Overriding superclass property with different type in Swift

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.