title: Could `dispatch_once` created singleton got released? If so, how to deal with it?
body: <p>As mentioned in this <a href="https://stackoverflow.com/questions/5720029/create-singleton-using-gcds-dispatch-once-in-objective-c">discussion</a>, <code>dispatch_once</code> is very useful in building singleton. However, could <code>dispatch_once</code> created instance got released and <code>dispatch_once</code> was not able to created that instance because it only execute once?</p>
  
  <p>If so, what is the best practice to deal with it?</p>