Re: [Vala] Create an instance of a derived class, from a base class method

[Tal Hadad <tal_hd hotmail com>]

I don't understand what is your purpose:
I don't understand why you don't use constructors, or static methods.

See, the type deceleration must be explicit at complication time.
However, if you don't want it, you can always use generics and the "dynamic" word.

In simple words, as far as I know, what you want to achieve isn't possible(but could be manually implicated).

Yours,
Tal

> Date: Sat, 18 Feb 2012 15:51:55 +0000
> From: blowback gmail com
> To: vala-list gnome org
> Subject: [Vala] Create an instance of a derived class,        from a base class method
>
> Hi,
>
> I am trying to craft a base class method, that is able to create an
> object of the same class as whatever derived-class object it was
> called on.
>
> For example, if classes Bar and Baz both derive from Foo, and Foo has
> a create() method, then bar.create() should give me an instance of
> Bar, and Baz.create() should give me an instance of Baz.
>
> I've spent so long happily experimenting with this that I forgot the
> original requirement. but I have come up with two approaches that
> almost work (below).
>
> Everything is fine at run-time, but I have to resort to ugly casts
> like "Derived obj = derived.foo() as Derived" to get valac to accept
> it.
>
> I think the return type ('Base') of foo() is my problem, but I'm at a
> loss as to what to replace it with to make this work.
>
> Is there a better way to achieve this sort of thing?
>
>
>
>
> public class Base : GLib.Object {
>     public Base foo() {
>         Type t = this.get_type();
>         return Object.new(t) as Base;
>     }
> }
>
> public class Derived : Base {
>     public void test() {
>         stdout.printf("I am an instance of Derived!\n");
>     }
> }
>
> /********************************************************************************/
>
> /* second attempt with abstract methods */
> public abstract class Base2 : GLib.Object {
>     public Base2 foo() {
>         return new_instance();
>     }
>     protected abstract Base2 new_instance();
>
> }
>
> public class Derived2 : Base2 {
>
>     /* return type has to be Base2 or vala won't allow the override */
>     public override Base2 new_instance() {
>         return new Derived2();
>     }
>
>     public void test() {
>         stdout.printf("Derived2: I am an instance of Derived2!\n");
>     }
> }
>
>
> /********************************************************************************/
>
>
> public static int main(string[] args) {
>
> #if false
>     Derived derived = new Derived();
>     Derived obj = derived.foo(); // 'cannot convert from Base to Derived?'
> #else
>     Derived derived = new Derived();
>     Derived obj = derived.foo() as Derived; // ugly!
> #endif
>
>     Type t = obj.get_type();
>     stdout.printf("Type of obj is %s\n", t.name());
>     obj.test();
>
>
>     /* second attempt with abstract methods */
> #if false
>     Derived2 derived2 = new Derived2();
>     Derived2 obj2 = derived2.foo(); // 'cannot convert from Base to Derived?'
> #else
>     Derived2 derived2 = new Derived2();
>     Derived2 obj2 = derived2.foo() as Derived2; // ugly!
> #endif
>
>     Type t2 = obj2.get_type();
>     stdout.printf("Type of obj2 is %s\n", t2.name());
>     obj2.test();
>
>
>     return 0;
> }
>
>
> cheers
> ant
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