How to cast an object from its base class into its subclass

Question

I have a class User which is a subclass of class PFUser:

class User: PFUser {
 var isManager = false
}

In one of my methods I receive a PFUser object and I want to cast it to a User object

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

Is this possible?

Answer 1

This type of casting is a downcast. Given an instance of a certain base class where you know there exist subclasses, you can try to downcast with the typecast operator as:

class Base {}
class Derived : Base {}

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

Keep in mind though, that a downcast can fail:

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

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

You can try a downcast using the optional form of the type as operator as?.

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.
}