Overriding superclass property with different type in Swift

Question

In Swift, can someone explain how to override a property on a superclass's with another object subclassed from the original property?

Take this simple example:

class Chassis {}
class RacingChassis : Chassis {}

class Car {
    let chassis = Chassis()
}
class RaceCar: Car {
    override let chassis = RacingChassis() //Error here
}

This gives the error:

Cannot override with a stored property 'chassis'

If I have chassis as 'var' instead, I get the error:

Cannot override mutable property 'chassis' of type 'Chassis' with covariant type 'RacingChassis'

The only thing I could find in the guide under "Overriding Properties" indicates that we have to override the getter and setter, which may work for changing the value of the property (if it's 'var'), but what about changing the property class?

Answer 1

Swift does not allow you to change the class type of any variables or properties. Instead you can create an extra variable in the subclass that handles the new class type:

class Chassis {}
class RacingChassis : Chassis {}

class Car {
    var chassis = Chassis()
}
class RaceCar: Car {
    var racingChassis = RacingChassis()
    override var chassis: Chassis {
        get {
            return racingChassis
        }
        set {
            if let newRacingChassis = newValue as? RacingChassis {
                racingChassis = newRacingChassis
            } else {
                println("incorrect chassis type for racecar")
            }
        }
    }
}

It seems one cannot declare a property with the let syntax and override it with var in it’s subclass or vice-versa, which may be because the superclass implementation might not be expecting that property to change once initialized. So in this case the property needs to be declared with ‘var’ in the superclass as well to match the subclass (as shown in the snippet above). If one cannot change the source code in the superclass then its probably best to destroy the current RaceCar and create a new RaceCar every time the chassis needs to be mutated.