[pro] Initialization of shared slots

[Martin Simmons]

>>>>> On Wed, 05 Jan 2011 16:17:09 +0100, Didier Verna said:
> 
>   Until now, I've been uncounsiously relying on something that the
> standard does not seem to specify: the fact that shared slots equipped
> with an initform are initialized before the first instance is created:
> 
> (defclass test () ((slot :allocation :class :initform t)))
> 
> (defmethod initialize-instance :before ((test test) &key)
>     (format t "Slot boundp: ~A~%" (slot-boundp test 'slot)))
> 
> 
> * (make-instance 'test)
> Slot boundp: T
> 
> * (make-instance 'test)
> Slot boundp: T
> 
> 
> I just realized that because with ABCL (and contrary to the other
> implementations I've tried), the result is NIL, and then T.
> 
> But now, I'm a little confused. Section 7.1.3 of CLHS says:
> 
>         An :initform form is used to initialize a slot only if no
>         initialization argument associated with that slot is given as an
>         argument to make-instance or is defaulted by :default-initargs.
> 
> So after re-reading this (the "only" part notably), it now appears to me
> somewhat abusive to initialize shared slots earlier than at first
> instance creation time (like most lisps seem to do), because there's no
> way to know, at that time, whether the fist call to make-instance will
> provide an initarg.
> 
> WDYT?

The initarg will always set the slot anyway, so the only difference is that it
has the "wrong" value transiently.


> Related to this is a question of style as well. I rarely use shared
> slots, but until now, I thought of it as a convenient way to encapsulate
> information that's supposed to be used by instances only (including the
> first one). Maybe this is not a good idea after all...

Overall, I think shared slots are a bad idea.  As discussed in a previous
topic, defclass by itself doesn't provide very good encapsulation.

Another gotcha is that reevaluating a defclass with a different value for the
initform doesn't reinitialize the slot.

-- 
Martin Simmons
LispWorks Ltd
http://www.lispworks.com/