//隐式可选型 可为nil，可有值

class City{

    var cityName: String

   unowned var country: Country

    init(cityName: String, country: Country) {

        self.cityName = cityName

        self.country = country

    }

}

class Country {

    let countryName: String

    var capitalCity: City?

    init(countryName: String, capitalCity:String) {

        self.countryName = countryName

        self.capitalCity = City(cityName: capitalCity, country: self)

    }

    func showInfo(){

        print("this is \(countryName)")

        print("the capital is \(String(describing: capitalCity?.cityName))")

    }

}

let china = Country(countryName: "China", capitalCity: "beiJing")

china.showInfo()