Class inheritance

The second difference between classes and structs is that you can create a class based on an existing class – it inherits all the properties and methods of the original class, and can add its own on top.

This is called class inheritance or subclassing, the class you inherit from is called the “parent” or “super” class, and the new class is called the “child” class.

Here’s the Dog class we just created:

class Dog {
    var name: String
    var breed: String

    init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

We could create a new class based on that one called Poodle. It will inherit the same properties and initializer as Dog by default:

class Poodle: Dog {

}

However, we can also give Poodle its own initializer. We know it will always have the breed “Poodle”, so we can make a new initializer that only needs a name property. Even better, we can make the Poodle initializer call the Dog initializer directly so that all the same setup happens:

class Poodle: Dog {
    init(name: String) {
        super.init(name: name, breed: "Poodle")
    }
}

For safety reasons, Swift always makes you call super.init() from child classes – just in case the parent class does some important work when it’s created.