下面是使用四种方式来完成同一个类的方法。
第一种是直接定义get和set方法；
第二种是通过define_method来动态的定义；
第三种是通过attr_accessor来动态的定义；
第四种也是通过attr_accessor来动态的定义，只是去除了冗余；

10.times{ |i| print("=")}
puts("Writing Code That Writes Code")
class CarModel
  def engine_info=(info)
    @engine_info = info
  end

  def engine_info
    @engine_info
  end

  def engine_price=(price)
    @engine_price = price
  end

  def engine_price
    @engine_price
  end

  def wheel_info=(info)
    @wheel_info = info
  end

  def wheel_info
    @wheel_info
  end

  def wheel_price=(price)
    @wheel_price = price
  end

  def wheel_price
    @wheel_price
  end

  def airbag_info=(info)
    @airbag_info = info
  end

  def airbag_info
    @airbag_info
  end

  def airbag_price=(price)
    @airbag_price = price
  end

  def airbag_price
    @airbag_price
  end

  def alarm_info=(info)
    @alarm_info = info
  end

  def alarm_info
    @alarm_info
  end

  def alarm_price=(price)
    @alarm_price = price
  end

  def alarm_price
    @alarm_price
  end

  def stereo_info=(info)
    @stereo_info = info
  end

  def stereo_info
    @stereo_info
  end

  def stereo_price=(price)
    @stereo_price = price
  end

  def stereo_price
    @stereo_price
  end
end

class CarModel2
  FEATURES = ["engine", "wheel", "airbag", "alarm", "stereo"]

  FEATURES.each do |feature|
    define_method("#{feature}_info=") do |info|
      instance_variable_set("@#{feature}_info", info)
    end

    define_method("#{feature}_info") do
      instance_variable_get("@#{feature}_info")
    end

    define_method "feature_price=" do |price|
      instance_variable_set("@#{feature}_price", price)
    end

    define_method("#{feature}_price") do
      instance_variable_get("@#{feature}_price")
    end
  end
end

class CarModel3
  attr_accessor :engine_info, :engine_price, :wheel_info, :wheel_price, :airbag_info, :airbag_price, :alarm_info, :alarm_price, :stereo_info, :stereo_price
end

class CarModel4
  # define a class macro for setting features
  def self.features(*args)
    args.each do |feature|
      attr_accessor "#{feature}_price", "#{feature}_info"
    end
  end

  # set _info and _price methods for each of these features
  features :engine, :wheel, :airbag, :alarm, :stereo
end

car = CarModel4.new
car.engine_info = "111"
puts car.engine_info
#list all methods 
car.methods.sort.each do |method|
  puts "method #{method} belong to " + car.method(:"#{method}").inspect
end

http://rubylearning.com/blog/2010/11/23/dont-know-metaprogramming-in-ruby/