X-Git-Url: https://git.distorted.org.uk/~mdw/lisp/blobdiff_plain/4b6a6387f82f1bb2058d642dcbd7040ce2578cea..20be6570feeace0e60cd9993eb3668ade3e8edc3:/mdw-mop.lisp

diff --git a/mdw-mop.lisp b/mdw-mop.lisp
index a669490..160eff9 100644
--- a/mdw-mop.lisp
+++ b/mdw-mop.lisp
@@ -27,21 +27,43 @@
 ;;; Packages.
 
 (defpackage #:mdw.mop
-  (:use #:common-lisp #:mdw.base #+cmu #:pcl)
-  (:export #:compatible-class
-	   #:copy-instance #:copy-instance-using-class
+  (:use #:common-lisp #:mdw.base #+cmu #:mop)
+  (:export #:copy-instance #:copy-instance-using-class
+	   #:compatible-class
 	   #:initargs-for-effective-slot #:make-effective-slot
 	   #:filtered-slot-class-mixin
 	     #:filtered-direct-slot-definition
 	     #: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)
 
 ;;;--------------------------------------------------------------------------
+;;; Copying instances.
+
+(defgeneric copy-instance-using-class (class object &rest initargs)
+  (:documentation
+   "Does the donkey-work behind copy-instance."))
+
+(defmethod copy-instance-using-class
+    ((class standard-class) object &rest initargs)
+  (let ((new (apply #'allocate-instance class initargs)))
+    (dolist (slot (class-slots class))
+      (setf (slot-value-using-class class new slot)
+	    (slot-value-using-class class object slot)))
+    (apply #'shared-initialize new nil initargs)
+    new))
+
+(defun copy-instance (object &rest initargs)
+  "Make a copy of OBJECT, modifying it by setting slots as requested by
+   INITARGS."
+  (apply #'copy-instance-using-class (class-of object) object initargs))
+
+;;;--------------------------------------------------------------------------
 ;;; Basic stuff.
 
 (defclass compatible-class (standard-class)
@@ -64,27 +86,6 @@
   (eq (class-of sub) (find-class 'standard-class)))
 
 ;;;--------------------------------------------------------------------------
-;;; Copying instances.
-
-(defgeneric copy-instance-using-class (class object &rest initargs)
-  (:documentation
-   "Does the donkey-work behind copy-instance."))
-
-(defmethod copy-instance-using-class
-    ((class standard-class) object &rest initargs)
-  (let ((new (apply #'allocate-instance class initargs)))
-    (dolist (slot (class-slots class))
-      (setf (slot-value-using-class class new slot)
-	    (slot-value-using-class class object slot)))
-    (apply #'shared-initialize new nil initargs)
-    new))
-
-(defun copy-instance (object &rest initargs)
-  "Make a copy of OBJECT, modifying it by setting slots as requested by
-   INITARGS."
-  (apply #'copy-instance-using-class (class-of object) object initargs))
-
-;;;--------------------------------------------------------------------------
 ;;; Utilities for messing with slot options.
 
 (defgeneric initargs-for-effective-slot (class direct-slots)
@@ -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.