title: Null and Undefined in QML and C++
body: <p>When extending QML with custom types written in C++, I started to wonder about differences between JS and C++.</p>
  
  <p>This time I was wondering, if <code>null</code>-checks on the C++-side are sufficiant, or if there is something like a "<code>QUndefined</code>" that I might need to consider as well. As far as I can see, this is not described <a href="http://doc.qt.io/qt-5/qtqml-cppintegration-data.html" rel="noreferrer">here</a>.</p>
  
  <p>Or to put it differently:  </p>
  
  <ul>
  <li>What happens on the C++ side when I set a property to <code>undefined</code> on the QML side</li>
  <li>What happens on the C++ side when I set a property to <code>null</code> on the QML side</li>
  <li>What happens on the QML side when I set a property to <code>null</code> on the C++ side</li>
  </ul>