Don't knock. You are accepted TL;DR: Avoid combining the Visitor pattern with instanceof checks. Problems 😔 Open/Closed principle violation
Tight Coupling
Maintainability
Code duplication
Poor readability
Brittle design
IFs Solutions 😃 Implement the Visitor pattern correctly.
Avoid instanceof checks.
Favor polymorphism.
Encapsulate behavior. Context 💬 When you use the Visitor pattern, you aim to separate algorithms from the objects they operate on. Combining it with instanceof checks breaks this separation, leading to a fragile and hard-to-maintain design. Sample Code 📖 Wrong 🚫 <?php

class SpaceObjectVisitor {
    public function visit(SpaceObject $object) {
        if ($object instanceof NeutronStar) {
            $this->visitNeutronStar($object);
        } elseif ($object instanceof Magnetar) {
            $this->visitMagnetar($object);
        } elseif ($object instanceof BlackHole) {
            $this->visitBlackHole($object);
        } 
        // Not closed for modification
    }

    private function visitNeutronStar(NeutronStar $star) {
        // Handle neutron star observation
    }

    private function visitMagnetar(Magnetar $magnetar) {
        // Handle magnetar observation
    }

    private function visitBlackHole(BlackHole $blackHole) {
        // Handle black hole observation
    }
} Right 👉 <?php

interface SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void;
}

class NeutronStar implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void {
        $visitor->visitNeutronStar($this);
    }
}

class Magnetar implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void {
        $visitor->visitMagnetar($this);
    }
}

class BlackHole implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor) {
        $visitor->visitBlackHole($this);
    }
}

class SpaceObjectVisitor {
    // Open for extension 
    // You can also convert it into an interface
    public function visitNeutronStar(NeutronStar $star): void {
        // Handle neutron star
    }

    public function visitMagnetar(Magnetar $magnetar): void {
        // Handle magnetar
    }

    public function visitBlackHole(BlackHole $blackHole): void {
        // Handle black hole
    }
} Detection 🔍 You can detect this smell by looking for instanceof checks within the Visitor pattern. Automated tools can flag these checks, and code reviews can help identify them. Manual Tags 🏷️ IFs Level 🔋 Intermediate Why the Bijection Is Important 🗺️ When you model real-world objects, you should maintain a one-to-one correspondence between the real-world entities and your code. Breaking this correspondence using instanceof checks (which do not exist on the model but the metamodel) leads to a mismatch between the real world and your program. AI Generation 🤖 AI generators might create this smell if they are not properly guided. AI Detection 🥃 AI can help fix this smell by suggesting refactoring steps to remove instanceof checks and use polymorphism instead. Try Them! 🛠 Remember: AI Assistants make lots of mistakes Without Proper Instructions

With Specific Instructions



ChatGPT

ChatGPT



Claude

Claude



Perplexity

Perplexity



Copilot

Copilot



Gemini

Gemini



DeepSeek

DeepSeek



Meta AI

Meta AI Conclusion ✔️ When you use the Visitor pattern, avoid combining it with iskindOf checks. Using the accept() method, the visitor class doesn't need to explicitly check the object type. Each object will call the appropriate visitor method honoring the Open/Closed principle. Favor polymorphism and encapsulation to keep your code clean and maintainable. Relations 👩‍❤️‍💋‍👨 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-v-evj3zs9 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-v-evj3zs9 More Information 📕 https://en.wikipedia.org/wiki/Open–closed_principle https://hackernoon.com/how-to-get-rid-of-annoying-ifs-forever-zuh3zlo?embedable=true Disclaimer 📘 Code Smells are my opinion. Credits 🙏 Photo by Braňo on Unsplash The Visitor pattern is a way to add new operations to a set of objects without changing their classes. Erich Gamma https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true This article is part of the CodeSmell Series. https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-i-xqz3evd?embedable=true Don't knock. You are accepted Don't knock. You are accepted TL;DR: Avoid combining the Visitor pattern with instanceof checks. TL;DR: Avoid combining the Visitor pattern with instanceof checks. instanceof Problems 😔 Open/Closed principle violation Tight Coupling Maintainability Code duplication Poor readability Brittle design IFs Open/Closed principle violation Tight Coupling Coupling Maintainability Code duplication Poor readability Brittle design IFs IFs Solutions 😃 Implement the Visitor pattern correctly. Avoid instanceof checks. Favor polymorphism. Encapsulate behavior. Implement the Visitor pattern correctly. Visitor pattern Avoid instanceof checks. instanceof instanceof Favor polymorphism. Encapsulate behavior. Context 💬 When you use the Visitor pattern, you aim to separate algorithms from the objects they operate on. Visitor separate algorithms Combining it with instanceof checks breaks this separation, leading to a fragile and hard-to-maintain design . instanceof fragile and hard-to-maintain design Sample Code 📖 Wrong 🚫 <?php

