swift 隐式可选型实例详解

时间:2022-06-29 14:45:26

1、隐式可选型的基本使用

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

隐式可选型的定义

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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

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// 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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原文链接:http://blog.csdn.net/u012903898/article/details/52929881