> "Matt" <matth...@simstor.co.uk> wrote in message
> news:3eb147c8@newsgroups.borland.com...

> > I am certain that the IDevice has not freed itself (as the form
> > owning it is still displayed on the screen and I have a
> > breakpoint on the destructor) so why do I need to call AddRef?

> As previous mentioned, COM interfaces are reference counted.  The count is
> initially 0, and is incremented everytime AddRef() is called.  You need to
> call AddRef() to keep the reference count above 0 so the interface stays
> alive while you're still using it.  As soon as the count drops to 0, the
> interface must free itself automatically.  Whenever you are done using an
> interface, you must Release() it so the reference count decrements
properly.
> It sounds like your C_COM_Object class is never calling FDevice->Release()
> when it is done with FDevice and doesn't need it anymore.

> It should also be mentioned that your AddDevice() method has another
design
> flaw in it - if AddDevice() is called multiple times, you are not
> Release()'ing the existing FDevice before reassigning it.  You need to do
> that as well.

> So, for example:

>     C_COM_Object::C_COM_Object()
>     {
>         FDevice = NULL;
>     }

>     C_COM_Object::~C_COM_Object()
>     {
>         if( FDevice )
>             FDevice->Release();
>     }

>     C_COM_Object :: AddDevice(IDevice *ADevice)
>     {
>         if( FDevice )
>             FDevice->Release();

>         FDevice = ADevice;

>         if( FDevice )
>         {
>             FDevice->AddRef();
>             FDevice -> DisplayMessage();
>         }
>     }

>     C_COM_Object::AnotherMethod()
>     {
>         if( FDevice )
>             FDevice->DisplayMessage();
>     }

> The easiest way to work around all of these reference counting issues is
to
> not use a raw pointer for your FDevice member to begine with.  Declare
your
> FDevice member using either the DelphiInterface or TComInterface wrapper
> class.  That way, the reference counting is handled automatically for you:

>     class C_COM_Object : ...
>     {
>     private:
>         Delphinterface<IDevice> FDevice;
>     };

>     C_COM_Object :: AddDevice(IDevice *ADevice)
>     {
>         FDevice = ADevice;
>         if( FDevice )
>             FDevice -> DisplayMessage();
>     }

>     C_COM_Object::AnotherMethod()
>     {
>         if( FDevice )
>             FDevice->DisplayMessage();
>     }

> > Why would this work in Delphi without the AddRef call needed but not in
C?

> Becaise the Delphi Pascal language has native support for interfaces and
it
> handles the reference counting for you behind the scenes.

> > When I pass an interface to one COM object to another
> > is just a pointer passed?

> More or less, yeah.

> Gambit