How to add slot to defclass

add-slot-to-defclass.lisp

(defclass foo-class (standard-class) ;; we first create a metaclass
  ()) ;; no slots required

(defmethod initialize-instance :before ((class foo-class) &key my-argument)
  (print my-argument)) ;; when we first define the class, just print the arguments

(defmethod reinitialize-instance :before ((class foo-class) &key my-argument)
  (print my-argument)) ;; when we later redefine the class, just print the arguments again

(defmethod validate-superclass ((class foo-class) (super standard-class))
  t) ;; we need to tell CLOS that STORAGE-CLASS and STANDARD-CLASS are compatible
     ;; see http://metamodular.com/CLOS-MOP/validate-superclass.html for more information on this

(defclass bar () ;; superclass: STANDARD-OBJECT
  () ;; no slots required
  (:metaclass foo-class) ;; we use the storage-class as the metaclass here
  (:my-argument 1 2 three four omg))