Visitor

Revision as of 06:16, 19 August 2008

Introduction

The Visitor is a behavioural pattern. It allows the designer to define new operations on an object structure and its elements, without modifying the object structure itself.

Structure

The Visitor describes the relations between three distinct classes:

• ObjectStructure - Provides a visitor with access to its elements
• Element - Accepts visits.
• Visitor - Accesses an element to perform operations.

Figure 1 graphically describes these Visitor pattern relationships.

Figure 1: A UML 2.1 class diagram describing the Visitor structure

Communication

The Visitor pattern allows a visitor object to access and perform operations on an element object. A summary of the Visitor pattern communications is given in Fig. 2. Here the following interactions are shown:

• Sequence 1 - The object aVisitor1 performs an operation on anElementA
• Sequence 2 - The object aVisitor2 performs an operation on anElementB
• Sequence 3 - The object aVisitor1 performs an operation on anElementA
• Sequence 4 - The object aVisitor2 performs an operation on anElementB

Figure 2: A UML 2.1 communication diagram describing the interactions between objects in the Visitor pattern

Consequences

Benefits

1. Adding new operations is easy. Just add a new Visitor class
2. Separates unrelated behaviours and gathers related ones
3. Visit across more than one class hierarchy

Liabilities

1. Adding a new ConreteElement is difficult
2. Elements may expose otherwise private attributes