title: boost::flat_map and its performance compared to map and unordered_map
body: <p>It is common knowledge in programming that memory locality improves performance a lot due to cache hits. I recently found out about <code>boost::flat_map</code> which is a vector based implementation of a map. It doesn't seem to be nearly as popular as your typical <code>map</code>/<code>unordered_map</code> so I haven't been able to find any performance comparisons. How does it compare and what are the best use cases for it?</p>
  
  <p>Thanks!</p>