Thanks, `plist-get' does work.


2011/8/9 Andreas Schwab <schwab@linux-m68k.org>

> 小江沈 <xiaojiang@siteshen.com> writes:
>
> > (progn (put 'defun 'x "out")
> >        (let ((old (plist-member (symbol-plist 'defun) 'x)))
> >          (message "old: %s." old)
> >          (put 'defun 'x "in")
> >          (message "old: %s." old)
> >          nil))
> >
> > When I eval this form, I get something like this in *message* buffer:
> > old: (x out).
> > old: (x in).
> > nil
>
> This has nothing to do with let.  plist-member returns a tail of the
> property list, and when the value of an existing property is changed
> only the cdr of the cons cell is overwritten by put, so the reference to
> the cons cell in `old' will follow the change.
>
> > If it is not a bug, how can I save the prev symbol property?
>
> Use plist-get or get to extract the property value, or make a copy.
>
> Andreas.
>
> --
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
> "And now for something completely different."
>