Practical Uses of the Visitor Pattern
When I first read about the visitor pattern I did not understand why it worked or how it would be useful. Now, many years later, I finally understand why it works, but until recently I haven't encountered a situation where the visitor pattern was superior to alternative patterns. Today I came across a problem in which the visitor pattern actually seemed to offer some value in real code. A data contract specifies a message containing a member with an abstract data type. The caller is expected to provide a concrete implementation of that abstract type (one of several known DTO types) in the message. When the message is received, the concrete object must be mapped onto a corresponding domain object. Then after the request has completed, the domain object must be mapped back onto the DTO type for inclusion in the response message. Both the DTO type and the domain object type are polymorphic, so the concrete implementations are not at all important to the domain. The goals ar