title: Namespace alias in c++
body: <p>I use c++11 while I need some classes from c++17 library. When using boost from which classes were added I wish to do the following:    </p>
  
  <pre><code>#if __cplusplus &lt; CPP17
  using std::any = boost::any;  
  #endif
  </code></pre>
  
  <p>Such alias is not allowed. Also extending the std namespace causes  <a href="http://en.cppreference.com/w/cpp/language/extending_std" rel="nofollow noreferrer">undefined behaviour</a>. I wish my code to look the same regardles of the c++ version. Is there a clear way?</p>