如何在Swift中保留基类数据的子类？

class Vehicle {
    var make: String
    var model: String
    var year: Int
    
    init(make: String, model: String, year: Int) {
        self.make = make
        self.model = model
        self.year = year
    }
    
    func start() {
        print("The vehicle is starting.")
    }
    
    func stop() {
        print("The vehicle has stopped.")
    }
}

class Car: Vehicle {
    var numberOfDoors: Int
    
    init(make: String, model: String, year: Int, numberOfDoors: Int) {
        self.numberOfDoors = numberOfDoors
        super.init(make: make, model: model, year: year)
    }
    
    override func start() {
        print("The car is starting.")
    }
    
    func lockDoors() {
        print("The car doors are locked.")
    }
}

class Car: Vehicle {
    override var make: String {
        didSet {
            print("Make has been changed to \(make)")
        }
    }
    
    init(make: String, model: String, year: Int, numberOfDoors: Int) {
        self.make = make
        super.init(make: make, model: model, year: year)
    }
}

class Car: Vehicle {
    override func start() {
        super.start()
        print("The car is starting.")
    }
}