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Window Procedure inside a class - Problems with static - Non-static and access to other class functions

Posted on 2003-10-27
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Last Modified: 2012-05-04
Hi people. Here is my problem.
My problem basicaly is: Im creating a class for a window, and i have a window procedure inside this class. But inside the WNDCLASSEX    lpfnWndProc i must declare the MyWindow::WinProc function. And the compiler just dont accept this, only if i declare WinProc as STATIC. Declaring static, all looks ok, but now i cant access the other class functions from inside WinProc procedure. Oh my, there´s is no solution for this?

Thanks a lot for your help!!

Here is the code:

class MyWindow
  {
     HWND window, button1, button2, BckImage;
     void RegClass(HINSTANCE);
     void Create(HWND,HINSTANCE);
     void Show();
     void Hide();
     static LRESULT CALLBACK WinProc(HWND,UINT,WPARAM,LPARAM);
  };

void MyWindow::RegClass(HINSTANCE h)
  {
     WNDCLASSEX w;
      w.lpfnWndProc = WinProc;  
     // here is all the variables of WNDCLASSEX
  }

void Create(HWND parent, HINSTANCE h)
  {
       // here comes the creation of windows, etc...
  }

void MyWindow::Show()
  {ShowWindow(window,1);}
voidMyWindow::Hide()
  {ShowWindow(window,0);}

LRESULT CALLBACK MyWindow::WinProc(HWND hwnd, UINT msg, WPARAM w, LPARAM l)
  {
       // here is the window procedure code,etc...
  }
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Question by:richardomus
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4 Comments
 
LVL 86

Accepted Solution

by:
jkr earned 500 total points
ID: 9628611
The usual technique is to store the 'this' pointer of the corresponding 'MyWindow' object in the 'GWL_USERDATA' storage area that is associated with every window, e.g.

SetWindowLong ( window, GWL_USERDATA, ( LONG) this);

and in your 'WinProc()', use

MyWindow* pWnd = (MyWindow*) GetWindowLong ( hwnd, GWL_USERDATA);

pWnd->CallAnyFunctionofMyWindow ();
0
 
LVL 6

Expert Comment

by:GaryFx
ID: 9628647
The solution is to create the windows procedure outside the class, making sure that the relevant object is somehow embedded inside one of the parameters.

The problem is that Windows callbacks are essentially C functions, not C++.  As such, they don't know how to deal with the implicit object required by non-static methods.  Or, to put it another way, they have no way of setting the value for "this."

So, short of not using a class at all, the only solution is to pass the object via the parameters,and then inside your new windows procedure call something to get the real thing.

For example,

LRESULT CALLBACK MyWinProc(HWND hwnd, UINT msg, WPARAM w, LPARAM l)
 {
      // here is the window procedure code,etc...
      MyWindow* pMywindow = (MyWindow*)l;
      pMywindow->realWinProc(hwnd, msg, w);
 }

This assumes that the LPARAM is available to you for this purpose.  If not (and I don't know off the top of my head), you'll have to pass it some other way.

Gary
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LVL 86

Expert Comment

by:jkr
ID: 9628746
>>making sure that the relevant object is somehow embedded inside one of the parameters

That's the main problem - you cannot make that sure, as a couple of window messages use *both* parameters, a very common one being e.g. WM_COMMAND.
0
 
LVL 1

Author Comment

by:richardomus
ID: 9629283
Thanks!! This solution is incredible, and smart. Thank you both for helping me. It works really well!!! Bye!!!
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