From: James Coglan <jcoglan@...>
Date: 2008-07-07T21:54:50+09:00
Subject: Re: inheritance v passing object through the constructor

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2008/7/7 <b.candler@pobox.com>:

> box ||= Box.new
>> class ErrorBox
>>  def initialize(box)
>>  end
>> end
>>
>
> That won't do anything as written; the two 'box' local variables are
> distinct.
> I'm guessing you mean something like this:
> class ErrorBox
>  def initialize(box)
>    @box = box
>  end
>  def draw
>    @box.draw
>  end
> end
> eb = ErrorBox.new(Box.new)
> eb.draw


If ErrorBox really is a more specialized kind of Box, then subclassing makes
sense in terms of design. If it's not really a new kind of Box but wants to
use some of Box's behaviour then use delegation or consider mixins. Reg
Braithewaite has a decent article on this topic:

http://weblog.raganwald.com/2008/04/is-strictly-equivalent-to.html

Also, seek out the decorator in a book on design patterns. It's usually
presented as a way of extending the behaviour of objects in many orthogonal
ways in situations where subclassing would result in combinatorial
explosions of new subclasses. This is another case where mixins can also
make sense.

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