Visitor

Intent, Motivation, Applicability, Structure, Participants, Collaborations, Consequences, Implementation, Sample Code, Known Uses, Related Patterns

Known Uses

The Smalltalk-80 compiler has a Visitor class called ProgramNodeEnumerator. It's used primarily for algorithms that analyze source code. It isn't used for code generation or pretty-printing, although it could be.

IRIS Inventor is a toolkit for developing 3-D graphics applications. Inventor represents a three-dimensional scene as a hierarchy of nodes, each representing either a geometric object or an attribute of one. Operations like rendering a scene or mapping an input event require traversing this hierarchy in different ways. Inventor does this using visitors called ``actions.'' There are different visitors for rendering, event handling, searching, filing, and determining bounding boxes.

To make adding new nodes easier, Inventor implements a double-dispatch scheme for C++. The scheme relies on run-time type information and a two-dimensional table in which rows represent visitors and columns represent node classes. The cells store a pointer to the function bound to the visitor and node class.

Mark Linton coined the term ``Visitor'' in the X Consortium's Fresco Application Toolkit specification.

Known Uses of Abstract Factory, Adapter, Bridge, Builder, Chain of Responsibility, Command, Composite, Decorator, Facade, Factory Method, Flyweight, Interpreter, Iterator, Mediator, Memento, Observer, Prototype, Proxy, Singleton, State, Strategy, Template Method, Visitor