weakref.rb

require "delegate"

# 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

  @@__map = ::ObjectSpace::WeakMap.new

  ##
  # Creates a weak reference to +orig+

  def initialize(orig)
    case orig
    when true, false, nil
      @delegate_sd_obj = orig
    else
      @@__map[self] = orig
    end
    super
  end

  def __getobj__ # :nodoc:
    @@__map[self] or defined?(@delegate_sd_obj) ? @delegate_sd_obj :
      Kernel::raise(RefError, "Invalid Reference - probably recycled", Kernel::caller(2))
  end

  def __setobj__(obj) # :nodoc:
  end

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

  def weakref_alive?
    !!(@@__map[self] or defined?(@delegate_sd_obj))
  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