title: c++ exceptions, can what() be NULL?
body: <p>Can a caught std::exception ever have what() being NULL?  </p>
  
  <p>Is the checking for e.what() below overhead? </p>
  
  <pre><code>//...
  }
  catch (const std::exception&amp; e)
  {
    std::string error;
    if(e.what())
      error = e.what();
  }
  </code></pre>