added-slots - a list.

discarded-slots - a list.

property-list - a list.

initargs - an initialization argument list.

result - an object.

The system-supplied primary method on update-instance-for-redefined-class checks the validity of initargs and signals an error if an initarg is supplied that is not declared as valid. This method then initializes slots with values according to the initargs, and initializes the newly added-slots with values according to their :initform forms. It does this by calling the generic function shared-initialize with the following arguments: the instance, a list of names of the newly added-slots to instance, and the initargs it received. Newly added-slots are those local slots for which no slot of the same name exists in the old version of the class.

When make-instances-obsolete is invoked or when a class has been redefined and an instance is being updated, a property-list is created that captures the slot names and values of all the discarded-slots with values in the original instance. The structure of the instance is transformed so that it conforms to the current class definition. The arguments to update-instance-for-redefined-class are this transformed instance, a list of added-slots to the instance, a list discarded-slots from the instance, and the property-list containing the slot names and values for slots that were discarded and had values. Included in this list of discarded slots are slots that were local in the old class and are shared in the new class.

The value returned by update-instance-for-redefined-class is ignored.

 (defclass position () ())
 
 (defclass x-y-position (position)
     ((x :initform 0 :accessor position-x)
      (y :initform 0 :accessor position-y)))
 
;;; It turns out polar coordinates are used more than Cartesian 
;;; coordinates, so the representation is altered and some new
;;; accessor methods are added.
 
 (defmethod update-instance-for-redefined-class :before
    ((pos x-y-position) added deleted plist &key)
   ;; Transform the x-y coordinates to polar coordinates
   ;; and store into the new slots.
   (let ((x (getf plist 'x))
         (y (getf plist 'y)))
     (setf (position-rho pos) (sqrt (+ (* x x) (* y y)))
           (position-theta pos) (atan y x))))
  
 (defclass x-y-position (position)
     ((rho :initform 0 :accessor position-rho)
      (theta :initform 0 :accessor position-theta)))
  
;;; All instances of the old x-y-position class will be updated
;;; automatically.
 
;;; The new representation is given the look and feel of the old one.
 
 (defmethod position-x ((pos x-y-position))  
    (with-slots (rho theta) pos (* rho (cos theta))))
 
 (defmethod (setf position-x) (new-x (pos x-y-position))
    (with-slots (rho theta) pos
      (let ((y (position-y pos)))
        (setq rho (sqrt (+ (* new-x new-x) (* y y)))
              theta (atan y new-x))
        new-x)))
 
 (defmethod position-y ((pos x-y-position))
    (with-slots (rho theta) pos (* rho (sin theta))))
 
 (defmethod (setf position-y) (new-y (pos x-y-position))
    (with-slots (rho theta) pos
      (let ((x (position-x pos)))
        (setq rho (sqrt (+ (* x x) (* new-y new-y)))
              theta (atan new-y x))
        new-y)))