From: Tim Bates <tim@...>
Date: 2003-08-11T22:12:37+09:00
Subject: Re: WeakRef and caches

On Mon, Aug 11, 2003 at 09:56:44PM +0900, Robert Klemme wrote:
> That's the expected behavior: the weakest element in the strongest path
> determines the availability of an instance.  So an instance won't be GC'ed
> as long as there is at least one strong ref to it.  This has to be so, in
> order to not change the semantics of strong refs.  If they could be cleared
> out of nothing many programs would not work as expected any more.
> 
> Your cache implementation is exactly as it should be, but the code outside
> should only hold on to an instance as long as it is needed, typically a
> method invocation's duration.  The only thing the outside world should store
> is the id, which is needed to access the instance via the cache.  A typical
> usage pattern looks like this
 
Yes, except that it doesn't work:

[tim@zaphod:3 ~/ruby]$ cat weakref_test.rb 
require 'weakref'

o = Object.new
q = o
o = WeakRef.new(o)
ObjectSpace.garbage_collect
p o
p q
q = nil  # o is now the only reference to the object.
ObjectSpace.garbage_collect
puts o
[tim@zaphod:3 ~/ruby]$ ruby weakref_test.rb
#<Object:0x40028ba0>
#<Object:0x40028ba0>
#<Object:0x40028ba0>
[tim@zaphod:3 ~/ruby]$ 


The final "puts o" should give a RefError, and indeed it does if I
remove the line 'p q':

[tim@zaphod:3 ~/ruby 1]$ cat weakref_test.rb 
require 'weakref'

o = Object.new
q = o
o = WeakRef.new(o)
ObjectSpace.garbage_collect
p o
#p q
q = nil  # o is now the only reference to the object.
ObjectSpace.garbage_collect
puts o
[tim@zaphod:3 ~/ruby]$ ruby weakref_test.rb 
#<Object:0x40028c04>
weakref_test.rb:11:in `puts': Illegal Reference - probably recycled (WeakRef::RefError)
        from weakref_test.rb:11
[tim@zaphod:3 ~/ruby 1]$ 


But it seems that doing anything with a strong reference to an object
causes the weakref to stop functioning as expected. Why is this the
case?

Tim Bates
-- 
tim@bates.id.au