From: matz@... (Yukihiro Matsumoto)
Date: 2003-08-22T00:49:43+09:00
Subject: Re: Class variables - a surprising result

Hi,

In message "Re: Class variables - a surprising result"
    on 03/08/21, David Heinemeier Hansson <david@loudthinking.com> writes:

|> Class variable allocation is per class scope, not per subclass scope.
|> Sub1, Sub2 and Sup really do share their class variables. Now don't
|> ask me for the "why exactly"...
|
|I was fooled by this as well. I don't know why I intuitively would 
|think that a subclass would get its own scope, but I did. Having your 
|own scope as a subclass would be a really neat addition, though. But 
|perhaps a reason from Matz would shed light?

They are variables.  If you can't update values, how they can be
variables.

    class Sup
     @@x = "A"          # declare @@x, set @@x as "A"
     def test 
      print @@x 
     end
    end
    
    class Sub1 < Sup
     @@x = "B"          # set @@x as "B"
    end
    
    class Sub2 < Sup
     @@x = "C"          # set @@x as "C"
    end

    Sup.new.test        # print the value of @@x ("C")
    Sub1.new.test       # ditto.
    Sub2.new.test       # ditto.

Class variables are like global variables whose scope is limited to
the inheritance tree.

							matz.