EXTERNAL POLYMORPHISM

		   An Object Structural Pattern for
		       Transparently Extending
		       C++ Concrete Data Types

			    Chris Cleeland
			  chris@envision.com

	      Douglas C. Schmidt and Timothy H. Harrison
	    schmidt@cs.wustl.edu and harrison@cs.wustl.edu

		    Department of Computer Science
			Washington University
			   Campus Box 1045
			 One Brookings Drive
		    St. Louis, Missouri 63130-4899
			(TEL): (314) 935-7538
				   
			       ABSTRACT

This paper describes the External Polymorphism pattern, which allows
classes that are not related by inheritance and/or have no virtual
methods to be treated polymorphically.  The pattern capitalizes on C++
language features combined with other basic patterns (e.g., Adapter
and Decorator ) to give the appearance of polymorphic behavior on
otherwise unrelated classes.  This pattern has been used in a number
of C++ frameworks such as ACE and the OSE class library.