meta.rb

module Quackable
  def self.included(base)
    base.extend ClassMethods
  end

  module ClassMethods
    def has_voice(word)
      @words ||= []
      @words << word
    end
  end

  def quack
    self.class.instance_variable_get("@words").each { |w| puts "#{self.class} says #{w}" }
  end
end

class Dog
  include Quackable

  has_voice :bow
  has_voice :wow
end

class Duck
  include Quackable
  has_voice :quack
  has_voice :qwwack
end

class BullTerrier < Dog
  has_voice :growl
end

Dog.new.quack
Duck.new.quack
Dog.new.quack
Duck.new.quack
BullTerrier.new.quack

# Cannot use @@variables because then it will be shared with all
# Solution is to use class_attribute
# http://apidock.com/rails/Class/class_attribute
puts "======="

require 'active_support'
require 'active_support/core_ext/class/attribute'
module Quackable2
  def self.included(base)
    base.class_eval do
      base.extend ClassMethods
      class_attribute :words
      self.words = []
    end
  end

  module ClassMethods
    def has_voice2(word)
      self.words += [word]
    end
  end

  def quack2
    self.class.words.each { |w| puts "#{self.class} says #{w}" }
  end
end

class Dog
  include Quackable2
  has_voice2 :bow
  has_voice2 :wow
end

class BullTerrier
  has_voice2 :growl
end

Dog.new.quack2
BullTerrier.new.quack2