Re: virtual inheritance and Glib::Interface

[Murray Cumming <murrayc murrayc com>]

On Wed, 2006-04-05 at 20:38 -0500, Jonathon Jongsma wrote: 
> On 4/5/06, Chris Vine <chris cvine freeserve co uk> wrote:
> > On Wednesday 05 April 2006 22:57, Jonathon Jongsma wrote:
> > > Yes, I was not trying to suggest that ListStore inherit from TreeModel
> > > virtually, I was suggesting that since ListStore inherits from 4
> > > classes with a common ancestor (of which TreeModel is one), those 4
> > > classes should use virtual inheritance to inherit from Interface,
> > > which they don't appear to do.
> >
> > But then if they have anything other than a default constructor you can make
> > them practically unusable, because of the need to invoke the constructor from
> > the most derived class.  If it doesn't matter having multiple Interface
> > objects (and I do not know if it does or it doesn't) you may be better off
> > leaving things as they are.
> >
> 
> I'm not quite sure I understand your explanation.

With 
- DerivedDerived : public Derived
- Derived : virtual public Base
- Base

You must do
DerivedDerived::DerivedDerived(int something)
: Derived(something),
  Base(something)

Or else there must be a default Base() constructors that can be chosen
automatically.

If I do 

Derived::Derived(int something)
: Base(5)

then that call to Base(5) will simply be ignored (or similar) when I
instantiate a DerivedDerived.

So, if Interface was inherited virtually then, to avoid making things
very difficult, we'd have to add an Interface() constructor. But then
the gtype-interface-adding code in the other constructor would not run. 

I think people would then need to explicitly call
TreeModel::add_interface() when creating their own TreeModel, which I
don't think they need to do now.

It might be more complicated than that.

This is why we avoid virtual inheritance whenever possible. However, maybe 
there's a way to make it work and save the space.

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