MemCheck tutorial: find bad interface calls

1. Create a new class which implements IUnknown (warning: this is for the lesson only - this code is not good for real use)
       TMyUnknown = class(TObject, IUnknown)
           protected
               fCount: Cardinal;
   
           public
               function QueryInterface(const IID: TGUID; out Obj): HResult; stdcall;
               function _AddRef: Integer; stdcall;
               function _Release: Integer; stdcall;
           end;
   
       function TMyUnknown._AddRef: Integer;
           begin
               fCount:= fCount + 1;
           end;
   
       function TMyUnknown._Release: Integer;
           begin
               fCount:= fCount - 1;
               if fCount = 0 then
                   Destroy;
           end;
   
       function TMyUnknown.QueryInterface(const IID: TGUID; out Obj): HResult;
           begin
           end;
2. Write the BadInterf procedure
       procedure BadInterf;
           var
               i: IUnknown;
               o: TMyUnknown;
           begin
               o:= TMyUnknown.Create;
               i:= o;
               o.destroy;
           end;
3. From the dpr's body call BadInterf
       begin
           MemChk;
           BadInterf;
       end.
4. MemCheck tells you that you are calling a method of an interface while the underlying object has been destroyed (the compiler has generated a call to ReleaseRef on exit of BadInterf). Of course, this feature of MemCheck is less often useful than others; but when it is, you'll thank MemCheck !

Last update: March 2001