1、隐式可选型的基本使用
var errorMessage: String? = nil errorMessage = "Not Found" "The message is " + errorMessage!
隐式可选型的定义
var errorMessage: String! = nil errorMessage = "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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