10.7 Summary

This chapter discussed the Visitor pattern and the problems it addresses. Essentially, Visitor allows you to add virtual functions to a class hierarchy without modifying the classes in that hierarchy. In some cases, Visitor can lead to a clever, extensible design.

However, Visitor has a bunch of problems, to the point that its use with any but the most stable class hierarchies is unjustifiably hard. Acyclic Visitor is of help, at the expense of a decrease in efficiency.

By using careful design and advanced implementation techniques, you can ensure that the generic Visitor components get the best out of the Visitor pattern. Although the implementation fully preserves Visitor's power, it mitigates most of its shortcomings.

In an application, Acyclic Visitor should be the variant of choice, unless you need all the speed you can get. If speed is important, you can use a generic implementation of the GoF Visitor that keeps maintenance low (a single, clear point of maintenance) and compile-time cost reasonable.

The generic implementation leverages advanced C++ programming techniques, such as dynamic casts, typelists, and partial specialization. The outcome is that all the common, repetitive parts of implementing visitors for any concrete need have been absorbed by the library.