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Filename : weakref.rb
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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            # create a new object instance
#   p foo.to_s                  # original's class
#   foo = WeakRef.new(foo)      # reassign foo with WeakRef instance
#   p foo.to_s                  # should be same class
#   GC.start                    # start the garbage collector
#   p foo.to_s                  # should raise exception (recycled)
#
# == Example
#
# With help from WeakRef, we can implement our own redimentary WeakHash class.
#
# We will call it WeakHash, since it's really just a Hash except all of it's
# keys and values can be garbage collected.
#
#     require 'weakref'
#
#     class WeakHash < Hash
#       def []= key, obj
#         super WeakRef.new(key), WeakRef.new(obj)
#       end
#     end
#
# This is just a simple implementation, we've opened the Hash class and changed
# Hash#store to create a new WeakRef object with +key+ and +obj+ parameters
# before passing them as our key-value pair to the hash.
#
# With this you will have to limit your self to String key's, otherwise you
# will get an ArgumentError because WeakRef cannot create a finalizer for a
# Symbol. Symbols are immutable and cannot be garbage collected.
#
# Let's see it in action:
#
#   omg = "lol"
#   c = WeakHash.new
#   c['foo'] = "bar"
#   c['baz'] = Object.new
#   c['qux'] = omg
#   puts c.inspect
#   #=> {"foo"=>"bar", "baz"=>#<Object:0x007f4ddfc6cb48>, "qux"=>"lol"}
#
#   # Now run the garbage collector
#   GC.start
#   c['foo'] #=> nil
#   c['baz'] #=> nil
#   c['qux'] #=> nil
#   omg      #=> "lol"
#
#   puts c.inspect
#   #=> WeakRef::RefError: Invalid Reference - probably recycled
#
# You can see the local variable +omg+ stayed, although it's reference in our
# hash object was garbage collected, along with the rest of the keys and
# values. Also, when we tried to inspect our hash, we got a WeakRef::RefError,
# this is because these objects were also garbage collected.

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+
  #
  # Raises an ArgumentError if the given +orig+ is immutable, such as Symbol,
  # Fixnum, or Float.

  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