title: In unordered_map of C++11, how to update the value of a particular key?
body: <p>In Java's hashmap:</p>
  
  <pre><code>map.put(key, new_value) 
  </code></pre>
  
  <p>will update the entry of key=key with new_value if it exists in the hashmap.</p>
  
  <p>What's the correct way to do the similar thing in unordered_map of C++11?</p>
  
  <p>I haven't found an API like updateXXX, and the documentation says the <b>unordered_map::insert</b> function will succeed only when there isn't any such pair with a key.</p>