Polymorphism when overriding methods or properties in Swift - swift

Question

I want to ensure by compiler that CarViewController only receives a Car in the vehicle property.
Given the following swift example code:
class Vehicle {
func doSomething(){}
}
class Car: Vehicle {
func doCarThings(){}
}
class VehicleViewController : UIViewController {
var vehicle : Vehicle!;
override func viewDidLoad() {
super.viewDidLoad();
vehicle.doSomething();
}
}
class CarViewController:VehicleViewController {
var vehicle: Car!
override func viewDidLoad() {
super.viewDidLoad();
vehicle.doCarThings();
}
}
I get the following error: Cannot override mutable property 'vehicle' of type 'Vehicle!' with covariant type 'Car!'
I tried with a generics-based approach:
class Vehicle {
func doSomething(){}
}
class Car: Vehicle {
func doCarThings(){}
}
class VehicleViewController<T:Vehicle> : UIViewController {
var vehicle : T!;
override func viewDidLoad() {
super.viewDidLoad();
vehicle.doSomething();
}
}
class CarViewController:VehicleViewController<Car> {
override func viewDidLoad() {
super.viewDidLoad();
vehicle.doCarThings();
}
}
It is correct but using generics in storyboard classes results in errors (since they get compiled to objective-c).
How can I do this without using generics?
Thanks!

Answer 1

I'm really not sure about the design here, but to accomplish what you want you could do:
class CarViewController: VehicleViewController {
var vehicleAsCar: Car { return self.vehicle as! Car }
override func viewDidLoad() {
super.viewDidLoad();
vehicleAsCar.doCarThings();
}
}
But this seems very smelly. Safer night be:
class CarViewController: VehicleViewController {
override var vehicle: Vehicle! {
didSet {
assert(vehicle is Car, "Attempt to set vehicle to non-Car")
}
}
var vehicleAsCar: Car { return self.vehicle as! Car }
override func viewDidLoad() {
super.viewDidLoad();
vehicleAsCar.doCarThings();
}
}

Answer 2

Taken from here:
Overriding Property Getters and Setters
You can provide a custom getter (and setter, if appropriate) to
override any inherited property, regardless of whether the inherited
property is implemented as a stored or computed property at source.
The stored or computed nature of an inherited property is not known by
a subclass—it only knows that the inherited property has a certain
name and type. You must always state both the name and the type of the
property you are overriding, to enable the compiler to check that your
override matches a superclass property with the same name and type.
Seems like you cant do that.