title: std::map - erase last element
body: <p>My map is defined as such: 
  <code>map&lt;string, LocationStruct&gt; myLocations;</code> where the key is a time string</p>
  
  <p>I am only keeping 40 items in this map, and would like to drop off the last item in the map when i reach 40 items.  I know that i can't do <code>myLocations.erase(myLocations.end())</code>, so how do i go about this?</p>
  
  <p>I do intend for the last item in the map to be the oldest, and therefore FIFO.  The data will be coming in rather quick (about 20Hz), so i'm hoping that the map can keep up with it. I do need to look up the data based on time, so i really do need it to be the key, but i am open to alternate methods of accomplishing this.</p>
  
  <p>The format of the string is a very verbose "Thursday June 21 18:44:21:281", though i can pare that down to be the seconds since epoch for simplicity.  It was my first go at it, and didn't think too much about the format yet.</p>