATE and WM_CREATE messages as part of the CREATE­STRUCT structure. One of the first messages passed to a window is WM_NC­CREATE, so that’s where we’ll grab the reference data and save it for later.

You can follow along in this simple C++ program: The static window procedure handles the WM_NC­CREATE message by extracting the lpCreate­Params from the CREATE­STRUCT and saving it in the GWLP_USER­DATA window bytes. That value is a special per-window storage location provided for the benefit of the window procedure, and most people use it to store their context parameter for safekeeping.

If the message is not WM_NC­CREATE, then we retrieve the context parameter from where we had stashed it.

Either way, we end up with a copy of the context parameter. If you want your window procedure to be a member function, the natural choice for the context parameter is the this pointer for the instance. The static window procedure therefore tends to look like this:

LRESULT CALLBACK MyWindowClass::s_WndProc(
    HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
 MyWindowClass *pThis; // our "this" pointer will go here
 if (uMsg == WM_NCCREATE) {
  // Recover the "this" pointer which was passed as a parameter
  // to CreateWindow(Ex).
  LPCREATESTRUCT lpcs = reinterpret_cast<LPCREATESTRUCT>(lParam);
  pThis = static_cast<MyWindowClass*>(lpcs->lpCreateParams);
  // Put the value in a safe place for future use
  SetWindowLongPtr(hwnd, GWLP_USERDATA,
                   reinterpret_cast<LONG_PTR>(pThis));
 } else {
  // Recover the "this" pointer from where our WM_NCCREATE handler
  // stashed it.
  pThis = reinterpret_cast<MyWindowClass*>(
              GetWindowLongPtr(hwnd, GWLP_USERDATA));
 }
 if (pThis) {
  // Now that we have recovered our "this" pointer, let the
  // member function finish the job.
  return pThis->WndProc(hwnd, uMsg, wParam, lParam);
 }
 // We don't know what our "this" pointer is, so just do the default
 // thing. Hopefully, we didn't need to customize the behavior yet.
 return DefWindowProc(hwnd, uMsg, wParam, lParam);
}

You pass the the this pointer to Create­Window as the last parameter, so that the window procedure can pick it up.

hwnd = CreateWindow(... other parameters..., this);

For dialog boxes, you can do basically the same thing. It’s just that the bookkeeping is slightly different.

• The ...Param versions of the dialog box functions are the ones which let you pass reference data.

• The dialog procedure receives the reference data in the lParam passed with the WM_INIT­DIALOG.

• The system-provided secret hiding place for dialog boxes is called DWLP_USER.

INT_PTR CALLBACK MyDialogClass::s_DlgProc(
    HWND hdlg, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
 MyDialogClass *pThis; // our "this" pointer will go here
 if (uMsg == WM_INITDIALOG) {
  // Recover the "this" pointer which was passed as the last parameter
  // to the ...Dialog...Param function.
  pThis = reinterpret_cast<MyDialogClass*>(lParam);
  // Put the value in a safe place for future use
  SetWindowLongPtr(hdlg, DWLP_USER,
                   reinterpret_cast<LONG_PTR>(pThis));
 } else {
  // Recover the "this" pointer from where our WM_INITDIALOG handler
  // stashed it.
  pThis = reinterpret_cast<MyDialogClass*>(
              GetWindowLongPtr(hdlg, DWLP_USER));
 }
 if (pThis) {
  // Now that we have recovered our "this" pointer, let the
  // member function finish the job.
  return pThis->DlgProc(hwnd, uMsg, wParam, lParam);
 }
 // We don't know what our "this" pointer is, so just do the default
 // thing. Hopefully, we didn't need to customize the behavior yet.
 return FALSE; // returning FALSE means "do the default thing"
}

The above code should look really familiar, since it’s the same as the window procedure case, just with slightly different bookkeeping.

The resulting classes look like this:

class MyWindowClass
{
 ... other class stuff goes here ...
 // This is the static callback that we register
 static LRESULT CALLBACK s_WndProc(
    HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
 // The static callback recovers the "this" pointer and then
 // calls this member function.
 LRESULT WndProc(
    HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
};
void MyWindowClass::SomeMemberFunction()
{
 // to register the class
 WNDCLASS wc;
 ... fill out the window class as normal ...
 wc.lpfnWndProc = MyWindowClass::s_WndProc;
 wc.lpszClassName = TEXT("MyWindowClass");
 RegisterClass(&wc);
 // to create a window
 hwnd = CreateWindow(TEXT("MyWindowClass"),
                     ... other parameters as usual ...,
                     this);
}
class MyDialogClass
{
 ... other class stuff goes here ...
 // This is the static callback that we register
 static INT_PTR CALLBACK s_DlgProc(
    HWND hdlg, UINT uMsg, WPARAM wParam, LPARAM lParam);
 // The static callback recovers the "this" pointer and then
 // calls this member function.
 INT_PTR DlgProc(
    HWND hdlg, UINT uMsg, WPARAM wParam, LPARAM lParam);
};
void MyDialogClass::SomeMemberFunction()
{
 // to create the dialog box
 DialogBoxParam(...  other parameters as usual ...,
                reinterpret_cast<LPARAM>(this));
}

Okay, I’ll try to write something more interesting for next week. But at least I wrote this part down so I can point people at it in the future.

Bonus chatter: As commenter Ben noted last week, DDEML is another component that uses the implicit reference data model. In the DDEML case, you use Dde­Set­User­Handle to set the reference data and Dde­Query­Conv­Info to retrieve it.

(Various errors have been corrected based on comments, thanks everybody!)