Conclusion

The VISITOR family of patterns provides us with a number of ways to modify the behavior of a hierarchy of classes without having to change them. Thus, they help us maintain the Open/Closed Principle. They also provide mechanisms for segregating various kinds of functionality, keeping classes from getting cluttered with many different functions. As such, they help us maintain the Common Closure Principle. It should be clear that LSP and DIP are also applied to the structure of the VISITOR family.

The VISITOR patterns are seductive. It is easy to get carried away with them. Use them when they help, but maintain a healthy skepticism about their necessity. Often, something that can be solved with a VISITOR can also be solved by something simpler.^[5]

^[5] Now that you've read this chapter, you may wish to go back to Chapter 9 and solve the problem of ordering the shapes.