foo.lisp

(defclass symbolic-object-class (standard-class)
  ())

(defmethod allocate-instance 
           ((class symbolic-object-class) &rest args &key)
  (let ((inst (gensym "SYMBOLIC-OBJECT-")))
    (setf (get inst :class) class)
    (dolist (slot (class-slots class) inst)
      (setf (get inst (clos:slot-definition-name slot)) :unbound))))

(defmethod shared-initialize ((inst symbol) slot-names &rest args
                              &key)
  (if (or (eq T inst)
          (not (typep (get inst :class) 'class)))
      (call-next-method)
      (dolist (slot (class-slots (get inst :class)) inst)
        (setf (get inst (clos:slot-definition-name slot))
              (slot-definition-initform slot)))))

(defmethod c2mop:slot-value-using-class 
           ((class (eql (find-class 'symbol))) inst slot-name)
  (get inst slot-name))

(defmethod (setf c2mop:slot-value-using-class) 
           (newval (class (eql (find-class 'symbol))) inst slot-name)
  (setf (get inst slot-name) newval))

(defclass foo () 
  ((a :initform 0)
   (b :initform 1)
   (c :initform 2))
  (:metaclass symbolic-object-class))

(let ((inst (make-instance 'foo)))
  (setf (slot-value inst 'a) 42)
  (symbol-plist inst))
;→ (c 2 b 1 a 42 :class #<symbolic-object-class foo 41A0085653>)