Re: virtual inheritance and Glib::Interface

[Murray Cumming <murrayc murrayc com>]

On Wed, 2006-04-05 at 13:37 -0500, Jonathon Jongsma wrote:
> Just so I understand, what do you think is not true?  The fact that
> multiple Interface objects might be created?

Yes. Not true, I think. Here's an attempted explanation:

Gtk::TreeStore inherits non-virtually from TreeModel, TreeSortable,
TreeDragSource, and TreeDragDest

Because each of these inherits virtually from Interface, TreeStore has
only one copy of Interface. That's kind out counterintuitive, but I
think it's how VI works. It took me (with Daniel Elstner's help) a long
to figure it out. There is great potential for this to be a huge thread
full of misunderstanding and re-explanation. What's often most
informative is to make the change and see how the compiler complains.
And feel free to test the theory with std::out in constructors.


It would only be useful to, for instance, inherit TreeModel virtually
from TreeStore and ListStore, if I was going to create classes like
this:

DerivedA : public TreeStore
DerivedB : public ListStore 

DerivedDerived : public TreeStore, public ListStore

Then DerivedDerived would share one TreeModel. At the moment I think
there would be two TreeModels.

So, in general, virtual inheritance is used to make a base class
virtual, meaning shared.

So VI requires you to have some hindsight. In theory, you could make all
kinds of inheritance possible by always inheriting virtually, and I
think there is a request to add virtual inheritance somewhere else in
libsigc++ or glibmm, but it also causes all kinds of other problems and
inefficiences.

-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com
www.openismus.com