title: Common Qt questions
body: <p>I have learned the basics of Qt and now interested in the depths of that pretty library. Please help me to understand:</p>
  
  <ol>
  <li>Are the all classes derived from <strong>QObject</strong>?</li>
  <li>Why is it possible to paint on a <strong>QWidget</strong> (and derived classes)?</li>
  <li>What does the line <code>return app.exec();</code> mean? What does <strong>exec()</strong> method do?</li>
  <li>Are there <strong>virtual slots</strong>? And <strong>virtual signals</strong>?</li>
  </ol>
  
  <p>Thanks.</p>