title: How do I make a call to what() on std::exception_ptr
body: <p>This is the code I have.    </p>
  
  <pre><code>try
  {
  // code throws potentially unknown exception
  }
  catch (...)
  {
      std::exception_ptr eptr =  std::current_exception();
          // then what ?
  }
  </code></pre>
  
  <p>Ideally, I would like to get the string associated to the exception if it is a std::exception.</p>