title: Q_OBJECT macro and meta-object code
body: <p>This link: <a href="http://doc.trolltech.com/4.5/moc.html#moc" rel="nofollow">http://doc.trolltech.com/4.5/moc.html#moc</a> says </p>
  
  <blockquote>
    <p>The moc tool reads a C++ header file.
    If it finds one or more class
    declarations that contain the Q_OBJECT
    macro, it produces a C++ source file
    containing the meta-object code for
    those classes.</p>
  </blockquote>
  
  <p>What is a <strong><em>meta</em></strong> object code?</p>
  
  <p><strong>EDIT 1</strong></p>
  
  <p>How to know in which classes I should write the Q_OBJECT? One example is the signals and slots, any other cases where that needs to be used?</p>