From: Andy Gimblett <A.M.Gimblett@...>
Date: 2006-04-21T17:30:26+09:00
Subject: Re: how to figure out what class a method is in for a *class method*?

I have a related question - a similar problem to the one just given,
but that solution doesn't fit here because I don't just want the class
name - I want an attribute associated with the class, which subclasses
can override.

I've tried doing this using class variables, but it doesn't work:

--

class Animal
  @@sound = "nothing"
  def to_s
    "I say " + @@sound
  end
end

class Cat < Animal
  @@sound = "miaow"
end

class Dog < Animal
  @@sound = "woof"
end

fudge = Cat.new()
puts fudge

tip = Dog.new()
puts tip

--

This is an abstraction of my real situation but it captures the
essense: each subclass has some attribute which needs to be
manipulated at some point; the code to manipulate the attribute is
common across all subclasses, so it should live in the superclass.
(In fact, ideally, Animal is "abstract" and I don't want to declare a
@@sound there at all - but if I do that, to_s complains about
"uninitialized class variable".)

Alas:

[gimbo@lotus learn] ruby -v subclass_names.rb
ruby 1.8.4 (2005-12-24) [i386-freebsd6]
subclass_names.rb:9: warning: already initialized class variable @@sound
subclass_names.rb:13: warning: already initialized class variable @@sound
I say woof
I say woof

So I guess I'm misusing class variables.

I also tried it with constants (which would be even better since the
value won't ever change) but got "I say nothing" twice instead.

If anyone can help me understand what's going on and how this _should_
be done, I'd be grateful.  I'm still thinking in python a bit too
much, I think.

Thanks,

-Andy

-- 
Andy Gimblett
Computer Science Department
University of Wales Swansea
http://www.cs.swan.ac.uk/~csandy/