From: Jim Weirich <jweirich@...>
Date: 2002-08-13T20:41:12+09:00
Subject: Re: Sibling rivalry and class variables

>>>>> "Kent" == Kent Dahl <kentda@stud.ntnu.no> writes:

    Philip> What would you guys suggest I do if I want a variable set
    Philip> in the parent that the child may or may not override?

    Kent> Depending on what you wish to do with the variable, I'd suggest:
[... several suggestions elided ...]
    Kent> - Use real class instance variables instead of class variables. (Might
    Kent> be hard to do if you still want to propagate the variables downwards,
    Kent> since a subclass is a totally different object.)

A class instance variable is just a variable that belongs to the class
object rather than an instance of the class.  You have to do a little
work to reference a class instance variable from an object of that
class (setting up a class method is sufficient, perhaps there is an
easier way).

Elaborating on this suggestion, you could do the following ...

    class Parent
      @@cv = "Parent Class Variable"
      @civ = "Parent Class Instance Variable"
      def show_cv
        puts @@cv
      end
      def show_civ
        self.type.show_civ
      end
      def self.show_civ
        puts @civ
      end
    end
    
    class Son < Parent
      @@cv = "Son Class Variable"
      @civ = "Son Class Instance Variable"
      def self.show_civ
        puts @civ
      end
    end
    
    class Daughter < Parent
      @@cv = "Daughter Class Variable"
      @civ = "Daughter Class Instance Variable"
      def self.show_civ
        puts @civ
      end
    end
    
    puts "Son ..."
    s = Son.new
    s.show_cv
    s.show_civ
    
    puts
    puts "Daughter ..."
    d = Daughter.new
    d.show_cv
    d.show_civ
    
The output is ...

    Son ...
    Daughter Class Variable
    Son Class Instance Variable

    Daughter ...
    Daughter Class Variable
    Daughter Class Instance Variable

Notice that the class instance variable is unique to each subclass and
is not shared in the class heirarchy like the @@ class variable.

-- 
-- 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)