[comp.lang.clos] How to get class slots with an instance at each class

konstan@elmer-fudd.berkeley.edu (Joe Konstan) (04/03/91)

Is there an easy way to accomplish the following:

(Assume I have a typical inheritance tree as follows:

	A
       / \
      Aa  Ab             etc.
     /  \   \
   Aa1  Aa2 Ab1


I want to define a slot named "default" that is a class slot for all of these
classes but has a separate value in each class.  In other words, I want 
(defclass A ()
 ((default :allocation :class :initform 10 ...)
  ...))

but also want each subclass to have its own slot named default that can 
be changed separately.

I know this can be done by adding a new slot definition in the defclass for
each subclass, but this is undesirable (for several reasons).  Is there an
alternative?

Joe Konstan

Gregor@parc.xerox.com (Gregor Kiczales) (04/03/91)

   Organization: Picasso Research Group, UC Berkeley CS Division
   Date: 	Tue, 2 Apr 1991 15:07:05 PST
   From: konstan@elmer-fudd.berkeley.edu (Joe Konstan)


   I want to define a slot named "default" that is a class slot for all
   of these classes but has a separate value in each class.  In other
   words, I want
   (defclass A ()
    ((default :allocation :class :initform 10 ...)
     ...))

   but also want each subclass to have its own slot named default that
   can be changed separately.


It's funny you should bring this up just now.  I wanted something
similar myself earlier today.  I can think of two ways to do this.

1) With a special macro, like DEFINE-FROBBOZ, that takes the same
arguments as DEFCLASS and expands to a DEFCLASS with the additional
slot.  Given this macro, you would write:

(define-frobboz Aa1 (Aa) ())

Which saves the extra slot from appearing throughout your code.

2) With a special class metaobject class (metaclass) that simply adds
the extra class slot as a direct slot in each of its instances.  In the
final metaobject protocol, this would be written as:

(defclass frobboz-class (standard-class) ())

(defmethod initialize-instance ((class frobboz-class) &key direct-slots
                                                      &rest keys)
  (apply #'call-next-method
         :direct-slots (cons '(:name default :allocation :class)
                             direct-slots)
         keys))

Depending on what version of PCL you are using, you will have to adjust
this a bit.  Given this, you would write:

(defclass Aa1 (Aa) () (:metaclass frobboz-class))

Which also saves the extra slot from appearing throughout your code, in
a slightly different way.

If all you want to associate with the class is the extra slot, the first
solution is probably better.  In the case I was working with earlier
today, I am likely to choose the second solution because there is more I
want to associate with the class.