Thread-Safe Singleton Pattern in Swift

Singleton.swift

/* 1. Class Constant : Lazy initialization is supported. Officially recommanded way */

class Singleton {
    static let shared = Singleton()
    private init() { } // prevent creating another instances.
}

/* 2. Nested Struct : Workaround for the lack of static class constants in Swift 1.1 and earlier. still useful 
in functions, where static constants and varialbles cannot be used. */

class Singleton {
    class var shared: Singleton {
        struct Static {
            static let instance: Singleton = Singleton()
        }
        return Static.instance
    }
}

/* 3. dispatch_once : Traditional Objective-C style approach. */

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static var onceToken: dispatch_once_t = 0
            static var instance: Singleton? = nil
        }
        dispatch_once(&Static.onceToken) {
            Static.instance = Singleton()
        }
        return Static.instance!
    }
}

Is the recommended implementation really thread-safe?

Apparently so, according to https://krakendev.io/blog/the-right-way-to-write-a-singleton