How to add initializers for classes

Paul Hudson    @twostraws    October 12th 2021

Updated for Xcode 15

Class initializers in Swift are more complicated than struct initializers, but with a little cherrypicking we can focus on the part that really matters: if a child class has any custom initializers, it must always call the parent’s initializer after it has finished setting up its own properties, if it has any.

Like I said previously, Swift won’t automatically generate a memberwise initializer for classes. This applies with or without inheritance happening – it will never generate a memberwise initializer for you. So, you either need to write your own initializer, or provide default values for all the properties of the class.

Let’s start by defining a new class:

class Vehicle {
    let isElectric: Bool

    init(isElectric: Bool) {
        self.isElectric = isElectric
    }
}

That has a single Boolean property, plus an initializer to set the value for that property. Remember, using self here makes it clear we’re assigning the isElectric parameter to the property of the same name.

Now, let’s say we wanted to make a Car class inheriting from Vehicle – you might start out writing something like this:

class Car: Vehicle {
    let isConvertible: Bool

    init(isConvertible: Bool) {
        self.isConvertible = isConvertible
    }
}

But Swift will refuse to build that code: we’ve said that the Vehicle class needs to know whether it’s electric or not, but we haven’t provided a value for that.

What Swift wants us to do is provide Car with an initializer that includes both isElectric and isConvertible, but rather than trying to store isElectric ourselves we instead need to pass it on – we need to ask the super class to run its own initializer.

Here’s how that looks:

class Car: Vehicle {
    let isConvertible: Bool

    init(isElectric: Bool, isConvertible: Bool) {
        self.isConvertible = isConvertible
        super.init(isElectric: isElectric)
    }
}

super is another one of those values that Swift automatically provides for us, similar to self: it allows us to call up to methods that belong to our parent class, such as its initializer. You can use it with other methods if you want; it’s not limited to initializers.

Now that we have a valid initializer in both our classes, we can make an instance of Car like so:

let teslaX = Car(isElectric: true, isConvertible: false)

Tip: If a subclass does not have any of its own initializers, it automatically inherits the initializers of its parent class.

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