継承

/* 継承 */
class Vehicle {
    var numberOfWheels: Int?
    var maxPassengers: Int?
    init() {
    }
    func description() -> String {
//      return "numberOfWheels = \(numberOfWheels), maxPassengers = \(maxPassengers)" // warning: string interpolation produces a debug description for an optional value; did you mean to make this explicit?
        return "numberOfWheels = \(numberOfWheels ?? 0), maxPassengers = \(maxPassengers ?? 0)"
    }
}
 
class Bicycle: Vehicle {
    // init() { // error: overriding declaration requires an 'override' keyword
    override init() { // error: overriding declaration requires an 'override' keyword
        super.init()
        numberOfWheels = 2
    }
}