optionals.swift

class Country {
    let name:String
    var capitalCity:City?
    init (name:String, capitalName:String) {
        self.name = name
        self.capitalCity = City(name: capitalName, country: self)
    }
}

class City {
    let name:String
    unowned let country:Country
    init (name:String, country:Country) {
        self.name = name
        self.country = country
    }
}

var country = Country(name: "Korea", capitalName: "Seoul")
print("\(country.capitalCity!.name)")