TITLE:    VARIATIONS ON THE VISITOR PATTERN

AUTHOR:   Martin E. Nordberg III

E-MAIL:   102571.1562@compuserve.com

ADDRESS:  Quintessoft Engineering, Inc.
          2640 Towner Blvd.
          Ann Arbor, MI  48104

PHONE:    313-971-7917

ABSTRACT:

     A short catalog of variations on the Visitor pattern is presented.  

     A Polymorphic Visitor provides default handlers for cases where the
polymorphism of the hierarchy of elements can reduce the cost of code 
maintenance.  A Dynamic Visitor implements double dispatch with run time 
type information instead of Accept( ) methods.  With the same machinery it 
is possible to test the feasibility of a particular visit before performing 
its operation.  Design  tradeoffs determine the choice of the standard, 
polymorphic, or dynamic visitor pattern in an application.

     A Multiple Visitor extends double dispatch to multiple dispatch, 
generally in response to different design forces.  A Traversal Visitor 
introduces a two-layered structure of visitors to handle object structure 
traversals that depend upon starting point.

OUTLINE:

     Introduction
     Polymorphic Visitor
     Dynamic Visitor
     Choosing a Visitor Pattern Variation
     Multiple Visitor
     Traversal Visitor
     Concluding Thoughts