From: David Alan Black <dblack@...>
Date: 2001-08-18T20:48:36+09:00
Subject: [ruby-talk:19972] Re: problem understanding class relationships

Hello --

On Sat, 18 Aug 2001, Yukihiro Matsumoto wrote:

> Hi,
>
> In message "[ruby-talk:19960] problem understanding class relationships"
>     on 01/08/18, Tobias Reif <tobiasreif@pinkjuice.com> writes:
>
> |Thus all classes are in the same family
> |and thus @@ variables would be shared among all objects.
> |
> |Where am I erring?
>
> Like constants, class variables must be assigned at the class/module
> top level, and shared among its subclasses.
>
>   class Foo
>     @@foo = 43	# @@foo is assigned (declared) here.
>   end
>
>   class Bar<Foo
>     def bar
>       p @@foo	# @@foo is available.
>     end
>   end
>
>   class Baz<Object
>     def baz
>       p @@foo	# @@foo is NOT available here.
>     end
>   end

Another permutation -- sort of useless, but I found it interesting to
see how this works:

   class A
   end

# Assign to a class variable in a subclass *first*.

   class B < A
     @@cv = "in B"
     def cv; @@cv; end
   end

   a = A.new
   b = B.new

   puts b.cv            # "in B"

# Now go back and assign in the superclass.

   class A
     @@cv = "in A"
     def cv; @@cv; end
   end

# Result: A and B<A now have different @@cv class variables

   puts a.cv            # "in A"
   puts b.cv            # "in B"

   class A
     def chcv
       @@cv = "changed"
     end
   end

# B's #chcv is really A's #chcv, so it affects A's @@cv, not B's

   b.chcv
   puts a.cv            # "changed"
   puts b.cv            # "in B"


Well, I don't recommend it in real life, but I guess I found the
per-class behavior I was looking for at the beginning of all this :-)


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav