title: C++ reduce string memory usage
body: <p>I need to store a lot of short strings with a constant length.
  I noticed that a string object allocates 8 bytes, even if it holds some chars, which let me run into memory trouble.</p>
  
  <p>Is there a way to tell C++ that it should allocate only n (constant) bytes for the string? Or do I have to use char arrays?</p>