Re: Hash-table elements

[Nordlöw <per.nordlow@gmail.com>]

On Sep 29, 3:17 pm, p...@informatimago.com (Pascal J. Bourguignon)
wrote:
> Nordlöw <per.nord...@gmail.com> writes:
> > Can hash-tables contain references other "global" structures?
>
> > I have a hash-table that maps filenames to their metadata,file-scan-
> > hits/misses.
> > I don't want to pull-change-push the whole meta-data value whenever I
> > change parts of these metadata/hits.
>
> > Is this possible somehow?
>
> Yes.  Only do not modify the keys!  You wouldn't be able to retrieve
> them, since changing the state of a key would probably change its hash
> value, and therefore the bucket where the hash-table stored it.
> But you can do whatever you want on the value.
>
> (require 'cl)
> (let ((h (make-hash-table)))
>
>    (let ((value (list 1 2 3))
>          (key   :my-list))
>
>       (setf (gethash key h) value)
>       (setf (car value) 0
>             (cdr value) 'z)
>       (gethash key h)))
> --> (0 . z)
>
> --
> __Pascal Bourguignon__

So with the use of setf() we realize the reference pattern in Emacs-
Lisp as we use pointers in C?

So the value of (gethash key h) is actually a reference to "value" and
when "value" goes out of scope the interpreter knows that it is
referenced from another structure and does not delete its contents?

I have understood it correctly?

/Nordlöw