#:filtered-effective-slot-definition
#:predicate-class-mixin
#:abstract-class-mixin #:instantiate-abstract-class
- #:mdw-class #:abstract-class
+ #:singleton-class-mixin
+ #:mdw-class #:abstract-class #:singleton-class
#:print-object-with-slots))
(in-package #:mdw.mop)
(error 'instantiate-abstract-class :class class))
;;;--------------------------------------------------------------------------
+;;; Singleton classes.
+
+(defclass singleton-class-mixin (compatible-class)
+ ((instance :initform nil :type (or null standard-object)))
+ (:documentation
+ "A class which has only one instance. All calls to `make-instance' return
+ the same object."))
+
+(defmethod allocate-instance ((class singleton-class-mixin) &key)
+ "If the class already has an instance, return it; otherwise allocate one,
+ store it away, and return that."
+ (with-slots (instance) class
+ (or instance
+ (setf instance (call-next-method)))))
+
+;;;--------------------------------------------------------------------------
;;; Useful classes.
(defclass mdw-class (filtered-slot-class-mixin
features."))
(defclass abstract-class (mdw-class abstract-class-mixin) ())
+(defclass singleton-class (mdw-class singleton-class-mixin) ())
;;;--------------------------------------------------------------------------
;;; Printing things.