title: What is the use for buckets interface in std::unordered_map?
body: <p>I've been watching this <a href="http://www.youtube.com/watch?v=fHNmRkzxHWs">video</a> from CppCon 2014 and <a href="https://youtu.be/fHNmRkzxHWs?t=2850">discovered</a> that there is an interface to access buckets underneath <code>std::unordered_map</code>. Now I have a couple of questions:</p>
  
  <ul>
  <li>Are there any reasonable examples of the usage of this interface?</li>
  <li>Why did the committee decide to define this interface, why typical STL container interface wasn't enough?</li>
  </ul>