oop - 如何将对象从其基类转换为子类

Question

我有一个 User 类，它是 PFUser 类的子类:

class User: PFUser {
 var isManager = false
}

在我的一个方法中，我收到一个 PFUser 对象，我想将它转换为一个 User 对象

func signUpViewController(signUpController: PFSignUpViewController!, didSignUpUser user: PFUser!) {
 currentUser = user
}

这可能吗？

Answer 1

这种类型的转换是向下转换。给定一个您知道存在子类的基 类的实例，您可以尝试使用类型转换运算符as 进行向下转换:

class Base {}
class Derived : Base {}

let base : Base = Derived()
let derived = base as Derived

不过请记住，沮丧可能会失败:

class Base {}
class Derived : Base {}
class Other : Base {}

let base : Base = Other()
let derived = base as Derived  // fails with a runtime exception

您可以尝试使用类型为运算符as? 的可选形式 进行向下转型。

class Base {}
class Derived : Base {}
class Other : Base {}

let base : Base = Other()

// The idiomatic implementation to perform a downcast:

if let derived = base as? Derived {
    println("base IS A Derived")
}
else {
    println("base IS NOT A Derived")  // <= this will be printed.
}