Re: [Vala] Understanding object creation (and other things)

[Jürg Billeter <j bitron ch>]

On Son, 2007-04-22 at 11:55 +0100, Phil Housley wrote:
> > You can do this with construct-only properties without storing the value
> > anywhere.
> ...
> > The construct in the parameter of the creation method is just a short
> > version of setting the property x to the value of the argument x.
>
> Ok, I think I'm getting some of this.  To add a little complexity
> though, say I want to do something like a multiplier that stores the
> answer and not the terms.  This means I need two construct parameters
> to set one value:
>
> class Mul {
>       private int answer;
>
>       public int get_answer() {
>               return answer;
>       }
>
>       public void set_terms(int x, int y) {
>               answer = x * y;
>       }
>
>       public Mul(int x, int y) {
>               set_terms(x, y)
>       }
> }
>
> The closest I can get in vala seems to be:
>
> [...]
>       
> But that keeps the original x and y around for ever, even when the
> terms have been changed.  And is a lot of work also.
>
> Is there a way of cutting this back now, or is the syntax likely to
> shrink in the future maybe?

The only sensible possibility to make this simpler would be to allow
accessing the construction parameters in the constructor, maybe
something like that:

class Mul {
        private int answer;

        public int get_answer() {
                return answer;
        }

        public void set_terms(int x, int y) {
                answer = x * y;
        }

        public Mul(construct int x, construct int y) {
        }

        // implicitly declare construct-only parameters x and y
        // with a default value of -1
        construct (int x = -1, int y = -1) {
                set_terms(x, y)
        }
}

Would something like that make sense?

The way I'd implement such a class currently is by using a helper method
for construction - assuming that you don't want to create instances of
subtypes in the same manner.

class Mul {
        private int answer;

        public int get_answer() {
                return answer;
        }

        public void set_terms(int x, int y) {
                answer = x * y;
        }

        public static Mul! create_with_terms (int x, int y) {
                var mul = new Mul ();
                mul.set_terms (x, y);
                return mul;
        }
}

Jürg