Visitor

Imagine The Watcher from Marvel's "What If...?" series, who observes countless heroes and villains across the multiverse without interfering with their existence. He visits each character, assesses their abilities, and records his observations – all without changing who they are. This is essentially what the Visitor pattern does in software design.

The Visitor pattern allows you to add new operations to existing object structures without modifying those structures. It's like giving Nick Fury the ability to visit each Avenger, evaluate their unique powers, and create custom missions for them – all without changing anything about the heroes themselves.

What Problem Does It Solve?

The Visitor pattern addresses several challenging scenarios:

1. Adding operations without modifying classes: You need to perform new operations on elements of a stable object structure, but you don't want to change those objects
2. Operations that don't belong in the element classes: Some operations don't conceptually belong to the elements they operate on
3. Related operations spread across an object hierarchy: When similar operations need to be performed across different object types in a hierarchy
4. Accumulating state while traversing: When you need to gather information from multiple objects in a structure

It's like how S.H.I.E.L.D. needs to evaluate the Avengers without asking them to change their core abilities or self-assessment methods – they just send in an agent (a visitor) to collect the needed information.

Structure of the Visitor Pattern

The Visitor pattern consists of these key components:

1. Visitor: An interface declaring visit methods for each concrete element class
2. ConcreteVisitor: Implements the Visitor interface with specific behavior for each element type
3. Element: An interface with an accept method that takes a visitor
4. ConcreteElement: Implements the Element interface and the accept method
5. ObjectStructure: A collection or composite that holds elements and provides a way to iterate over them

The critical part is the "double dispatch" mechanism: when a visitor visits an element, the element calls back to the appropriate visit method on the visitor, based on the element's concrete type.

Basic Implementation Example

Let's implement a simple Visitor pattern for a superhero evaluation system:

In this example, we've created a system where different types of superheroes (Asgardian, Human, Enhanced) can be evaluated by different visitors (PowerEvaluator, TeamAssigner) without changing the superhero classes themselves. Each visitor provides specialized behavior for each superhero type through the double-dispatch mechanism.

Real-World Example: Document Export System

Here's a more practical example of using the Visitor pattern to export document elements to different formats:

This example shows how the Visitor pattern can be used to implement multiple export formats for a document structure without modifying the document elements themselves. Each exporter (visitor) knows how to convert each type of element to its specific format.

Benefits and Drawbacks

Benefits

• Separation of concerns: Algorithms are separated from the objects they operate on
• Open/Closed Principle: You can add new operations without modifying existing element classes
• Accumulating state: Visitors can maintain state across the traversal of a structure
• Related operations grouped: All related operations are in one visitor, not scattered across element classes
• Double dispatch: Enables polymorphic behavior based on both the visitor and element types

Drawbacks

• Breaking encapsulation: Visitors often need access to the internals of elements
• Element hierarchy changes: Adding or changing element classes requires updating all visitors
• Increases complexity: The double-dispatch mechanism can be confusing for developers unfamiliar with the pattern
• Not suitable for frequently changing element hierarchies: Every element change affects all visitors

When to Use the Visitor Pattern

Use the Visitor pattern when:

1. You have a stable class hierarchy that rarely changes, but you frequently need to add new operations
2. Similar operations need to be performed on objects of different types within a hierarchy
3. You want to perform operations on elements of a complex structure
4. The operations you're adding don't naturally belong to the element classes

Think of it like the Grandmaster from Thor: Ragnarok – he doesn't change the contestants, he just evaluates their abilities and assigns them to different contests!

Real-World Applications

The Visitor pattern is commonly used in:

1. Compilers and interpreters (visiting AST nodes)
2. Document object models for multiple export formats
3. Static code analysis tools
4. XML/JSON processors
5. GUI frameworks for rendering different elements

Related Patterns

• Composite: Visitor is often used with Composite to operate on a tree structure
• Iterator: Often used with Visitor to traverse the object structure
• Command: Both separate operations from the objects they operate on, but in different ways
• Strategy: Visitor is like multiple strategies applied based on element type

Just as The Watcher observes but doesn't interfere with the heroes of the multiverse, the Visitor pattern allows you to observe, evaluate, and process objects without altering their core identity. It maintains the separation between "what" objects are and "what's done" to them – a powerful concept when you need to add operations to stable object structures.