--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Singleton.st	Tue May 03 18:28:18 2005 +0200
@@ -0,0 +1,87 @@
+"{ Package: 'stx:libbasic2' }"
+
+Object subclass:#Singleton
+	instanceVariableNames:''
+	classVariableNames:'Lock'
+	poolDictionaries:''
+	category:'System-Support'
+!
+
+Singleton class instanceVariableNames:'theOnlyInstance'
+
+"
+ No other class instance variables are inherited by this class.
+"
+!
+
+!Singleton class methodsFor:'documentation'!
+
+documentation
+"
+    Subclasses of Singleton have only a single instance.
+    The instance creation methods ensure that there is only one instance.
+
+    Singletons that would have inherited from Object could inherit from
+    Singleton. For Singletons that do not inherit from Object, 
+    you have to copy the Singleton's code to your class.
+
+    [author:]
+        Stefan Vogel (stefan@zwerg)
+
+    [class instance variables:]
+        theOnlyInstance     Object          The only instance of this class
+
+    [class variables:]
+        Lock                RecursionLock   A global lock to protect agains races during instance creation
+"
+! !
+
+!Singleton class methodsFor:'initialization'!
+
+initialize
+    Lock := RecursionLock new name:#Singleton
+! !
+
+!Singleton class methodsFor:'instance creation'!
+
+basicNew
+    "allocate a singleton in a thread-safe way.
+     Do lazy locking here"
+    
+    theOnlyInstance isNil ifTrue:[
+        Lock critical:[
+            theOnlyInstance isNil ifTrue:[
+                theOnlyInstance := super basicNew.
+            ].
+        ]
+    ].
+    ^ theOnlyInstance.
+!
+
+basicNew:anInteger
+    "allocate a singleton in a thread-safe way.
+     Do lazy locking here"
+    
+    theOnlyInstance isNil ifTrue:[
+        Lock critical:[
+            theOnlyInstance isNil ifTrue:[
+                theOnlyInstance := super basicNew:anInteger.
+            ].
+        ]
+    ].
+    ^ theOnlyInstance.
+! !
+
+!Singleton class methodsFor:'accessing'!
+
+theOnlyInstance
+    ^ theOnlyInstance
+! !
+
+!Singleton class methodsFor:'documentation'!
+
+version
+    ^ '$Header: /cvs/stx/stx/libbasic2/Singleton.st,v 1.1 2005-05-03 16:28:18 stefan Exp $'
+! !
+
+Singleton initialize!