Garbage collect and buffer-undo-list

Garbage collect and buffer-undo-list

[Yuan Fu]

I know the garbage collector trims buffer-undo-list. Does it collect the cons cell if there are other references to that object (other than buffer-undo-list)?

I tried this:

(setq last1 (last buffer-undo-list))
(garbage-collect)

After the garbage collection, the entry referred by last1 is not in buffer-undo-list anymore, but still accessible by last1. That seems to support my theory.

Yuan

Re: Garbage collect and buffer-undo-list

[Stefan Monnier]

> I know the garbage collector trims buffer-undo-list. Does it collect the
> cons cell if there are other references to that object (other than
> buffer-undo-list)?

No, it can't.

If you want to keep a reference to some of those cons-cells but still
want the GC to collect them when the undo-log is trimmed, then you need
to use a weak reference.  We don't currently have plain "weak
references" in ELisp, but we do have hash-tables whose keys and/or
values can be weak references (that's what I used in
`undo-equiv-table`).


        Stefan

Re: Garbage collect and buffer-undo-list

[Yuan Fu]

> On Mar 13, 2021, at 2:01 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
>> I know the garbage collector trims buffer-undo-list. Does it collect the
>> cons cell if there are other references to that object (other than
>> buffer-undo-list)?
> 
> No, it can't.
> 
> If you want to keep a reference to some of those cons-cells but still
> want the GC to collect them when the undo-log is trimmed, then you need
> to use a weak reference.  We don't currently have plain "weak
> references" in ELisp, but we do have hash-tables whose keys and/or
> values can be weak references (that's what I used in
> `undo-equiv-table`).
> 

That’s cool, I do want the referenced object to survive. Thanks!

Yuan