Problems

• Sub classification for code reuse purposes.

• Liskov substitution violation (SOLID principle).

• Possible subclass overrides.

Solutions

1. Favor composition
2. Don't subclassify attributes.
3. Extract behavior to separate objects.
4. Use [traits](https://en.wikipedia.org/wiki/Trait_(computer_programming) (if available).

Wrong

<?

abstract class ElectronicDevice {

    protected $battery;

    public function __construct(OperatingSystem $battery) {
        $this->battery = $battery;
    }

}

abstract class Idevice extends ElectronicDevice {

    protected $operatingSystem;

    public function __construct(Battery $battery, OperatingSystem $ios) {
        $this->operatingSystem = $ios;
        parent::__construct($battery)
  }

}

final class Ipad extends Idevice {

    public function __construct(Battery $battery, OperatingSystem $ios) {
        parent::__construct($battery, $ios)
  }

}

final class Iphone extends Idevice {

  private $phoneModule;

  public __construct(Battery $batery, OperatingSystem $ios, PhoneModule $phoneModule) {
    $this->phoneModule = $phoneModule;
    parent::__construct($battery, $ios)
  }

}

Right

<?

interface ElectronicDevice {
    //...
}

interface PhoneCommunication {
    //...
}

final class Ipad implements ElectronicDevice {

    private $operatingSystem;
    private $battery;

    public function __construct(Battery $battery, OperatingSystem $ios) {
        $this->operatingSystem = $ios;
        $this->battery = $battery;
    }
}

final class Iphone implements ElectronicDevice, PhoneCommunication {

    private $phoneModule;
    private $operatingSystem;
    private $battery;

    public function __construct(Battery $battery, OperatingSystem $ios, PhoneModule $phoneModule) {
        $this->phoneModule = $phoneModule;
        $this->operatingSystem = $ios;
        $this->battery = $battery;
    }
}

Detection

In languages supporting protected attributes we can avoid them by policy or have a warning of this smell.

Tags

• Encapsulation

Conclusion

Protected attributes are yet another tool we should use carefully. Every decision is a smell, and we should be very careful with attributes and inheritance.

Relations

More Info

Traits on Wikipedia

Credits

Photo by Jonathan Farber on Unsplash

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Subclasses shouldn’t always share all characteristics of their parent class but will do so with inheritance. This can make a program’s design less flexible. It also introduces the possibility of calling methods on subclasses that don’t make sense or that cause errors because the methods don’t apply to the subclass.

Steve Klabnik

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This article is part of the CodeSmell Series.