title: boost::unique_lock vs boost::lock_guard
body: <p>I don't well understand the difference betweeen these two lock classes.
  In boost documentation it is said, <code>boost::unique_lock</code> doesn't realize lock automatically.</p>
  
  <p>Does it mean that the main difference between <code>unique_lock</code> and <code>lock_guard</code> is that with <code>unique_lock</code> we must call explicitly the <code>lock()</code> function ?</p>