title: How do you find the iterator in the middle of two iterators?
body: <p>I'm trying to convert my implementation of quicksort into a template that can be used with other containers besides a vector. </p>
  
  <p>Originally I used indexes to find the middle index, e.g. <code>(first + last) / 2</code>. How can I find the middle of two iterators?</p>