From: Michael Walter <cm@...>
Date: 2004-06-25T23:53:18+09:00
Subject: Re: Is it considered Harmful?

Heya,

Lennon Day-Reynolds wrote:
> That's exactly the problem -- by forcing one object into another
> class, you require that one or both classes have sufficient knowledge
> of the internal data representation of the other to be able to safely
> make the switch. That kind of low-level manipulation usually stays
> within a class for a reason.
That's why there are the _generic functions_ CHANGE-CLASS and 
UPDATE-INSTANCE-FOR-DIFFERENT-CLASS in Common Lisp.

> The only real reason I can see to dynamically change the class of an
> object would be in languages where there are static type constraints
> that have to be satisfied. As it is, in Ruby, you can always just wrap
> the original object in a proxy, or access it through an adapter that
> supports all the public methods of the new class; it fits the "duck
> typing" rule, without breaking encapsulation.
Another reason would be that it might be useful to change the object _in 
place_. That is, all existing references suddenly point to the new object.

Cheers,
Michael