visitor pattern

S

Visitor Pattern

Yateen L. Nikharge

Aravindh Manickavasagam

Ider Zheng

The VISITOR Family

The Visitor family allows new methods to be addedto existing hierarchies without modifying thehierarchies.

The Patterns in this family are:

S VISITOR

S ACYCLIC VISITOR

S DECORATOR

S EXTENSION OBJECT

Intent

S Represent an operation to be performed on the elements of an object structure.

S Visitor lets you define a new operation without changing the classes of the elements on which it operates.

Motivation

S Decouple the data structure and algorithms.

S Allow addition of new algorithms without changing the data structure.

S Example: Modem Configuration

S To configure a modem for Unix, create an instance of the visitor and pass it to accept.

S The appropriate derivative call visit(this)

S New OS configuration can be added by adding a new derivative of visitor.

Motivation (contd…)

Motivation(contd…)

Applicability

Use the pattern when-

S An object structure contains many classes of objects with differing interfaces…

S Many distinct and unrelated operations need to be performed on objects in an object structure…

S The classes defining the object structure rarely change, but you often want to define new operations over the structure…

Structure

Participants

S Visitor (ModemVisitor): Declares a Visit operation or each class…

S ConcreteVisitor (ConfigureUnixVisitor):

Implements each Visit…

S Element (Modem) :Defines an Accept operation…

S ConcreteElement(ZoomModem): Implements Accept …

Participants(Contd…)

S ObjectStructure (Program):

May provide a high-level interface to allow the visitor to visit its

elements .

Collaborations

S A client using the Visitor pattern must create a ConcreteVisitor

object and traverse the object structure, visiting each element

with the visitor.

S When an element is visited, it calls the visitor operation that

corresponds to its class. The element supplies itself as an

argument to this operation to let the visitor access its state, if

necessary.

Collaborations cont.

Consequences-Benefits

S Adding new operations

S Gathers related operations and separates unrelated ones

S Visitors can visit objects that don’t have a common parent

class

S Visitors can accumulate state as they visit each element

Consequences - Liabilities

S Adding new ConcreteElement classes is hard

S Allowing a Visitor to access the internal state of a

Concrete Element breaks encapsulation

Implementation - Issues

S Double Dispatch

S Who is Responsible for Traversing the Object Structure?

S Object Structure

S Visitor – whenever operations depend on other operations

on the object structure

S Iterator Object

Sam

ple

Cod

es

Implementation

Function Pointer

Implementation

Global Function

Implementation

Static Methods

Functor

Implementation

Graphic Project

From CSE 687 Objected Oriented Design

Graphic

“v1” “v3”

“v4” “v7”

“v2”

“v5”

“v6”“e8”

“e3”

“e6”

“e7”“e4”

“e1”

“e5”

“e9”“e2”

Known Uses

S Smalltalk-80 Compiler

S IRIS Inventor Toolkit

S X Consortium’s Fresco Application Toolkit

S Bistro Programming Language Compiler

Related Patterns

S Composite

S Interpreter

References

S Design Patterns: Elements of Reusable Object-Oriented

Software

By Erich Gamma, Richard Helm, Ralph

Johnson, and John Vlissides

Shttp://www.codeproject.com/KB/aspnet/SoftArch3.aspx