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From my short experience with Swift there are three approaches to implement the Singleton pattern that support lazy initialization and thread safety.

These approaches might change or become redundant as the language matures.

Global constant

let _SingletonSharedInstance = Singleton()

class Singleton  {

    class var sharedInstance : Singleton {

        return _SingletonSharedInstance

    }

}

We use a global constant because class constants are not yet supported.

This approach supports lazy initialization because Swift lazily initializes global constants (and variables), and is thread safe by virtue of let.

Nested struct

class Singleton {

    class var sharedInstance : Singleton {

        struct Static {

            static let instance : Singleton = Singleton()

        }

        return Static.instance

    }

}

Unlike classes, structs do support static constants. By using a nested struct we can leverage its static constant as a class constant.

dispatch_once

The traditional Objective-C approach ported to Swift. I'd say it's no longer necessary to use this approach but I'm putting it here anyway as I find the differences in syntax interesting.

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!

    }

}