Re: [Vala] Singleton and reference count

[Mike Massonnet <mmassonnet gmail com>]

2010/4/27 Sam Wilson <tecywiz121 hotmail com>:
> On Tue, 2010-04-27 at 10:46 -0300, Jonh Wendell wrote:
> > Hi, folks. I wrote a simple singleton:
> >
> > public class Prefs {
> >   private static Prefs instance;
> >
> >   public static Prefs default() {
> >     if (instance == null)
> >       instance = new Prefs ();
> >
> >     return instance;
> >   }

                public static unowned Preferences get_default () {
                        if (preferences == null) {
                                preferences = new Preferences ();
                                preferences.add_weak_pointer (&preferences);
                        }
                        else
                                preferences.ref ();

                        return preferences;
                }

To invoke such an object inside a variable I do it like this with the
unowned keyword:

        private unowned Xfmpc.Preferences preferences;
        ...
                this.preferences = Xfmpc.Preferences.get_default ();

That is taken from xfmpc[0], the difference is that, for a singleton,
it makes the pointer “weak” so that it is effectively reset to nil
once the last count is removed. Also note the return type that
includes the keyword unowned, that is needed for Vala. Just run the
compiler as valac -C to have a look at the differences, that teaches
always a lot more.

Hopefully this will get you a little closer to what you are trying to
achieve, I can't confirm the destructor is called, but I guess it
should.

[0] http://git.xfce.org/apps/xfmpc/tree/src/preferences.vala

Regards
Mike

> >   private Prefs () {
> >     stdout.printf ("constructor\n");
> >   }
> >
> >   ~Prefs () {
> >     stdout.printf ("destruction\n");
> >   }
> > }
> >
> > And in the other file, I do:
> >
> > var prefs = Prefs.default ();
> > .... use prefs var...
> >
> > The prefs var doesn't get destroyed, thus my destructor is never
> > called.
>
> You probably want to make default return an "unowned Prefs" instead of
> just "Prefs", that way the reference count is never increased.  I might
> be terribly wrong though, I am still not very fluid with vala's memory
> management.
>
> Sam
>
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-- 
Mike