Chapter 17, The Visitor Pattern and Multiple Dispatch

1. The Visitor pattern provides a way to separate the implementation of algorithms from the objects they operate on; in other words, it is a way to add operations to classes without modifying them by writing new member functions.
2. The Visitor pattern allows us to extend the functionality of class hierarchies. It can be used when the source code of the class is not available for modification or when such modifications would be difficult to maintain.
3. Double dispatch is the process of dispatching a function call (selecting the algorithm to run) based on two factors. Double dispatch can be implemented at runtime using the Visitor pattern (virtual functions provide the single dispatch) or at compile time using templates or compile-time visitors.
4. The classic visitor has a circular dependency between the visitor class hierarchy and the visitable class hierarchy. While the visitable classes do not need to be edited when a new...