Re: Casting from a wrapped class to a possible implemented interface

[José Alburquerque <jaalburquerque cox net>]

Murray Cumming wrote:
> On Sun, 2008-03-02 at 01:57 -0500, José Alburquerque wrote:
>   
> > Is it possible to cast a class that in its declaration 
> > does not by default implement a certain interface and then cast it's 
> > RefPtr to the interface (assuming that the C object implements the 
> > interface)?
> >     
>
> Generally, no.
>
>   
> > In gstreamer, the GstImplementsInterface 
> > <http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstImplementsInterface.html> 
> > has functionality to test if an Element implements an interface 
> > (gst_element_implements_interface 
> > <http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstImplementsInterface.html#gst-element-implements-interface>) 
> > and to cast to that interface if necessary 
> > (gst_implements_interface_cast() 
> > <http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstImplementsInterface.html#gst-implements-interface-cast>) 
> > and I thought maybe this functionality can be used to cast from a 
> > wrapped Gst::Element to a RefPtr of the interface.  Would this make
> > sense?
> >     
>
> This would cause us to have two C++ instances with the same underlying C
> instance. I wonder if that would have any bad side-effects. You would
> probably have to avoid using Glib::wrap(), because it assumes a 1-to-1
> mapping.
>
>   
I think I may have found a way to do this, as you said, not using 
Glib::wrap().  It may be a bit intricate (and not ideal), but I think it 
may work.  What I did is that I implemented a templated Gst::Element 
derived class (called ElementInterfaced) which implements the template 
argument (which would be the interface to be cast to).  The 
Gst::Interface::cast() method (templated with the interface to be cast) 
creates a new templated Gst::ElementInterfaced class with the target 
interface  and returns a RefPtr to the Gst::ElementInterfaced templated 
class.  Working with the returned cast is successful so this is positive 
(see the test-create-element.cc for example).

My concern, however, is that because the newly created 
Gst::ElementInterfaced class is not "wrapped", it doesn't look like it 
will be destroyed when all references to the underlying gobject are 
gone.  As a thought, I'm thinking that maybe the destruction of the 
underlying gobject might be "sensed" from the Gst::ElementInterfaced 
instance and then destroyed, but I'd have to look a bit into this.  In 
the meantime, I've checked in what I've done so far.  If it turns out 
not to be useful, it can be modified or reverted.

--
José Alburquerque
jaalburquerque cox net