title: How to get (unordered) set difference or symmetric difference in C++?
body: <p>I can't infer I can use <a href="http://www.cplusplus.com/reference/algorithm/set_difference/" rel="nofollow noreferrer">std::set_difference</a> from documentation, because it says sets should be ordered, which means they are not sets, but lists. Also all examples are about ordered lists, not sets.</p>
  
  <p>How to know the truth?</p>