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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