From: Joel VanderWerf <vjoel@...>
Date: 2004-08-24T06:43:53+09:00
Subject: Re: Circular References: How to prove that they are garbage collected

Karsten Meier wrote:
> Hello Ruby Fans
> 
> I'm converting a perl example which deals with the circular data 
> structures, and how to make sure they are destroyed properly.
> (For the pleac cookbook, pleac.sourceforge.net)
> 
> I have read several times that the garbage collector has no problems 
> with circular references, so I tried to prove it.
> But with my ruby 1.8.0 it looks like the garbage collector
> does not destroys the Node A, B and C, but only the lonely node:

"Only the lonely
  Node the way I feel tonight"

   ... hehe. Sorry Roy Orbison.

Anyway, try it with the following 3 additional lines. All four nodes 
print their message before the "Garbage colection called" message. I 
think what's going on is conservatism. Keeping the local vars in a 
method makes sure that stack frame is gone begore the GC starts.

> GC.enable
> 
> class RingNode
>   attr_accessor :next
>   attr_accessor :prev
>   attr_reader :name
> 
>   def initialize(aName)
>       @name = aName
>       ObjectSpace.define_finalizer(self,
>             self.class.method(:finalize).to_proc)
>   end
> 
>   def RingNode.finalize(id)
>       puts "Node #{id} dying"
>   end
> 
>   def RingNode.show_all_objects
>       ObjectSpace.each_object {|id|
>           puts id.name if id.class == RingNode
>       }
>   end
> end
> 
def main
> a = RingNode.new("Node A")
> b = RingNode.new("Node B")
> c = RingNode.new("Node C")
> a.next = b
> b.next = c
> c.next = a
> a.prev = c
> c.prev = b
> b.prev = a
> lonely = RingNode.new("Lonely node")
end
> 
> a = nil
> b = nil
> c = nil
> lonely = nil
> 
main
> GC.start
> puts "Garbage Collection called"
> RingNode.show_all_objects
> puts "End of Program"
> 
> 
> What happens is that Noda A,B,C get destroyed, but after the end of the 
> program. If I comment out the finalizer (in case I made an error with 
> it) and only use the ObjectSpace.each_object, I see also Node A,B,C 
> after the garbage collection.
>