subclassing

[rippin]

I am wondering if it is possible to subclass a window from within a class. Consider the following class:

class Base
{
public:
  Base() {}
  virtual ~Base() {}

  BOOL Subclass(HWND hWnd)
  {
    long lpfnNewProc = (long)this->MsgHandler; //<-- This is my error
    long lpfnOldProc = SetWindowLong(hWnd, GWL_WNDPROC, lpfnNewProc);

    return TRUE;
  }

  LRESULT CALLBACK MsgHandler(HWND hWnd,
                              UINT msg,
                              WPARAM wParam,
                              LPARAM lParam)
  {
    // Do something with the message
    return 0;
  }
};

Now, I realize this is not a complete class. I cannot get a function pointer to the MsgHandler function of the class. I realize this is because Windows wants (requires?) that the callback function be a global function and not an instance function, but is there some way around this? If not then I have to have a global MsgHandler and store all the hWnds and function pointers in global arrays and I really don't want to do this. If there is a way to pull off what I am trying to do, can somebody please let me know how.

Thanks

[parksie]

This is what seems to be the best option...

Make the MsgHandler function static, and also have an InternalMsgHandler function of the same type, just not static:

class Base {
    static LRESULT CALLBACK MsgHandler(...) { ... }
    LRESULT InternalMsgHandler(...) { ... }
};

You put the specific code into the InternalMsgHandler function.

This is where it gets clever When you subclass it, use SetWindowLong like so:

SetWindowLong(hWnd, GWL_USERDATA, (long)this);

This associates your class with the window, since the GWL_USERDATA property is a 4-byte long integer (how useful... ). Then, in MsgHandler, just use:

Base *pHandler = GetWindowLong(hWnd, GWL_USERDATA);
if(pHandler) {
    pHandler->InternalMsgHandler(hWnd, uMsg, wParam, lParam);
}
// Call the previous function here

It's a bit nasty I know...can you see what I'm getting at?

[rippin]

Ahhh...pretty clever indeed. Thank you very much.

You could also use the GetProp and SetProp functions to associate the pointer with the window.

Again thanks.

[parksie]

Proudly stolen from Microsoft's own samples

You can use the properties, but I think it's simpler this way. Oh, and for your information, I think this is called delegating...not sure though Looks a bit like the description.