class SpaceObjectVisitor {
    public function visit(SpaceObject $object) {
        if ($object instanceof NeutronStar) {
            $this->visitNeutronStar($object);
        } elseif ($object instanceof Magnetar) {
            $this->visitMagnetar($object);
        } elseif ($object instanceof BlackHole) {
            $this->visitBlackHole($object);
        } 
        // Not closed for modification
    }

    private function visitNeutronStar(NeutronStar $star) {
        // Handle neutron star observation
    }

    private function visitMagnetar(Magnetar $magnetar) {
        // Handle magnetar observation
    }

    private function visitBlackHole(BlackHole $blackHole) {
        // Handle black hole observation
    }
} <?php

class SpaceObjectVisitor {
    public function visit(SpaceObject $object) {
        if ($object instanceof NeutronStar) {
            $this->visitNeutronStar($object);
        } elseif ($object instanceof Magnetar) {
            $this->visitMagnetar($object);
        } elseif ($object instanceof BlackHole) {
            $this->visitBlackHole($object);
        } 
        // Not closed for modification
    }

    private function visitNeutronStar(NeutronStar $star) {
        // Handle neutron star observation
    }

    private function visitMagnetar(Magnetar $magnetar) {
        // Handle magnetar observation
    }

    private function visitBlackHole(BlackHole $blackHole) {
        // Handle black hole observation
    }
} Right 👉 <?php

interface SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void;
}

class NeutronStar implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void {
        $visitor->visitNeutronStar($this);
    }
}

class Magnetar implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void {
        $visitor->visitMagnetar($this);
    }
}

class BlackHole implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor) {
        $visitor->visitBlackHole($this);
    }
}

class SpaceObjectVisitor {
    // Open for extension 
    // You can also convert it into an interface
    public function visitNeutronStar(NeutronStar $star): void {
        // Handle neutron star
    }

    public function visitMagnetar(Magnetar $magnetar): void {
        // Handle magnetar
    }

    public function visitBlackHole(BlackHole $blackHole): void {
        // Handle black hole
    }
} <?php

interface SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void;
}

class NeutronStar implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void {
        $visitor->visitNeutronStar($this);
    }
}

class Magnetar implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor): void {
        $visitor->visitMagnetar($this);
    }
}

class BlackHole implements SpaceObject {
    public function accept(SpaceObjectVisitor $visitor) {
        $visitor->visitBlackHole($this);
    }
}

class SpaceObjectVisitor {
    // Open for extension 
    // You can also convert it into an interface
    public function visitNeutronStar(NeutronStar $star): void {
        // Handle neutron star
    }

    public function visitMagnetar(Magnetar $magnetar): void {
        // Handle magnetar
    }

    public function visitBlackHole(BlackHole $blackHole): void {
        // Handle black hole
    }
} Detection 🔍 You can detect this smell by looking for instanceof checks within the Visitor pattern. instanceof Automated tools can flag these checks, and code reviews can help identify them. Manual Manual Tags 🏷️ IFs IFs Level 🔋 Intermediate Intermediate Why the Bijection Is Important 🗺️ When you model real-world objects, you should maintain a one-to-one correspondence between the real-world entities and your code. real-world one-to-one Breaking this correspondence using instanceof checks (which do not exist on the model but the metamodel) leads to a mismatch between the real world and your program. instanceof (which do not exist on the model but the metamodel) mismatch AI Generation 🤖 AI generators might create this smell if they are not properly guided. AI Detection 🥃 AI can help fix this smell by suggesting refactoring steps to remove instanceof checks and use polymorphism instead. instanceof use polymorphism Try Them! 🛠 Remember: AI Assistants make lots of mistakes Remember: AI Assistants make lots of mistakes Without Proper Instructions

