Re: [Sbcl-help] Transient classes

[Christophe R. <cs...@ca...>]

Fredrik Tolf <fr...@do...> writes:

> I've tried creating a class in a couple of ways that I thought would work, 
> and then installing a finalizer function on it to show if it were GC'd, 
> but I can't seem to get them to be GC'd whatever I do, even if I hack away 
> all the references I've been able to find, including the following 
> obviously ugly ways:
>
> (let ((test-class (defclass foo (transient) ())))
>    (finalize test-class #'(lambda () (print "foo")))
>    (setf (sb-kernel:classoid-cell-pcl-class (sb-kernel:find-classoid-cell 'foo)) nil)
>    (sb-mop:remove-direct-subclass (find-class 'transient) test-class))

Well, this seems to be nearly there: if I run

  (let ((c (let ((test-class (defclass foo (transient) ())))
             (finalize test-class #'(lambda () (print "foo")))
             (setf (sb-kernel:classoid-cell-pcl-class (sb-kernel:find-classoid-cell 'foo)) nil)
             (sb-mop:remove-direct-subclass (find-class 'transient) test-class))))
    (dotimes (i 3) 
      (gc :full t)) 
    (sb-vm::map-referencing-objects #'print :dynamic c))

then I get as output

  #<SB-KERNEL:STANDARD-CLASSOID FOO> 
  (#<STANDARD-CLASS FOO> (24 . #<CLOSURE # {100295C189}>)) 

The first of these is obvious, I guess: we'd need an extra
  (setf (sb-kernel:classoid-pcl-class (sb-kernel:find-classoid 'foo)) nil)
to remove the reference from the classoid (which raises the question of
why the classoid and the classoid-cell both need a reference...

The second is more problematic; I couldn't convince myself that it came
from a real reference, and MAP-REFERENCING-OBJECTS isn't reliable enough
to do a fine dissection (adding the clearing of CLASSOID-PCL-CLASS made
the other printed reference go away).  On the other hand, I wasn't
getting the finalization message printed either, so more investigation
is needed.

Cheers,

Christophe