ios - 怎么被as的垂头丧气了!为同一类型工作？

Question

在下面的代码中，通过 as! 向下转型对于同一类型是失败的。Swift 是否不允许对同一类型进行向下转换？

class base {
    var a = 1
}

class derived : base {

}

let a: base? = base()
let b = a as! base     // error occur!
println("\(b.a)")

错误信息:
错误:从'base' 沮丧？ 'base' 只解包选项；您是要使用“!”吗？
让 b = a 作为!根据
~^~~~~


如果我像下面这样修改上面的代码，就没有问题。

let a: base? = derived()
let b = a as! derived
println("\(b.a)")

Answer 1

这是DownCasting在swift中的定义

Downcasting

A constant or variable of a certain class type may actually refer to an instance of a subclass behind the scenes. Where you believe this is the case, you can try to downcast to the subclass type with a type cast operator (as? or as!).

因此，如果类型相同，则不是 DownCasting。

Swift document