Ruby - 实例方法中未初始化的类变量

Question

我正在学习 RoR 课程，但在 irb 控制台中收到错误消息： "NameError: InstanceCounter::InstanceMethods 中未初始化的类变量@@instances"

我需要在 modules.rb 中更改什么？

控制台：

Evgenys-MacBook-Pro-2:lesson_05 admin$ irb
2.3.1 :001 > load 'modules.rb'
 => true 
2.3.1 :002 > load 'car.rb'
 => true 
2.3.1 :003 > car=Car.new
initialize
NameError: uninitialized class variable @@instances in InstanceCounter::InstanceMethods
Did you mean?  instance_methods
from modules.rb:35:in `initialize'
from (irb):3:in `new'
from (irb):3
from /Users/aku/.rvm/rubies/ruby-2.3.1/bin/irb:11:in `<main>'

modules.rb:

module InstanceCounter
  module ClassMethods

    @@instances = 0

    def instances
      puts "@@instances"
      @@instances
    end

  end

  module InstanceMethods

    def initialize
      puts "initialize"
      register_instance
      super
    end

    protected

    def register_instance
      @@instances += 1
      puts "@@instances += 1"
    end

  end

  def self.included(base)
    base.extend ClassMethods
    base.send :prepend, InstanceMethods
  end
end

汽车.rb：

class Car
  include InstanceCounter

  def initialize
    puts "initialize2"
  end  
end

class PassengerCar < Car
end

class CargoCar < Car
end

Answer 1

将类变量放在InstanceMethods 模块中。默认情况下，extend 似乎只向调用对象添加方法。

关于默认 ruby​​ 实现的文档供参考：extend 和 include