🚀 go-pugleaf

In article <GUMBY.90Dec19092440@Cygnus.COM> gumby@Cygnus.COM (David Vinayak Wallace) writes:
>   From: creon@nas.nasa.gov (Creon C. Levit)
>
>   I would like to define a method that gets called when a CLOS instance
>   gets garbage collected, sort of like a destructor method in c++.

>What do you want to happen?  For instance, an object may never BE
>collected, since the GC may never run.

That's a good point.  I expect that an example of what he's doing is
objects containing references to streams.  If the only reference to a
stream is in a particular instance, and the instance gets GCed, you would
like to know to close the stream.

The design of many modern GCs doesn't make such a feature easy to
implement.  Many systems use some form of copying GC, so the only objects
it sees are the live ones; the garbage gets left in oldspace, which is
then reclaimed in one fell swoop.  Searching through oldspace for all the
garbage instances would significantly slow down the GC; this would end up
as a hybrid of copying and mark-sweep (copy-and-sweep?).

In my experience, most objects that need this kind of special treatment are
amenable to solutions using explicit destruction (e.g. the CLOSE function)
and/or automatic destruction when leaving a dynamic environment (e.g.  the
WITH-OPEN-FILE macro).  Not surprisingly, these are the same operations
that invoke C++ destructors: using the "delete" operator on a pointer
returned by "new", and leaving a block that declared an automatic class
object.  In addition, Lisp generally allows you to invoke an explicit
destructor on an object allocated by a dynamic macro, which I'm not sure is
valid in C++.

--
Barry Margolin, Thinking Machines Corp.

barmar@think.com
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