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我发现实例 var( @var) 和attr_accessor. 我相信当在 中指定时attr_accessor，var=some_value其行为应该与@var=some_value. 但是，下面的代码片段（在第 10 行中不使用符号时，Ruby 中的 LinkedList 实现会中断@。谁能解释为什么会发生这种情况？我已经在 Ruby 2.6.1 和 2.6.5 中进行了测试。

class LinkedList
  def initialize
    self.head = nil
    self.tail = nil
  end

  def add_first(data)
    if @head.nil?
      @head = Node.new(data) # Break if I use head without @
      @tail = @head 
    else
      @head = Node.new(data, @head)
    end
  end

  def add_last(data); 
    if @tail
      @tail.next_node = Node.new(data)
      @tail = @tail.next_node
    else
      add_first(data)
    end
  end

  def to_s
    result_str = ""
    curr_node = @head
    until (curr_node.next_node.nil?)
      result_str << curr_node.data << ", "
      curr_node = curr_node.next_node
    end
    # Last data is a special case without a comma
    result_str << curr_node.data
  end

  protected
  attr_accessor :head, :tail
end

class Node
  attr_accessor :data, :next_node
  def initialize(data=nil, next_node=nil)
    self.data = data
    self.next_node = next_node
  end

  def to_s
    data.to_s
  end
end

head = LinkedList.new
head.add_first("First")
head.add_first("Second")
head.add_first("Third")
head.add_last("Chicago")
head.add_last("Vegas")

puts head
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