Re: [Vala] not null 'this' in constructors

[Виталий Кирсанов <krokoziabla gmail com>]

Ok, there was a bug in my code but the reaction of vala is curious. My code
did something like this:

public class Foo
{
    int i;

    public Foo.from(int i)
    {
        this.i = i;
    }
}

public class Bar
{
    Foo         f;

    public Bar()
    {
        assert( this != null );
        f.from(1);                         // this line causes the trouble
    }
}

int main(string[] argv)
{
    var b = new Bar();
    return 0;
}


This program fails in the following way:

**
ERROR:/net/tieto/home/vkirsan/sandbit/test.vala.c:305:bar_construct:
assertion failed: (this != null)
Abort


And vala generates the following interesting piece of code:

Bar* bar_construct (GType object_type) {
Bar* self = NULL;
Foo* _tmp0_ = NULL;
_vala_assert (self != NULL, "this != null");
_tmp0_ = self->priv->f;
self = (Bar*) foo_construct_from (object_type, 1);
return self;
}


So it looks like vala, compiling a constructor for a class, treats direct
calling constructor(s) of the object's field(s) as an instruction to
construct the object itself.

*Is it ok?* Or does it rather look like a bug?

Sorry, if I may not explain clearly enough, I'm no native speaker.


BR, Vitaly Kirsanov
skype: vkirsan

2014-11-11 13:52 GMT+03:00 Виталий Кирсанов <krokoziabla gmail com>:

> Hm, the following code works fine indeed:
>
> public class Foo
> {
>     public Foo()
>     {
>         assert( this != null );
>     }
> }
>
> int main(string[] argv)
> {
>     var f = new Foo();
>     return 0;
> }
>
>
> But in my other real program a similar assertion is false in a
> constructor. I need dig to the bottom of it, will be back with results.
>
> BR, Vitaly Kirsanov
> skype: vkirsan
>
> 2014-11-11 3:37 GMT+03:00 Nor Jaidi Tuah <norjaidi tuah ubd edu bn>:
>
> > > Is 'this' always null in constructors? The following code produces a
> > > run-time error:
> > >
> > > public class Foo
> > > {
> > >     Foo()
> > >     {
> > >         assert( this != null );
> > >     }
> > > }
> >
> > Are you sure about this? Perhaps you should
> > provide a small program that triggers this error.
> >
> > > Is there a way to get the true value of this in the constructor? I
> > guess,
> > > probably not. As far as I understand references in vala are somewhat
> > > similar to shared_ptr<> pointers in C++ and vala classes to
> > > enable_shared_from_this<> C++ subclasses. If the reasons are also
> > similar,
> > > it would be sad.
> >
> > Vala references are like C pointers -- raw,
> > unwrapped, direct and optimizer friendly (though
> > I notice that the vala compiler puts an
> > additional indirection for private variables,
> > incurring performance penalty in object construction
> > and subsequent accesses).
> >
> > C++ shared_ptr is an ugly wart. Its only similarity
> > with Vala references is ref counting.
> >
> > Do not let your experience with shared_ptr
> > discourage you from Vala.
> >
> > Nice day
> > Nor Jaidi Tuah
> >
> >
> >
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