Swift学习09：swift中as、as！、as？号的使用

关于 as

• 将子类转换为基类

class Auto {}
class Car : Auto {}
let car = Car()
let truck = car as Auto

• 数值类型的转换

let n1 = 56 as Int

• 模式匹配

swich Auto {
    case let car as Car:
        print("这里是Car类型对象");
    case let truck as Truck:
        print("这里是Truck类型对象");
    default: break
}

as!

向下强制类型转换，如果转换失败会报错

class Auto {}
class Car: Auto {}
let auto: Auto = Car()
let car = auto as! Car

as?

向下强制类型转换，如果转换失败会返回nil

let auto: Auto = Car()
if let car = auto as? Car {
   print("这是Car")
}
else {
  print("这不是Car")
}