Singleton pattern in Swift

Singleton.swift

// lazy Singleton: create the singleton object when needed
// Note: Global constants and variables are always computed lazily
// ref: https://docs.swift.org/swift-book/LanguageGuide/Properties.html#ID263
class SingletonGreed {
    static let shared = SingletonGreed()
    private init() {}
}

class SingletonThreadSafe {
    private static var shared: SingletonThreadSafe?
    private init() {}
    static func getInstance() -> SingletonThreadSafe {
        if shared == nil {
            synced(self) {
                if shared == nil {
                    shared = SingletonThreadSafe()
                }
            }
        }
        return shared!
    }

    static func synced(_ lock: Any, closure: () -> ()) {
        objc_sync_enter(lock)
        closure()
        objc_sync_exit(lock)
    }
}