In VC++ I have been able to redirect Window Messages (via Wndproc) to an instance of a class's specific message handler by storing the instance address with SetWindowLong. Found this detailed on the net somewhere. Worked Perfectly. Does anyone know why the same idea doesnt work (as I have it) in Delphi? At the moment, if I try to change a variable within the class the program just dies - no errors. It executes code with the class's message handler, but doesnt seem to like variable access. Code Follows: This is the main WndProc associated with the WndClass. As you can see, it retrieves a pointer with GetWindowLong, and if its not NULL it jumps to the class's own message handler, otherwise just passes it to DefWindowProc. This part of the code seems to execute quite happily. In another part of the code is a SetWindowLong which stores a pointer to 'self'.

function WndProc(hWnd: HWND; uMsg: UINT; wParam: WPARAM; lParam: LPARAM):LRESULT; stdcall;
var
  wndobj: pGLEngine;
begin
  wndobj:= pGLEngine(ptr(GetWindowLong(hWnd, GWL_USERDATA)));
  if wndobj = nil then
    result:= DefWindowProc(hWnd, uMsg, wParam, lParam)
  else
		result:= wndobj.MsgHandler(hWnd, uMsg, wParam, lParam)
end;

This is the class's 'internal' message handler. The problem occurs within the function. All the code seems to execute, as long as I don't touch a class variable. The case WM_ACTIVATE causes an error when it trys to set Active:=.... Active is declared in the private section of the GLEngine class. If I {..} that line out, its fine. If I add something trivial like 'Active:= true', the program dies without error. I have tested other variables as well that are within the class - all cause the program to die.

function GLEngine.MsgHandler(hWnd: HWND; uMsg: UINT; wParam: WPARAM; lParam: LPARAM):LRESULT;
begin
  case uMsg of
    WM_ACTIVATE:
    begin
      if (hi(wParam)=0) then active:= true else active:= false;
      result:= 0;
      exit;
    end;
    WM_SYSCOMMAND:
    begin
      if (wParam = SC_SCREENSAVE) or (wParam = SC_MONITORPOWER) then
      begin
        result:= 0;
        exit;
      end;
    end;
...
...
end;

Is there a trick to delphi's classes when it comes to variables? [Edited by - Sentient2k5 on July 4, 2006 3:02:59 PM]

I don't know much about delphi. But if you want your code to be readable...

Enclose it in either {code}{/code} or {source}{/source} brackets. (replace the squiggly-braces with square-braces)

Seems that the problem isn't with the code itself.

I changed the code to store the pointer in a global variable, and it works perfectly. Infact, with the old code, it works fine for a short while then seems to corrupt the data stored with SetWindowLong.

Does anyone know if Delphi stores data to GWL_USERDATA with SetWindowLong? This is the only reason I can see why it would change as I only have one call to SetWindowLong, and its in the WindowCreate function which is called only once in my program - I can only think Delphi is changing this data behind the scenes.

Grr, I now think this is a pointer problem.

Basically, I have GLEngine = class, and pGLEngine = ^GLEngine.

If I create a global variable MyGLE:GLEngine, and set MyGLE = self within the GLEngine.CreateWindow procedure, the code runs without problems.

If I create a global variable MyGLEP:pGLEngine, and set MyGLEP = @self, then it dies.

Help Please :)

Quote:Plus, "self" is a local variable, so @self will be illegal outside of the scope of the particular function.

Thanks for this Mikeman. Clears up the problem completely. I'm just confused as to why it runs through ~100 messages before it decides to die. Not that its a problem anymore. Cheers.