swift 隐式可选型实例详解

  • Z时代
  • 2024-01-10
  • 分类：综合

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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