as as! as?的使用方法和区别

as、as!、as?都是类型转换操作符

一、as

1.用来进行一般的向上类型转换

例如

class Person{}

class XiaoMing : Person{}

let xiaoMing = XiaoMing()

let person = xiaoMing as Person

在这个类里面就将子类对象xiaoMing转化为了父类Person对象并赋值给了person

2.进行数值类型限定转换，也是起到了一个说明的作用

let num1 =25 as CGFloat

let num2 =3 as Int

let num3 = (1 / 3) as CGFloat

let num4 =1.45 as Int

3.用在switch条件判断语句中

class Person{}

class XiaoMing : Person{}

class XiaoGang : Person{}

let xiaoMing = XiaoMing()

_ = XiaoGang()

let person = xiaoMing as Person

switch person {

case letming as XiaoMing:

  print (ming)

case let gang as XiaoGang:

  print (gang)

default: break

}

二、as!

进行强制向下的类型转换例如：（如果类型转换不成功会报runtime崩溃)

class Person{}

class XiaoMing : Person{}

let person : Person = XiaoMing()

let xiaoMing : XiaoMing = person as! XiaoMing

三、as?

as?如果类型转换不成功就会返回一个nil对象，不确定是否一定可以转化的时候可以使用as?

class Person{}

class XiaoMing : Person{}

let person : Person = XiaoMing()

if let xiaoMing = person as! XiaoMing{

    print("xiaoMing不为nil")

}else{

    print("xiaoMing为nil")

}

总结：as用于向上转换、数字类型转换、Switch类型判断

as!和as?用于向下类型转换。不确定成功用as?一定成功用as!