From: ts <decoux@...>
Date: 2002-12-06T22:49:33+09:00
Subject: Re: Class-instance variables access

>>>>> "F" == Francois GORET <fg@siamecommerce.com> writes:

F> I understand that @@x refers only to a (non existant) class variable of A. I 
F> don't catch why there is a distinction between a class instance variable and 
F> a class variable: the two are "stored" in the unique object representing the 
F> class A, no ? 

 Well, what you must understand is just this

pigeon% ruby -e 'p Array.ancestors'
[Array, Enumerable, Object, Kernel]
pigeon% 

 See Object in the inheritence.

 in ruby a class is an object like any other object.

 Now when you write this

   class A
      def initialize
         @a = 12
      end
   end

   a = A.new

 You have created an object of class A, and you have defined an instance
 variable for this object. Only this object can access its instance
 variable and if you want to make it "public" you define an accessor like
 this 

   class A
      attr_accessor :a
      def initialize
         @a = 12
      end
   end

   a = A.new
   a.a = 24


 This work for *all* objects, and because it work for all objects it work
 also for a class (a class is an object). this mean that you can write 

   class A
      @b = 12
   end

 and you have defined an instance variable for the object A, and only A can
 access it. Like previously, if you want to make it "public" you must
 create an accessor, something like this

   class A
      class << self
         attr_accessor :b  # attr_accessor is for the class, not the
                           #  instance, this is why I've used class << self
      end
      @b = 12
   end

   A.b = 24

 and now you can access the instance variable of A and you can write

   class A
      class << self
         attr_accessor :b
      end
      @b = 24

      attr_accessor :a
      def initialize
         @a = 12
      end

      def show
         puts "class instance variable #{self.class.b}"
         puts "instance variable #{self.a}"
      end
   end

   a = A.new
   a.show





Guy Decoux