<nikodemus@random-state.net> wrote:
+---------------
| Also, some implementations provide finalizers...
...
| [ Note: make sure your finalizer doesn't retain any references to the
|   object being finalized: then it'll never be GC'd. ]
+---------------

And in practice, this almost always means that you need another level
of indirection: the finalized object has to contain a pointer to the
"real" object you're worrying about, which the finalizer also contains
a pointer to (or is closed over, same thing). To extend your example:

    > (defmethod make-cleanup-thunk ((thing string))
	(lambda ()
	  (format t "The ~s has been cleaned up!~%" thing)))
    #<Standard-Method MAKE-CLEANUP-THUNK (STRING) {484C68A5}>
    > (let* ((x "Thing needing cleaning")
	     (y (list x)))
	(finalize y (make-cleanup-thunk x))
	'done)
    DONE
    > (gc)
    NIL
    > (gc)
    The "Thing needing cleaning" has been cleaned up!
    NIL
    >

[Note that it took two calls to GC in this case -- for no special
reason, just that the first one only needed to do a minor GC and
thus the finalized cons cell didn't get freed during the first one.]


-Rob

-----
Rob Warnock			<rpw3@rpw3.org>
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