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Ruby 1.9.2 代码：

def append_string_to_text(string_to_append)
  string_to_alter = 'starting_bit'
  p "OUTSIDE BLOCK: string_to_alter.object_id is #{string_to_alter.object_id}"
  Proc.new do
    p "****** START OF BLOCK: string_to_alter.object_id is #{string_to_alter.object_id}"
    p "defined?(new_string_created_in_block) is #{defined?(new_string_created_in_block) == true}"
    unless defined?(new_string_created_in_block)
      p "new_string_created_in_block is undefined. lets define it."
      new_string_created_in_block = 'test'
    end
    p "new_string_created_in_block.object_id is #{new_string_created_in_block.object_id}"
    string_to_alter = string_to_alter + string_to_append
    p "END OF BLOCK: string_to_alter.object_id is #{string_to_alter.object_id}"
    string_to_alter
  end
end

proc = append_string_to_text('_text_at_the_end')
p proc.call
p proc.call

输出：

"OUTSIDE BLOCK: string_to_alter.object_id is 70283335840820"
"****** START OF BLOCK: string_to_alter.object_id is 70283335840820"
"defined?(new_string_created_in_block) is false"
"new_string_created_in_block is undefined. lets define it."
"new_string_created_in_block.object_id is 70283335840520"
"END OF BLOCK: string_to_alter.object_id is 70283335840440"
"starting_bit_text_at_the_end"
"****** START OF BLOCK: string_to_alter.object_id is 70283335840440"
"defined?(new_string_created_in_block) is false"
"new_string_created_in_block is undefined. lets define it."
"new_string_created_in_block.object_id is 70283335840180"
"END OF BLOCK: string_to_alter.object_id is 70283335840100"
"starting_bit_text_at_the_end_text_at_the_end"

第一次运行块时，string_to_alter变量最初指向在append_string_to_text方法开始时创建的对象，因为块是一个闭包。该块创建一个新变量new_string_created_in_block，然后创建一个新的块局部变量string_to_alter，它遮蔽了外部变量string_to_alter。

第二次运行该块时，string_to_alter变量最初指向在第一次运行块时创建的对象。

为什么在第二次运行期间new_string_created_in_block未定义？它在第一次运行时被分配，string_to_alter变量的分配从第一次运行时就被持久化了，那么为什么不new_string_created_in_block持久化呢？

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1 回答 1

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string_to_alter被持久化，因为它在Proc块之外并且是闭包的一部分。然而Proc，块本身在每次调用时都会被实例化，因此块内定义的任何变量都缺乏持久性。

于 2013-09-04T14:37:39.260 回答