晋太元中，武陵人捕鱼为业。缘溪行，忘路之远近。忽逢桃花林，夹岸数百步，中无杂树，芳草鲜美，落英缤纷。渔人甚异之，复前行，欲穷其林。 　　林尽水源，便得一山，山有小口，仿佛若有光。便舍船，从口入。初极狭，才通人。复行数十步，豁然开朗。土地平旷，屋舍俨然，有良田、美池、桑竹之属。阡陌交通，鸡犬相闻。其中往来种作，男女衣着，悉如外人。黄发垂髫，并怡然自乐。 　　见渔人，乃大惊，问所从来。具答之。便要还家，设酒杀鸡作食。村中闻有此人，咸来问讯。自云先世避秦时乱，率妻子邑人来此绝境，不复出焉，遂与外人间隔。问今是何世，乃不知有汉，无论魏晋。此人一一为具言所闻，皆叹惋。余人各复延至其家，皆出酒食。停数日，辞去。此中人语云：“不足为外人道也。”(间隔 一作：隔绝) 　　既出，得其船，便扶向路，处处志之。及郡下，诣太守，说如此。太守即遣人随其往，寻向所志，遂迷，不复得路。 　　南阳刘子骥，高尚士也，闻之，欣然规往。未果，寻病终。后遂无问津者。 sh-3ll

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require "delegate"
require 'thread'

# Weak Reference class that allows a referenced object to be
# garbage-collected.  A WeakRef may be used exactly like the object it
# references.
#
# Usage:
#
#   foo = Object.new
#   foo = Object.new
#   p foo.to_s                  # original's class
#   foo = WeakRef.new(foo)
#   p foo.to_s                  # should be same class
#   ObjectSpace.garbage_collect
#   p foo.to_s                  # should raise exception (recycled)

class WeakRef < Delegator

  ##
  # RefError is raised when a referenced object has been recycled by the
  # garbage collector

  class RefError < StandardError
  end

  @@id_map =  {}                # obj -> [ref,...]
  @@id_rev_map =  {}            # ref -> obj
  @@mutex = Mutex.new
  @@final = lambda {|id|
    @@mutex.synchronize {
      rids = @@id_map[id]
      if rids
        for rid in rids
          @@id_rev_map.delete(rid)
        end
        @@id_map.delete(id)
      end
      rid = @@id_rev_map[id]
      if rid
        @@id_rev_map.delete(id)
        @@id_map[rid].delete(id)
        @@id_map.delete(rid) if @@id_map[rid].empty?
      end
    }
  }

  ##
  # Creates a weak reference to +orig+

  def initialize(orig)
    @__id = orig.object_id
    ObjectSpace.define_finalizer orig, @@final
    ObjectSpace.define_finalizer self, @@final
    @@mutex.synchronize {
      @@id_map[@__id] = [] unless @@id_map[@__id]
    }
    @@id_map[@__id].push self.object_id
    @@id_rev_map[self.object_id] = @__id
    super
  end

  def __getobj__ # :nodoc:
    unless @@id_rev_map[self.object_id] == @__id
      Kernel::raise RefError, "Invalid Reference - probably recycled", Kernel::caller(2)
    end
    begin
      ObjectSpace._id2ref(@__id)
    rescue RangeError
      Kernel::raise RefError, "Invalid Reference - probably recycled", Kernel::caller(2)
    end
  end

  def __setobj__(obj) # :nodoc:
  end

  ##
  # Returns true if the referenced object is still alive.

  def weakref_alive?
    @@id_rev_map[self.object_id] == @__id
  end
end

if __FILE__ == $0
#  require 'thread'
  foo = Object.new
  p foo.to_s                    # original's class
  foo = WeakRef.new(foo)
  p foo.to_s                    # should be same class
  ObjectSpace.garbage_collect
  ObjectSpace.garbage_collect
  p foo.to_s                    # should raise exception (recycled)
end