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忽略的基础性的问题，应该早点写出这个问题的解释比较好，毕竟 swift 好多的地方都用到了 as as！as？，是非常容易搞晕大脑的 1，as使用场合 class Animal {} class Cat: Animal {} let cat = Cat() let animal = cat as Animal （2）消除二义性，数值类型转换 let num1 = 42 as CGFloat let num2 = 42 as Int let num3 = 42.5 as Int let num4 = (42 / 2) as Doubl （3）switch 语句中进行模式匹配 switch animal { case let cat as Cat: print("如果是Cat类型对象，则做相应处理") case let dog as Dog: print("如果是Dog类型对象，则做相应处理") default: break } class Animal {} class Cat: Animal {} let animal :Animal = Cat() let cat = animal as! Cat let animal:Animal = Cat() if let cat = animal as? Cat{ print("cat is not nil") } else { print("cat is nil") }
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