title: Efficient way to move elements from a map to another
body: <p>I must find a efficient solution for this problem. I have two map. I must move some elements from map1 to map2 (namely erase from map1 and put into map2).
  I have the  keys through which I find the elements in map1 namely i'm doing for now:</p>
  
  <pre><code>bool temp;
  temp = map1[key1];
  map2[key1]=temp;
  map1.erase(key1)
  </code></pre>
  
  <p>I do it for each key(in a loop)</p>
  
  <p>My question is if there is a less expensive solution  of my
  (I use C++11 compiler)</p>