title: how to know window is open in qt
body: <p>I have created two Main windows. How to know whether 2nd window is open in QT?</p>
  
  <pre><code>QMainWindow *window1 = new QMainWindow();
  
  QMainWindow *window2 = new QMainWindow();
  </code></pre>
  
  <p>Now I have to find whther window2 is already opened in my one instance. How to know it</p>