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Weak hash table for attaching extra data to an object
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Date: 2007-08-14 (10:15)
From: Richard Jones <rich@a...>
Subject: Weak hash table for attaching extra data to an object

I have a module which I can't edit[1].  This module, let's call it
Document, stores a representation of an XML tree in internal node

  type intnode

What I want to do is -- completely outside Document -- attach my own
extra information to these nodes.  The key point is that if the
garbage collector collects an intnode, then I want my extra
information to be garbage collected too.

My proposed solution would use a weak hash table similar to / derived
from this one: http://remi.vanicat.free.fr/ocaml/hweak/.  I would
store in this hash table mappings from (weak) intnode pointers to my
extra data structure.

Remi's Hweak module isn't quite what I need because the data is also
weak.  Since nothing except the hash table itself is tracking my extra
data, the GC can potentially drop all of the bindings, and that's not
much help.

I tried to modify Hweak so that the data is not weak, but that proved
to be quite hard.

I wonder if anyone has done this, or can suggest a better way to solve
this problem?  The issue of attaching extra data to some existing
object would seem to be a fairly common one ...


[1] By "can't edit" I really mean "don't want to modify".  I want
Document to be general purpose.  Also, unlike PXP, I want multiple
callers to be able to individually attach data to nodes.

Richard Jones
Red Hat