Summary

In this chapter, we learned about the Visitor pattern and the different ways it can be implemented in C++. The classic object-oriented Visitor pattern allows us to effectively add a new virtual function to the entire class hierarchy without changing the source code of the classes. The hierarchy must be made visitable, but after that, any number of operations can be added, and their implementation is kept separate from the objects themselves. In the classic Visitor pattern implementation, the source code containing the visited hierarchy does not need to be changed, but it does need to be recompiled when a new class is added to the hierarchy. The Acyclic Visitor pattern solves this problem but at the cost of the additional dynamic cast. On the other hand, the Acyclic Visitor pattern also supports partial visitation - ignoring some visitor/visitable combinations - while the classic Visitor pattern requires that all combinations must at least be declared.

For all visitor variants...