Design Patterns #12: Let the Visitor In — A Deep Dive into the Visitor Pattern.

The Visitor pattern separates algorithms from object structures, allowing operations on complex class hierarchies without modification. An ordinary file system example involves creating base and concrete file/folder classes, each containing size and report methods. This approach is difficult to scale, requiring modifications to each class for new actions. The Visitor pattern defines interfaces for file system elements and visitors, delegating actions to concrete visitor implementations. File system elements accept visitors, which handle specific operations like size calculation and reporting. This approach enhances scalability, as new actions are added as new visitors without altering the core classes. The visitor design involves building a tree hierarchy and then applying specific visitors for actions like generating file reports and calculating total size. Using visitors allows you to add actions by creating new classes. The traditional approach requires modification to existing classes when new actions are needed. The Visitor pattern promotes a cleaner separation of concerns.