mop: New metaclass for singleton classes: ensures only one instance.

diff --git a/mdw-mop.lisp b/mdw-mop.lisp
index a669490..dc5eb87 100644 (file)
--- a/mdw-mop.lisp
+++ b/mdw-mop.lisp
@@ -36,7 +36,8 @@
             #: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)
@@ -256,6 +257,22 @@
   (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
@@ -269,6 +286,7 @@
     features."))
 
 (defclass abstract-class (mdw-class abstract-class-mixin) ())
+(defclass singleton-class (mdw-class singleton-class-mixin) ())
 
 ;;;--------------------------------------------------------------------------
 ;;; Printing things.