swift 隐式可选型

来源：互联网发布：sql语句自动优化工具编辑：程序博客网时间：2024/05/09 00:08

1、隐式可选型的基本使用

var errorMessage: String? = nilerrorMessage = "Not Found""The message is " + errorMessage!

隐式可选型的定义

var errorMessage: String! = nilerrorMessage = "Not Found""The message is " + errorMessage

隐式可选型不需要解包，所以隐式可选型容易出错
以上程序当errorMessage为nil时程序会报错

2、隐式可选型的实际应用

// 主要应用在类的成员变量的初始化上class City{    let 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 \(capitalCity.cityName).")    }}let china = Country(countryName: "China", capitalCity: "Beijing")china.showInfo()

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