Re: [Vala] Virtual methods in interfaces

[James Moschou <james moschou gmail com>]

On 25 January 2011 23:36, Abderrahim Kitouni <a kitouni gmail com> wrote:
> Hi,
>                   في ث، 25-01-2011 عند 21:35 +1030 ، كتب James Moschou:
> > Is is possible to mark methods as virtual in interfaces, as a
> > mechanism of providing default implementations?
> >
> > If I do
> >
> > interface Interface {
> >  public virtual void function () {
> >    // Default
> >  }
> > }
> >
> > class Class : Object, Interface {
> >  public override void function () {
> >    // Custom
> >  }
> > }
> >
> > The compiler says that no method to override was found.
>
> but if you remove the override keyword, it works ;-)
>
> So now the question is : is this intentional? I always thought that you
> don't need override when implementing an abstract method (and need
> override when there is already an implementation), but it seems I was
> wrong. This probably needs an answer from Jürg (and the rationale to be
> added somewhere in the documentation).
>
> HTH,
> Abderrahim

I'm aware that it works if you don't use the override keyword, and it
does seem unintentional, considering that the behaviour of
abstract/virtual/override/new is pretty well defined for normal class
inheritance, and vala emits warnings if you don't do it correctly.

This issue is actually a precursor to my real question, which I don't
think has come up before on this list exactly, although similar
threads have touched on it.

I really want to be able to define a class as implementing an
interface with default implementations of virtual methods, and have
subclasses of that class override the virtual methods. So:

interface Interface {
 public virtual void function () {
   // Default
 }
}

class Class : Object, Interface {

}

class Subclass : Class {
 public override function () {
   // Subclass implementation
 }
}

But I suspect this is impossible with the way GObject is designed,
which would be a shame.

Regards,
James