From: Nat Pryce <nat.pryce@...13media.com>
Date: 2001-08-13T19:22:43+09:00
Subject: [ruby-talk:19630] Re: Forced garbage collection

From: <jweirich@one.net>
> In ruby, the finalizer runs *after* the object is GC'ed.  There is no
> more object, so the funky "save the object reference in a global"
> problem can't happen.  So, how do we get to the data formally stored
> in that object?  Well, that's what the block closure is for.  We make
> sure we create the closure in such a way that it "remembers" anything
> we need to dispose of at finalization time.  This also means that we
> had better be careful not to accidently create a reference to the
> object.  1) It won't be there when finalization is done, and 2) the
> reference keeps the object from being collected.

Excellent explanation. Thanks.  I've got my WeakHash working now.

The way to avoid holding a reference to the object in its finalizer is
to use a factory method to create finalizers.  That method should take 
as arguments all the data required to finalise the object, but *not* a 
reference to the object itself.  Therefore the closure does not refer to 
the object and so will not keep the object from being garbage collected.

E.g. from my WeakHash:

def []=( key, value )
    @hash[key] = value.__id__
    ObjectSpace.define_finalizer( value, create_finalizer(key) )
end

def create_finalizer( key )
    proc { |id| remove_key(key) }
end

def remove_key( key )
    @hash.delete( key )
end