Re: [Vala] Generics: constraints and 'new T ()' on type parameters?

[Yu Feng <rainwoodman gmail com>]

Hi Sam,

On Thu, 2009-02-26 at 19:52 -0500, Sam Danielson wrote:
> I'm writing some ORM where a database view is wrapped in a factory
> object that produces records. Ideally I would think a Factory <T>
> should be able to return new T's but I can't figure out how to do
> this. The work around is to return the Record base class and then
> upcast, or put virtual upcasting functions in all of the
> derived Factory classes. Yuck.
>
> Is there a fundamental reason why this cant work?
>
> class Foo {}
> class Factory <T> {
>       public static T make () {
>               return new T ();
>       }
> }

Suppose Foo is the root of all the products from your factory, I suggest
using this instead of those ugly generics.

class Foo:Object {
   
}

class Foo1:Foo {

}
class Factory {
    public static Foo make(Type type) {
          return Object.new (type) as Foo;
    }
}
The types can be obtained with typeof(Foo1). The constraints can be done
at run-time with Type.is_a() or =.

BTW: shouldn't a factory has one method for making each type of objects?

Yu

> And is there a plan to implement type parameter constraints? Something like...
>
> class Factory <T:Object> { ... }
>
> The compiler may then prove that it is okay to 'new T ()'
>
> PS. I can slip the following by the type system without a complaint. Should I
> file a bug?
>
> class Foo {}
> class Factory <T> {
>       public static T make () {
>               return new Foo ();
>       }
> }
>
> LLAP,
> Sam Danielson
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