title: Is auto& or auto&& prefered for modifying a range with range based foor loop?
body: <p>STL's(person) recent proposal for implicit auto&amp;&amp; in range based for loop made me wonder what it the proper way to iterate and modify elements of a sequence</p>
  
  <pre><code>for(auto&amp;&amp; elem: cont)
  {
  }
  </code></pre>
  
  <p>or </p>
  
  <pre><code>for(auto&amp; elem: cont)
  {
  }
  </code></pre>