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 extant object structures without modifying the structures. It is one way to follow the open/closed principle.
Contents
- Definition
- Motivation
- Details
- C example
- Classic visitor
- Dynamic Visitor
- Output
- Java example
- Sources
- Notes
- Related design patterns
- References
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.
Definition
The Gang of Four defines the Visitor as:
Represent 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.
Motivation
Consider the design of a 2D computer-aided design (CAD) system. At its core there are several types to represent basic geometric shapes like circles, lines, and arcs. The entities are ordered into layers, and at the top of the type hierarchy is the drawing, which is simply a list of layers, plus some added properties.
A fundamental operation on this type hierarchy is saving a drawing to the system's native file format. At first glance it may seem acceptable to add local save methods to all types in the hierarchy. But it is also useful to be able to save drawings to other file formats. Adding ever more methods for saving into many different file formats soon clutters the relatively pure original geometric data structure.
A naive way to solve this would be to maintain separate functions for each file format. Such a save function would take a drawing as input, traverse it, and encode into that specific file format. As this is done for each added different format, duplication between the functions accumulates. For example, saving a circle shape in a raster format requires very similar code no matter what specific raster form is used, and is different from other primitive shapes. The case for other primitive shapes like lines and polygons is similar. Thus, the code becomes a large outer loop traversing through the objects, with a large decision tree inside the loop querying the type of the object. Another problem with this approach is that it is very easy to miss a shape in one or more savers, or a new primitive shape is introduced, but the save routine is implemented only for one file type and not others, leading to code extension and maintenance problems.
Instead, the Visitor pattern can be applied. It encodes a logical operation on the whole hierarchy into one class containing one method per type. In the CAD example, each save function would be implemented as a separate Visitor subclass. This would remove all duplication of type checks and traversal steps. It would also make the compiler complain if a shape is omitted.
Another motive is to reuse iteration code. For example, iterating over a directory structure could be implemented with a visitor pattern. This would allow creating file searches, file backups, directory removal, etc., by implementing a visitor for each function while reusing the iteration code.
Details
The visitor pattern requires a programming language that supports single dispatch. Under this condition, consider two objects, each of some class type; one is termed the element, and the other is visitor. An element has an accept method that can take the visitor as an argument. The accept method calls a visit method of the visitor; the element passes itself as an argument to the visit method. Thus:
accept method is called in the program, its implementation is chosen based on both:visit method is called, its implementation is chosen based on both:accept method, which is the same as the dynamic type of the element. (As a bonus, if the visitor can't handle an argument of the given element's type, then the compiler will catch the error.)visit method is chosen based on both:In this way, one algorithm can be written to traverse a graph of elements, and many different kinds of operations can be performed during that traversal by supplying different kinds of visitors to interact with the elements based on the dynamic types of both the elements and the visitors.
Some languages (e.g., C# via the Dynamic Language Runtime (DLR)) support multiple dispatch, which greatly simplifies implementing the Visitor pattern (a.k.a. Dynamic Visitor) by allowing use of simple function overloading to cover all the cases being visited. A dynamic visitor, provided it operates on public data only, conforms to the open/closed principle (since it does not modify extant structures) and to the single responsibility principle (since it implements the Visitor pattern in a separate component).
C# example
This example shows how to print a tree representing a numeric expression involving literals and their addition. The same example is presented using both classic and Dynamic Language Runtime implementations.
Classic visitor
A classic visitor where the Print operations for each type are implemented in one ExpressionPrinter class as a number of overloads of the Visit method.
Dynamic Visitor
This example declares a separate ExpressionPrinter class that takes care of the printing. Note that the expression classes have to expose their members to make this possible.
Output
dispatching ArchivedFile dispatching SplitFile dispatching ExtractedFileJava example
The following example is in the language Java, and shows how the contents of a tree of nodes (in this case describing the components of a car) can be printed. Instead of creating print methods for each node subclass (Wheel, Engine, Body, and Car), one visitor class (CarElementPrintVisitor) performs the required printing action. Because different node subclasses require slightly different actions to print properly, CarElementPrintVisitor dispatches actions based on the class of the argument passed to its visit method. CarElementDoVisitor, which is analogous to a save operation for a different file format, does likewise.
Sources
A more flexible approach to this pattern is to create a wrapper class implementing the interface defining the accept method. The wrapper contains a reference pointing to the ICarElement that could be initialized through the constructor. This approach avoids having to implement an interface on each element. See article Java Tip 98 article below
Output
Visiting front left wheel Visiting front right wheel Visiting back left wheel Visiting back right wheel Visiting body Visiting engine Visiting car Kicking my front left wheel Kicking my front right wheel Kicking my back left wheel Kicking my back right wheel Moving my body Starting my engine Starting my carOutput
"front-left-wheel" "front-right-wheel" "rear-right-wheel" "rear-right-wheel" "body" "engine" kicking wheel "front-left-wheel" 42 times kicking wheel "front-right-wheel" 42 times kicking wheel "rear-right-wheel" 42 times kicking wheel "rear-right-wheel" 42 times don't know how "body" and 42 should interact starting engine "engine" 42 times kicking wheel "front-left-wheel" symbolically using symbol ABC kicking wheel "front-right-wheel" symbolically using symbol ABC kicking wheel "rear-right-wheel" symbolically using symbol ABC kicking wheel "rear-right-wheel" symbolically using symbol ABC don't know how "body" and ABC should interact starting engine "engine" symbolically using symbol ABCNotes
The other-object parameter is superfluous in traverse. The reason is that it is possible to use an anonymous function that calls the desired target method with a lexically captured object:
Now, the multiple dispatch occurs in the call issued from the body of the anonymous function, and so traverse is just a mapping function that distributes a function application over the elements of an object. Thus all traces of the Visitor Pattern disappear, except for the mapping function, in which there is no evidence of two objects being involved. All knowledge of there being two objects and a dispatch on their types is in the lambda function.