title: Is it valid to define a pure virtual signal in C++/Qt?
body: <p>I am making an abstract-base-class and was thinking I <em>might</em>  want a pure virtual signal. But when I compiled I get a warning for the pure virtual signals I have defined:</p>
  
  <pre><code>../FILE1.h:27: Warning: Signals cannot be declared virtual
  ../FILE1.h:28: Warning: Signals cannot be declared virtual
  </code></pre>
  
  <p><strong>Is it valid to define a pure virtual signal in C++/Qt? Is it valid to define a virtual signal?</strong></p>
  
  <p><a href="http://qt-project.org/doc/qt-4.8/signalsandslots.html" rel="noreferrer">Qt's signal and slot documentation page</a> says you can define virtual slots but doesn't talk about signals. I can't seem to find good information on pure virtual signals.</p>