From: jweirich@...
Date: 2001-08-11T21:49:17+09:00
Subject: [ruby-talk:19525] Re: the way class variables work

>>>>> "David" == David Alan Black <dblack@candle.superlink.net> writes:

    David> I've been puzzling over Ruby's treatment of class variables
    David> -- not so much the "what" as the "why".

[... example elided ...]

    David> Class variables seem to be not so much per class as per
    David> class hierarchy or family.  I can't quite figure out why
    David> they work this way, rather than working like instance
    David> methods (so that redefinition in a child class stops the
    David> search process up the inheritance path).

    David> Insights, opinions, and/or words of wisdom welcome.

In Smalltalk, there are two kinds of "class" variables.  There are
class variables which work the way Ruby class variables work (shared
across an inheritance hierarchy).  But there are also class *instance*
variables which are unique to the individual class object (instance)
where they are defined.

It turns out that both types of class variables are supported in Ruby
as well...

    class A
      class << self
        def class_instance_var
          @class_instance_var
        end
        def class_var
          @@class_var
        end
      end
    
      @@class_var = 0		# Shared across hierarchy
      @class_instance_var = 10	# Unique to class
    end
    
    class B < A
      @@class_var = 1           # Shared
      @class_instance_var = 11  # Unique
    end
    
    puts A.class_var           # => 1
    puts B.class_var           # => 1
    puts A.class_instance_var  # => 10
    puts B.class_instance_var  # => 11

Class variable mimic "static" variables in Java or C++.  Class
instance variables are instance variables in the class object.

I hope this helps.

-- 
-- Jim Weirich     jweirich@one.net    http://w3.one.net/~jweirich
---------------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct, 
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)