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
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?
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."
What solution does the Visitor design pattern describe?

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:
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.

Uses

Moving operations into visitor classes is beneficial when
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.

Use case example

Consider the design of a 2D computer-aided design 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.

Structure

UML class and sequence diagram

In the above UML class diagram, the ElementA class doesn't implement a new operation directly.
Instead, ElementA implements a dispatching operation accept that "dispatches" a request to the "accepted visitor object". The Visitor1 class implements the operation.
ElementB then implements accept by dispatching to visitor.visitElementB. The Visitor1 class implements the operation.
The UML sequence diagram
shows the run-time interactions: The Client object traverses the elements of an object structure and calls accept on each element.
First, the Client calls accept on
ElementA, which calls visitElementA on the accepted visitor object.
The element itself is passed to the visitor so that
it can "visit" ElementA.
Thereafter, the Client calls accept on
ElementB, which calls visitElementB on the visitor that "visits" ElementB.

Class diagram

Details

The visitor pattern requires a programming language that supports single dispatch, as common object-oriented languages do. Under this condition, consider two objects, each of some class type; one is termed the element, and the other is visitor.
The visitor declares a visit method, which takes the element as an argument, for each class of element. Concrete visitors are derived from the visitor class and implement these visit methods, each of which implements part of the algorithm operating on the object structure. The state of the algorithm is maintained locally by the concrete visitor class.
The element declares an accept method to accept a visitor, taking the visitor as an argument. Concrete elements, derived from the element class, implement the accept method. In its simplest form, this is no more than a call to the visitor's visit method. Composite elements, which maintain a list of child objects, typically iterate over these, calling each child's accept method.
The client creates the object structure, directly or indirectly, and instantiates the concrete visitors. When an operation is to be performed which is implemented using the Visitor pattern, it calls the accept method of the top-level element.
When the accept method is called in the program, its implementation is chosen based on both the dynamic type of the element and the static type of the visitor. When the associated visit method is called, its implementation is chosen based on both the dynamic type of the visitor and the static type of the element, as known from within the implementation of the accept method, which is the same as the dynamic type of the element.
Thus, the implementation of the visit method is chosen based on both the dynamic type of the element and the dynamic type of the visitor. This effectively implements double dispatch. For languages whose object systems support multiple dispatch, not only single dispatch, such as Common Lisp or C# via the Dynamic Language Runtime, implementation of the visitor pattern is greatly simplified 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 and to the single responsibility principle.
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.

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.

Dynamic visitor

This example declares a separate ExpressionPrinter class that takes care of the printing. The expression classes must expose their members to make this possible.

using System;
using System.Text;
namespace Wikipedia

Smalltalk example

In this case, it is the object's responsibility to know how to print itself on a stream. The visitor here is then the object, not the stream.

"There's no syntax for creating a class. Classes are created by sending messages to other classes."
WriteStream subclass: #ExpressionPrinter
instanceVariableNames:
classVariableNames:
package: 'Wikipedia'.
ExpressionPrinter>>write: anObject
"Delegates the action to the object. The object doesn't need to be of any special
class; it only needs to be able to understand the message #putOn:"
anObject putOn: self.
^ anObject.
Object subclass: #Expression
instanceVariableNames:
classVariableNames:
package: 'Wikipedia'.
Expression subclass: #Literal
instanceVariableNames: 'value'
classVariableNames:
package: 'Wikipedia'.
Literal class>>with: aValue
"Class method for building an instance of the Literal class"
^ self new
value: aValue;
yourself.
Literal>>value: aValue
"Setter for value"
value := aValue.
Literal>>putOn: aStream
"A Literal object knows how to print itself"
aStream nextPutAll: value asString.
Expression subclass: #Addition
instanceVariableNames: 'left right'
classVariableNames:
package: 'Wikipedia'.
Addition class>>left: a right: b
"Class method for building an instance of the Addition class"
^ self new
left: a;
right: b;
yourself.
Addition>>left: anExpression
"Setter for left"
left := anExpression.
Addition>>right: anExpression
"Setter for right"
right := anExpression.
Addition>>putOn: aStream
"An Addition object knows how to print itself"
aStream nextPut: $.
Object subclass: #Program
instanceVariableNames:
classVariableNames:
package: 'Wikipedia'.
Program>>main
| expression stream |
expression := Addition
left:
right: )
right:.
stream := ExpressionPrinter on:.
stream write: expression.
Transcript show: stream contents.
Transcript flush.

C++ example

Output


dispatching ArchivedFile
dispatching SplitFile
dispatching ExtractedFile

Go example

Go does not support overloading, so the visit methods need different names.

Output


visiting car
visiting engine
visiting body
visiting front left wheel
visiting front right wheel
visiting back left wheel
visiting back right wheel

Java example

The following example is in the language Java, and shows how the contents of a tree of nodes can be printed. Instead of creating print methods for each node subclass, one visitor class 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.

Diagram

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 car

Common Lisp example

Output

"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 ABC

Python example

Python does not support method overloading in classical sense, so the "visit" methods for the different model types need to have different names.

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 car.

Abstraction

If one is using Python 3 or above, they can make a general implementation of the accept method:

class Visitable:
def accept:
lookup = "visit_" + type.__qualname__.replace
return getattr

One could extend this to iterate over the class's method resolution order if they would like to fall back on already-implemented classes. They could also use the subclass hook feature to define the lookup in advance.

Related design patterns