title: Header inclusion optimization
body: <p>Is there an automatic way to optimize inclusion of header files in C++, so that compilation time is improved ? With the word "automatic" I mean a tool or program. Is it possible to find which headers files are obsolete (e.g exposed functionality is not used) ? </p>
  
  <p><strong>Edit</strong>: Having each include header "included only once is one important thing, but is there a way to even change the contents of files so that frequently used "functionality" is on specific includes and less frequently used functionality is on other includes? Am i asking too much ? Unfortunately, we are talking about an existing code base with thousands of files. Could it be a refactoring tool what I am actually asking for ? </p>
  
  <p>Thank you.</p>