ANSI Common Lisp 7 Objects 7.7 Dictionary of Objects
||Standard Generic Function
instance new-class &key &allow-other-keys
- Method Signatures:
- Arguments and Values:
instance - an object.
new-class - a class designator.
initargs - an initialization argument list.
The generic function change-class changes the
class of an instance to new-class.
It destructively modifies and returns the instance.
If in the old class there is any slot of the
same name as a local slot in the new-class,
the value of that slot is retained. This means that if
the slot has a value, the value returned by slot-value
after change-class is invoked is eql to the
value returned by slot-value before change-class is
invoked. Similarly, if the slot was unbound, it remains
unbound. The other slots are initialized as described in
Section 7.2 Changing the Class of an Instance.
After completing all other actions, change-class invokes
generic function update-instance-for-different-class can be used
to assign values to slots in the transformed instance.
See Section 7.2.2 Initializing Newly Added Local Slots.
If the second of the above methods is selected,
that method invokes change-class
on instance, (find-class new-class),
and the initargs.
(defclass position () ())
(defclass x-y-position (position)
((x :initform 0 :initarg :x)
(y :initform 0 :initarg :y)))
(defclass rho-theta-position (position)
((rho :initform 0)
(theta :initform 0)))
(defmethod update-instance-for-different-class :before ((old x-y-position)
;; Copy the position information from old to new to make new
;; be a rho-theta-position at the same position as old.
(let ((x (slot-value old 'x))
(y (slot-value old 'y)))
(setf (slot-value new 'rho) (sqrt (+ (* x x) (* y y)))
(slot-value new 'theta) (atan y x))))
;;; At this point an instance of the class x-y-position can be
;;; changed to be an instance of the class rho-theta-position using
(setq p1 (make-instance 'x-y-position :x 2 :y 0))
(change-class p1 'rho-theta-position)
;;; The result is that the instance bound to p1 is now an instance of
;;; the class rho-theta-position. The update-instance-for-different-class
;;; method performed the initialization of the rho and theta slots based
;;; on the value of the x and y slots, which were maintained by
;;; the old instance.
- See Also:
Section 7.2 Changing the Class of an Instance
The generic function change-class has several semantic
difficulties. First, it performs a destructive operation that can be
invoked within a method on an instance that was used to select that
When multiple methods are involved because methods are being
combined, the methods currently executing or about to be executed may
no longer be applicable. Second, some implementations might use
compiler optimizations of slot access, and when the class of an
instance is changed the assumptions the compiler made might be
violated. This implies that a programmer must not use
change-class inside a method if any
methods for that generic function
access any slots, or the results are undefined.
- Allegro CL Implementation Details: