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 +
From: blowb...@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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