swift 隐式可选型实例详解

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

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

// 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()