title: Qt:signal slot pass by const reference
body: <p>Many qt examples did the following things: </p>
  
  <pre><code>signal: passByConstRef(const QImage &amp;)
  slot: receivedByConstRef( const QImage &amp;)
  </code></pre>
  
  <p>They are connected through queued connection in different threads.
  In thread 1:</p>
  
  <pre><code>{QImage image; 
   image = fromData(...);
   emit passByConstRef(image);
  }
  </code></pre>
  
  <p>In thread 2:</p>
  
  <pre><code>receivedByConstRef(const QImage&amp; image){
     ...
     pixmap = fromImage(image);
     ...
   }
  </code></pre>
  
  <p>Since the signal slot is passing the QImage by const Ref which means there's no copy happening, will it be possible that when "pixmap" is being constructed, the life time of "image" in thread 1 is over and leads to the failure of constructing "pixmap" in thread 2.</p>