The Visitor Pattern and Multiple Dispatch

The Visitor pattern is another classic object-oriented design pattern, one of the 23 patterns introduced in the book Design Patterns – Elements of Reusable Object-Oriented Software, by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides. It was one of the more popular patterns during the golden age of object-oriented programming since it can be used to make large class hierarchies more maintainable. In recent years, the use of Visitor in C++ declined, as large complex hierarchies became less common, and the Visitor pattern is a fairly complex pattern to implement. Generic programming—in particular, the new language features added in C++11 and C++14—makes it easier to implement and maintain the Visitor classes, while the new applications of the old pattern have served to rekindle some of the fading interest...