public class ThreadSafeValue<T> {
    private let serialQueue = dispatch_queue_create("ThreadSafeValue queue", DISPATCH_QUEUE_SERIAL)

    private var value: T
    public init(_ v: T) { value = v }

    public func get<V>(action: (T) -> V) -> V {
        var v: V?
        dispatch_sync(serialQueue) {
            v = action(self.value)
        }
        return v!
    }
    public func access(action: (inout T) -> ()) {
        dispatch_sync(serialQueue) {
            action(&self.value)
        }
    }

    public func get() -> T {
        return get { $0 }
    }
    public func set(v: T) {
        access { $0 = v; return }
    }
}

let value = ThreadSafeValue("my precious")

// On one thread
let v = value.get()

// On another thread
value.set("asfsdfasd")

// On yet another thread
let count = value.get { countElements($0) }

// On yet yet another thread
let count = value.access { $0.doSomethingOnStringThatDoesNotReturnAValue() }

class MyClass {
    var string = ""
}

let value = ThreadSafeValue("my precious")

// On one thread
let v = value.get().string = "ksjadhf" // UNSAFE