Re: [chuck-users] Casting behaviour

17 Oct 2014

      Gonzalo wrote:
...
On 18/10/2014 1:40 am, Michael Heuer wrote:
...
Gonzalo wrote:
...
I have this (unexpected?) behaviour when casting. I can cast a object to
its
parent class, but the methods that get executed are still their own (and
viceversa). I.e.:
----------------------------------
public class Parent {
   fun void test() {
     <<< "Parent" >>>;
   }
}
class Child extends Parent {
   fun void test() {
     <<< "Child" >>>;
   }
}
Parent p;
Child @ ch;
p $ Child @=> ch;
ch.test(); // prints Parent
This shouldn't work, because p !instanceof Child, the cast should be a
compiler error.
Thanks Michael. You're right, and in fact it doesn't work at all. Now that
I've tried, ch can not call methods exclusive to Child either (Assertion
failed: (m_offset < obj->vtable->funcs.size()), function execute, file
chuck_instr.cpp, line 4765).
...
...
Is this intended behavior? What's the point of casting then?
I typically only use casting when pulling object references out of
data structures that hold arrays of Object references, e.g. an
ArrayList
https://github.com/heuermh/lick/blob/master/Freeze.ck#L73
Of course, I see. I was trying to use casting to bypass the lack of
constructors, bad misuse.
...
...
I might describe too what I'm trying to do, maybe there's a better
approach.
I have a .create method in Parent that returns a Parent object, and I
wanted
to create that same method in Child, but I can't do it because the
arguments
are the same, and it won't let me overwrite the method when only the
return
type changes. So to create a child with the .create method, I do this:
Parent.create(...) $ Child @=> Child ch;
Which works, but then it still behaves as a parent, so it defeats the
whole
purpose.
Having proper constructors in ChucK would help here
https://github.com/spencersalazar/chuck/issues/32
A workaround might be to include the classname in your create methods
class Parent {
   fun static Parent createParent(...) { }
}
class Child extends Parent {
   fun static Child createChild(...) { }
}
Yes, that might be the way to go. I use init() methods as a workaround for
constructors, so my normal instantiations look like:
MyClass c;
c.init(...);
Not very nice, which is why I was trying to have the static 'create'
methods.
Yep, that's very similar to what I do, init() method + static create
methods, for example

https://github.com/heuermh/lick/blob/master/Freeze.ck#L159
...
I'm thinking of having a global Make class, and put all my static
initializers there. Which would look something like:
class Make {
  fun static Parent Parent(args) {
    Parent p;
    p.init(args);
    return p;
  }
  ....
}
Make.Parent(...) @=> Parent p;
Ugly class, but nicer to call than Parent.createParent(args) ?
That's not a bad idea.

The reason I grumble about not having constructors is that for most
stuff I can just do

adc => Foo foo => Bar bar => dac;

but the classes that require special initialization I have to remember to do

Baz.create() @=> Baz baz;
Qux.create() @=> Qux qux;

adc => baz => qux => dac;

because

adc => Foo foo => Bar bar => dac;

foo.init();
bar.init();

might not necessarily work.

   michael