Code for blog post http://jpmoral.com/ruby/2015/08/09/metaprogramming-class-instance-variables-and-inheritance.html

class_instance_variables_and_inheritance.rb

module Echoer
  def echoes(*phrases)
    @known_phrases.concat(phrases)

    @known_phrases.each do |phrase|
      next if self.respond_to?(phrase.to_sym)
      define_method("#{phrase}") { phrase.to_s }
    end
  end

  def known_phrases
    @known_phrases.map do |phrase|
      phrase.to_s
    end
  end

  def self.extended(mod)
    mod.instance_variable_set(:@known_phrases, [])
    mod.send :include, InstanceMethods
  end

  def inherited(subclass)
    subclass.instance_variable_set(:@known_phrases, @known_phrases)
  end

  module InstanceMethods
    def echo(phrase)
      phrase.to_s
    end

    def known_phrases
      self.class.known_phrases
    end
  end
end

class Person
  extend Echoer
  echoes :hello, :goodbye
end

class Shopper < Person
  echoes :charge
end