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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 rudimentary 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 keys, 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 its 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.key?(self) or defined?(@delegate_sd_obj)
end
end
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