class Vehicle {

    var numberOfWheels: Int

    var maxPassengers: Int

    func description() -> String {

        return "\(numberOfWheels) wheels; up to \(maxPassengers) passengers"

    }

    init() {

        numberOfWheels = 1;

        maxPassengers = 3;

    }

}

class Bicycle: Vehicle {

    init() {

        super.init();

        numberOfWheels = 2;

    }

}

class Tandem: Bicycle {

    init() {

        super.init();

        maxPassengers = 2;

    }

}

//override func

class Car : Vehicle {

    var speed : Double = 0.0;

    init() {

        super.init()

        maxPassengers = 5

        numberOfWheels = 4;

    }

    override func description() -> String {

        return super.description() + ";" + "speed at \(speed) mph";

    }

}

//override properties

class SpeedLimit : Car {

    override var speed: Double {

    get {

        return super.speed

    }

    set {

        super.speed = max(newValue, 40.0)

    }

    }

}

let limit = SpeedLimit();

limit.speed = 220.0

println("LimitSpeed:\(limit.description())")

//重写属性观察器

class AutomaticCar: Car {

    var gear = 1;

    override var speed: Double {

    didSet {

        gear = Int(speed/10) + 1

    }

    }

    override func description() -> String {

        return super.description() + " in gead at \(gear)";

    }

}

let automaticCar = AutomaticCar()

automaticCar.speed = 50.0

println(automaticCar.description())