title: QT Signal and Slot between windows
body: <p>I want to connect to windows in QT.</p>
  
  <p>I know how to connect two widgets in same window.</p>
  
  <p><code>using QObject::connect()</code> in <code>ui_a.h</code>;</p>
  
  <p>But now, I have two windows.</p>
  
  <p>Six files.</p>
  
  <p>For example:</p>
  
  <p><code>ui_a.h</code>(Window A)</p>
  
  <p><code>a.h</code>   <code>a.cpp</code> (<code>Widget</code> in window A)</p>
  
  <p><code>ui_b.h</code>(Window B)</p>
  
  <p><code>b.h</code>   <code>b.cpp</code> (<code>Widget</code> in window B)</p>
  
  <p>How to connect two widget which are in different windows?</p>
  
  <p>Thank you.</p>