From: Joel VanderWerf <vjoel@...>
Date: 2004-06-28T04:51:11+09:00
Subject: Re: accessing a subclass

Florian Weber wrote:
> hi!
> 
> im having two classes.. fruit and apple.
> 
> i wanna do something like
> 
> class Apple < Fruit
>    add_something "foobar"
> end
> 
> in the fruit class i have a add_something method
> which i want to save the "foobar" to a static hash
> in the apple class. the hash however should also
> not be declared in the apple class.
> 
> is this somehow possible? i guess not, or? just
> making sure, since ruby definitely has a lot of suprises =)
> 
> i guess the only other way would be to make a static
> hash in the fruit class which is 2d, the first dimension being
> the subclass..

You can make use of the fact that class methods are inherited, but class 
instance variables are not:

class Fruit
   class << self
     def somethings
       @somethings ||= {}
     end
     def add_something(thing)
       somethings[thing] = true
     end
   end
end

class Apple < Fruit
   add_something :spherical
end

p Fruit.somethings[:spherical]  # ==> nil
p Apple.somethings[:spherical]  # ==> true