title: Can we use a user defined class for the key in a STL map?
body: <p>I need a key in the map, however, I found it should be multiple data. Can I put these data in one user defined class and put the whole class as a key in the map?</p>
  
  <p>Will it impact the time efficiency?</p>
  
  <p>What other concerns should be applied here?</p>