null object pattern

null_object.rb

class NullObject
  def method_missing(*)
    self
  end

  def respond_to_method_missing?(name)
    true
  end

  def nil?
    true
  end
end

class NullEngine < NullObject
  def start
    puts "null engine cannot start"
  end
end

def Actual(object)
  case object
  when NullObject then nil
  else object
  end
end

class Car
  def initialize(engine = NullEngine.new())
    @engine = engine
  end

  def run
    @engine.start()
    @engine.run()
  end

  def engine
    Actual(@engine)
  end
end

def main
  car = Car.new()
  car.run()
  engine = car.engine
  if engine
    puts engine.description
  end
end

main()