Re: [Vala] Virtual methods in interfaces

[Abderrahim Kitouni <a kitouni gmail com>]

Hello,

                   في خ، 27-01-2011 عند 16:13 +1030 ، كتب James Moschou:
> On 25 January 2011 23:36, Abderrahim Kitouni <a kitouni gmail com> wrote:
> > 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).
>
> 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.

I'm not sure it's completely unintentional, Vala can be such an awkward
language at some times (but we still love it ;-)). See below.


> 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:
> [...]
> But I suspect this is impossible with the way GObject is designed,
> which would be a shame.
Nothing is impossible in Vala, things just happen to be weird ;-p

You won't believe the answer is so easy, I've tried many ways before
finding the correct answer (I've even written a long paragraph trying to
argue how it could be possible in GObject).

You just need to declare that your subclass implements the interface
(and of course remove the override keyword). So the following code:

interface Interface {
 public virtual void function () {
        print ("interface\n");
 }
}

class Class : Object, Interface {}

class Subclass : Class, Interface {
 public void function () {
   print ("subclass\n");
 }
}

var bc = new Class();
bc.function();

var sc = new Subclass();
sc.function();

prints:

interface
subclass

HTH,
Abderrahim