Best practice to implement a failable initializer in Swift

Question

With the following code I try to define a simple model class and it's failable initializer, which takes a (json-) dictionary as parameter. The initializer should return nil if the user name is not defined in the original json.

1. Why doesn't the code compile? The error message says:

All stored properties of a class instance must be initialized before returning nil from an initializer.

That doesn't make sense. Why should I initialize those properties when I plan to return nil?

2. Is my approach the right one or would there be other ideas or common patterns to achieve my goal?

class User: NSObject {

    let userName: String
    let isSuperUser: Bool = false
    let someDetails: [String]?

    init?(dictionary: NSDictionary) {
        if let value: String = dictionary["user_name"] as? String {
            userName = value
        }
        else {
           return nil
        }

        if let value: Bool = dictionary["super_user"] as? Bool {
            isSuperUser = value
        }

        someDetails = dictionary["some_details"] as? Array

        super.init()
    }
}

Answer 1

That doesn't make sense. Why should I initialize those properties when I plan to return nil?

According to Chris Lattner this is a bug. Here is what he says:

This is an implementation limitation in the swift 1.1 compiler, documented in the release notes. The compiler is currently unable to destroy partially initialized classes in all cases, so it disallows formation of a situation where it would have to. We consider this a bug to be fixed in future releases, not a feature.

Source

EDIT:

So swift is now open source and according to this changelog it is fixed now in snapshots of swift 2.2

Designated class initializers declared as failable or throwing may now return nil or throw an error, respectively, before the object has been fully initialized.