Visitor pattern
In object-oriented programming and software engineering, the visitor design pattern is a way of separating an algorithm from an object structure on which it operates. A practical result of this separation is the ability to add new operations to existing object structures without modifying the structures. It is one way to follow the open/closed principle.
In essence, the visitor allows adding new virtual functions to a family of classes, without modifying the classes. Instead, a visitor class is created that implements all of the appropriate specializations of the virtual function. The visitor takes the instance reference as input, and implements the goal through double dispatch.
Overview
The Visitor [1] design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
What problems can the Visitor design pattern solve?[2]
- It should be possible to define a new operation for (some) classes of an object structure without changing the classes.
When new operations are needed frequently and the object structure consists of many unrelated classes, it's inflexible to add new subclasses each time a new operation is required because "[..] distributing all these operations across the various node classes leads to a system that's hard to understand, maintain, and change."[1]
What solution does the Visitor design pattern describe?
- Define a separate (visitor) object that implements an operation to be performed on elements of an object structure.
- Clients traverse the object structure and call a dispatching operation accept (visitor) on an element — that "dispatches" (delegates) the request to the "accepted visitor object". The visitor object then performs the operation on the element ("visits the element").
This makes it possible to create new operations independently from the classes of an object structure by adding new visitor objects.
See also the UML class and sequence diagram below.
Definition
The Gang of Four defines the Visitor as:
Represent[ing] an operation to be performed on elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
The nature of the Visitor makes it an ideal pattern to plug into public APIs, thus allowing its clients to perform operations on a class using a "visiting" class without having to modify the source.[3]
Uses
Moving operations into visitor classes is beneficial when
- many unrelated operations on an object structure are required,
- the classes that make up the object structure are known and not expected to change,
- new operations need to be added frequently,
- an algorithm involves several classes of the object structure, but it is desired to manage it in one single location,
- an algorithm needs to work across several independent class hierarchies.
A drawback to this pattern, however, is that it makes extensions to the class hierarchy more difficult, as new classes typically require a new visit method to be added to each visitor.
Related design patterns
- Iterator pattern – defines a traversal principle like the visitor pattern, without making a type differentiation within the traversed objects
- Church encoding – a related concept from functional programming, in which tagged union/sum types may be modeled using the behaviors of "visitors" on such types, and which enables the visitor pattern to emulate variants and patterns.
See also
References
- ^ a b Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides (1994). Design Patterns: Elements of Reusable Object-Oriented Software. Addison Wesley. pp. 331ff. ISBN 0-201-63361-2.
{{cite book}}: CS1 maint: multiple names: authors list (link) - ^ "The Visitor design pattern - Problem, Solution, and Applicability". w3sDesign.com. Retrieved 2017-08-12.
- ^ Visitor pattern real-world example
External links
- The Visitor Family of Design Patterns at the Wayback Machine (archived October 22, 2015). Additional archives: April 12, 2004, March 5, 2002. A rough chapter from The Principles, Patterns, and Practices of Agile Software Development, Robert C. Martin, Prentice Hall
- Visitor pattern in UML and in LePUS3 (a Design Description Language)
- Article "Componentization: the Visitor Example by Bertrand Meyer and Karine Arnout, Computer (IEEE), vol. 39, no. 7, July 2006, pages 23-30.
- Article A Type-theoretic Reconstruction of the Visitor Pattern
- Article "The Essence of the Visitor Pattern" by Jens Palsberg and C. Barry Jay. 1997 IEEE-CS COMPSAC paper showing that accept() methods are unnecessary when reflection is available; introduces term 'Walkabout' for the technique.
- Article "A Time for Reflection" by Bruce Wallace – subtitled "Java 1.2's reflection capabilities eliminate burdensome accept() methods from your Visitor pattern"
- Visitor Pattern using reflection(java).
- PerfectJPattern Open Source Project, Provides a context-free and type-safe implementation of the Visitor Pattern in Java based on Delegates.
- Visitor Design Pattern