Static member functions as window and dialog procedures

Hi there,

I just solved (after searching online) a tricky problem with the concept mentioned in the thread title.

I had this line in a window procedure (which I found out later was declared as static):

Code:

---

switch (wmId)
{
case IDM_ABOUT:
    DialogBoxW(hInstance, MAKEINTRESOURCE(IDD_ABOUTBOX), hwnd, DemoApp::About);
    break;
}

---

The DemoApp::About parameter kept on throwing errors. The compiler stated that it could not be used as a type DLGPROC. When I declared the function as static in the class declaration header file it was fine.

I'm wondering why this is the case? Thanks :)

If a member function is not static then it takes a hidden parameter, a pointer to the object that the function is called upon. So for a non-static member function, the call

Code:

---

  obj.func(1, 2);

---

is turned into

Code:

---

  func(&obj, 1, 2); // &obj is called 'this' in the member function

---

Obviously that would screw up a window procedure.
A static member function does not have that extra parameter and so is basically just a regular function.

Thanks very much again John :)