require 'test/unit'
 
module InheritIV
  def inherit_iv *args
    @ih ||= []
    @ih += args
    args.each do |iv|
      self.class_eval("class << self; attr_accessor :#{iv}; end")
    end
  end
 
  def inherited klass
    @ih.each do |iv|
      klass.send(:"#{iv}=", send(iv))
    end
    klass.instance_variable_set(:@ih, @ih)
  end
end
 
class First
  extend InheritIV
  inherit_iv :foo
 
  @foo = 1
 
  def foo
    self.class.foo
  end
 
  def bar
    self.class.bar
  end
end
 
class Second < First
  inherit_iv :bar
 
  @foo = 2
  @bar = 3
end
 
class Third < Second
end
 
class Testy < Test::Unit::TestCase
  def setup
    @first  = First.new
    @second = Second.new
    @third  = Third.new
  end
 
  def test_first
    assert_equal 1, @first.foo
  end
 
  def test_second
    assert_equal 2, @second.foo
    assert_equal 3, @second.bar
  end
 
  def test_third
    assert_equal 2, @third.foo
    assert_equal 3, @third.bar
  end
end