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Visitor

The Visitor design pattern is a behavioral pattern that enables operations to be performed on elements of an object structure without modifying their classes. It achieves this by separating algorithms from the object structure, allowing new operations to be added without changing the existing class hierarchy. Visitor implements double dispatch.

Designing a Vehicle Inspection System

Imagine you are developing a smart vehicle inspection system that mechanics and car service providers can use to analyze different car components—wheels, body, and engine. Instead of modifying each car part's code every time a new inspection method is introduced, you want a flexible design where new operations (like checking for damage, printing reports, or applying maintenance tasks) can be added without changing the core car structure.

The following UML describes a Car structure where different parts (Wheel, Body, Engine) accept visitors implementing operations such as printing details (CarElementPrintVisitor) or performing maintenance (CarElementDoVisitor).

This pattern is useful when operations change more frequently than the object structure, ensuring better maintainability and extensibility in C++ applications.

Key Components:

  1. Visitor Interface (CarElementVisitor) – Declares visit methods for different element types.
  2. Concrete Visitors (CarElementDoVisitor, CarElementPrintVisitor) – Implement specific behaviors for each element type.
  3. Element Interface (CarElement) – Declares an accept method that takes a visitor.
  4. Concrete Elements (Wheel, Body, Engine, Car) – Implement accept by calling visitor->visit(this).
  5. Object Structure (Car) – Contains elements and provides a way to iterate over them.

PlantUML model plantuml code

What's Happening in Our Code? What Are We Achieving?

Our software models a car as a collection of parts (Wheel, Body, Engine), where each part can be visited by different inspection or action handlers (visitors).

  • A Print Visitor (CarElementPrintVisitor) logs the details of each part.
  • A Maintenance Visitor (CarElementDoVisitor) performs specific actions on each part (e.g., kicking the wheels to check stability, starting the engine, moving the body).
  • New visitors (like a Damage Assessment Visitor or a Cost Estimator Visitor) can be easily added in the future without modifying the core car structure.

This approach ensures extensibility and maintainability in our car inspection system.

How It Works

  1. Car Components (CarElement) Are Defined

    • Wheel, Body, Engine, and Car inherit from CarElement.
    • Each component has an accept function, allowing visitors to interact with them.

  2. Visitors Implement Operations on Components

    • CarElementPrintVisitor logs car part details.
    • CarElementDoVisitor performs operations like checking the engine or kicking the wheels.
    • Future visitors could inspect tire pressure, detect rust, or estimate repair costs.

  3. Car Accepts a Visitor

    • When the accept function is called on Car, it loops through all its components and applies the visitor logic to each one.

  4. Double Dispatch Ensures Correct Behavior

    • Each visitor automatically detects the correct car part type (e.g., visit(Wheel), visit(Engine)), ensuring type-safe, structured operations.

Key Benefits of This Approach

✅ Extensibility – Easily add new operations (visitors) without modifying car components.
✅ Separation of Concerns – Keeps data (car parts) separate from behavior (inspection, logging, maintenance).
✅ Scalability – New visitors can be added for different functionalities (e.g., AI-driven diagnostics, cost estimation).
✅ Reusability – The same visitor pattern can be applied to other vehicle models (trucks, motorcycles, airplanes).
✅ Maintainability – Modifications to operations don’t require changes to car parts, reducing risk of breaking existing code.

✅When to Use the Visitor Pattern:

  1. When You Need to Perform Operations on a Complex Object Structure

    • If you have a hierarchical structure (e.g., a composite tree) where you need to apply multiple operations without modifying the structure.
    • Example: A Car consisting of Wheel, Body, and Engine, where each part requires multiple operations like printing or maintenance.

  2. When You Want to Add New Operations Without Modifying Existing Classes

    • If adding operations directly to element classes would violate Open/Closed Principle (OCP).
    • Example: You need to add logging, analytics, or transformation functions to elements without changing their class definitions.

  3. When You Need Double Dispatch

    • If operations depend on both the type of visitor and the type of visited element.
    • Example: Different visitors (CarElementPrintVisitor, CarElementDoVisitor) should behave differently for each part (Wheel, Engine).

❌When NOT to Use the Visitor Pattern:

  1. When the Object Structure Changes Frequently

    • If the set of elements (e.g., Wheel, Body, Engine) changes often, you will need to update all visitors, making maintenance difficult.
    • Example: If you're frequently adding/removing components in a CAD software system.

  2. When There’s No Need for Multiple Operations on the Same Object Structure

    • If you only need one operation, embedding it directly in the element class is simpler and avoids unnecessary complexity.

  3. When Type Safety is a Concern

    • In languages like C++, using dynamic_cast or type-checking mechanisms in visitors can introduce runtime overhead and potential safety issues.

🔗Related Patterns:

  1. Composite

    • Used when elements form a hierarchical structure (e.g., a Car is composed of Wheels, Body, and Engine).
    • The Visitor pattern works well with Composite by allowing traversal and operations on the structure.

  2. Iterator

    • Used to traverse elements in a collection.
    • Visitor can be combined with Iterator to apply operations while iterating over elements.

  3. Strategy

    • Defines interchangeable algorithms (like visitors) without modifying the structure.
    • The difference: Strategy focuses on algorithm selection, while Visitor focuses on operation dispatch based on element types.

  4. Interpreter

    • If the structure represents a language (e.g., an AST for parsing), Visitor can define operations like evaluation or optimization.

By considering these factors, you can decide if Visitor is the right choice for your application! 🚀

Source code examples: car visitor, color visitor, shopping item_visitor, document visitor, file visitor, stock visitor, expression printer