class_variable_and_instance_variable.rb

class Polygon
  @@sides = 10
  def self.sides
    @@sides
  end
end

puts Polygon.sides # => 10

class Triangle < Polygon
  @@sides = 3
end

puts Triangle.sides # => 3
puts Polygon.sides # => 3

class Rectangle < Polygon
  @@sides = 4
end

puts Polygon.sides # => 4

class Polygon
  @sides = 10
end

[Polygon, Rectangle].each do |klass|
  puts klass.to_s + ".sides: " + klass.sides.to_s
  puts
end

inheritable_attributes.rb

module ClassLevelInheritableAttributes
  def self.included(base)
    base.extend(ClassMethods)    
  end
  
  module ClassMethods
    def inheritable_attributes(*args)
      @inheritable_attributes ||= [:inheritable_attributes]
      @inheritable_attributes += args
      args.each do |arg|
        class_eval %(
          class << self; attr_accessor :#{arg} end
        )
      end
      @inheritable_attributes
    end
    
    def inherited(subclass)
      @inheritable_attributes.each do |inheritable_attribute|
        instance_var = "@#{inheritable_attribute}"
        subclass.instance_variable_set(instance_var, instance_variable_get(instance_var))
      end
    end
  end
end

inheritable_attributes_example.rb

require 'inheritable_attributes'

class Polygon
  include ClassLevelInheritableAttributes
  inheritable_attributes :sides, :coolness
  @sides    = 8
  @coolness = 'Very'
end

class Triangle < Polygon
  @sides = 3
end

class Octogon < Polygon; end

[Polygon, Octogon].each do |klass|
  puts klass.to_s + ".instance_variables: " + klass.instance_variables.inspect
  puts klass.to_s + ".sides: " + klass.sides.to_s
  puts klass.to_s + ".coolness: " + klass.coolness.to_s
  puts
end