1.4.0

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@520 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/singleton.rb

@@ -0,0 +1,37 @@
+#  Singleton module that ensures only one object to be allocated.
+#
+# Usage:
+#   class SomeSingletonClass
+#     include Singleton
+#    #....
+#   end
+#   a = SomeSingletonClass.instance
+#   b = SomeSingletonClass.instance	# a and b are same object
+#   p [a,b]
+#   a = SomeSingletonClass.new		# error (`new' is private)
+
+module Singleton
+  def Singleton.append_features(klass)
+    klass.private_class_method(:new)
+    klass.instance_eval %{
+      def instance
+	unless @__instance__
+	  @__instance__ = new
+	end
+	return @__instance__
+      end
+    }
+  end
+end
+
+if __FILE__ == $0
+  class SomeSingletonClass
+    include Singleton
+    #....
+  end
+
+  a = SomeSingletonClass.instance
+  b = SomeSingletonClass.instance	# a and b are same object
+  p [a,b]
+  a = SomeSingletonClass.new		# error (`new' is private)
+end