Swift中的可选链接应用

代码：

import Foundation

class Person{

    var residence: Residence?

}

class Residence {

    var city:String = "Shanghai"

}

let litao = Person()

if let cityname = litao.residence?.city {

    print("City is: \(cityname)")

} else {

    print("There is no city info")

}

let zhangsan = Person()

zhangsan.residence = Residence()   //实例化

if let zhangcity = zhangsan.residence?.city {

    print("ZhangSan's City is \(zhangcity)")

} else {

    print("ZhangSan has no city info")

}

输出：

There is no city info

ZhangSan's City is Shanghai

Program ended with exit code: 0