With Specific Instructions



ChatGPT

ChatGPT



Claude

Claude



Perplexity

Perplexity



Copilot

Copilot



Gemini

Gemini



DeepSeek

DeepSeek



Meta AI

Meta AI Without Proper Instructions

With Specific Instructions ChatGPT

ChatGPT Claude

Claude Perplexity

Perplexity Copilot

Copilot Gemini

Gemini DeepSeek

DeepSeek Meta AI

Meta AI Without Proper Instructions With Specific Instructions Without Proper Instructions Without Proper Instructions With Specific Instructions With Specific Instructions ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT ChatGPT Claude Claude Claude Claude Claude Claude Claude Claude Perplexity Perplexity Perplexity Perplexity Perplexity Perplexity Perplexity Perplexity Copilot Copilot Copilot Copilot Copilot Copilot Copilot Copilot Gemini Gemini Gemini Gemini Gemini Gemini Gemini Gemini DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek DeepSeek Meta AI Meta AI Meta AI Meta AI Meta AI Meta AI Meta AI Meta AI Conclusion ✔️ When you use the Visitor pattern, avoid combining it with iskindOf checks. iskindOf Using the accept() method, the visitor class doesn't need to explicitly check the object type . accept() explicitly check type Each object will call the appropriate visitor method honoring the Open/Closed principle . Open/Closed principle Favor polymorphism and encapsulation to keep your code clean and maintainable. polymorphism and encapsulation clean Relations 👩‍❤️‍💋‍👨 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-v-evj3zs9 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-v-evj3zs9 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-v-evj3zs9 https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-v-evj3zs9 More Information 📕 https://en.wikipedia.org/wiki/Open–closed_principle https://en.wikipedia.org/wiki/Open–closed_principle https://hackernoon.com/how-to-get-rid-of-annoying-ifs-forever-zuh3zlo?embedable=true https://hackernoon.com/how-to-get-rid-of-annoying-ifs-forever-zuh3zlo?embedable=true Disclaimer 📘 Code Smells are my opinion . opinion Credits 🙏 Photo by Braňo on Unsplash Braňo Unsplash The Visitor pattern is a way to add new operations to a set of objects without changing their classes. The Visitor pattern is a way to add new operations to a set of objects without changing their classes. Erich Gamma Erich Gamma https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true https://hackernoon.com/400-thought-provoking-software-engineering-quotes?embedable=true This article is part of the CodeSmell Series. https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-i-xqz3evd?embedable=true https://hackernoon.com/how-to-find-the-stinky-parts-of-your-code-part-i-xqz3evd?embedable=true

The code in this story is for educational purposes. The readers are solely responsible for whatever they build with it.

Walkthroughs, tutorials, guides, and tips. This story will teach you how to do something new or how to do something better.

How to Implement the Visitor Pattern Correctly

About Author

Comments

TOPICS

THIS ARTICLE WAS FEATURED IN

Related Stories

Untitled Story

10 Predictions About the Future of Finance

106 Stories To Learn About Clean Code

100 Pieces of Programming Advice from the Book Clean Code by Robert Martin

110 Stories To Learn About Refactoring

3 Things Coding And Prose Writing Have In Common According To Cory House

10 Predictions About the Future of Finance

106 Stories To Learn About Clean Code

100 Pieces of Programming Advice from the Book Clean Code by Robert Martin

110 Stories To Learn About Refactoring

3 Things Coding And Prose Writing Have In Common According To Cory House

Light-Mode

Classic

Newspaper

Dark-Mode

Neon Noir

Minty

HN StartUps