The Visitor design pattern is perhaps not the best-loved of the GoF patterns. One indication of its unpopularity is its position as the last pattern in the GoF Design Patterns book. But I have found it useful in several projects, and would like to share some Visitor tricks I've learned along the way.
In C++, Visitor is an abstract base class, with a set of one-argument virtual functions -- one for each class in a forest of hierarchies -- thereby providing "multiple-dispatch" of some piece of functionality. Virtual functions are an example of single-dispatch: based on the type of an object, the correct virtual function is called. Multiple-dispatch means that a function is selected based on more than one type: in this case, the type of the visitor, and the type of the class it visits.
Have you ever been writing some code, and thought: "I wish I could add a virtual function to these classes, without changing the class interfaces"? If so, that is exactly what Visitor allows. Of course, you have to have designed the class to accept a Visitor first, but after that, you have this "external virtual function" capability.
Another way in which Visitor is helpful in big C++ projects, is that it allows you to have good source file hygiene -- as described in our coding standard -- but still keep related functionality in one place. An example is pretty-printing of an Expression hierarchy: to make consistency easier to maintain, you would like all the printing functions to be in a single file, which would violate our source file standards if implemented by virtual functions in the classes.
Sounds good, why is Visitor unpopular? There are a couple of valid complaints about the pattern. First, it can make for difficulties when adding a new class to a visited hierarchy. This is somewhat mitigated by the Default Visitor variant, and we can also use some preprocessor tricks to create a compile- or link-time error for Visitors which must be extended for some or all new classes. It's also worth mentioning that if the Visitor functionality were instead implemented as a pure virtual function, the same amount of work is required (except for tracking down all the Visitors).
Secondly, effective use of Visitors may imply a loss of encapsulation. If a Visitor is to do meaningful work on an object, it may need access to implementation details that -- all other things being equal -- we would prefer to hide inside the class.
Nevertheless, Visitor can be a very useful design pattern. In coming posts I will present some practical C++ examples for getting the most out of it.
Here is the complete list of posts in this series:
- C++ Visitor Pattern: Motivation (this post)
- C++ Visitors: Basic Implementation
- Easier-to-Use C++ Visitor
- The Default Visitor Idiom
- Handy C++ Visitor Macros