title: What is QString::QString(QString &&other) used for?
body: <p>I realized there exists another constructor besides normal QString::QString(QString &amp;other). What is it used for? I cannot understand the type QString &amp;&amp;. Does it mean the reference to a reference variable? But the following code could not compile:</p>
  <pre><code>QString s1(&quot;abc&quot;);
  QString &amp;s3=s1;
  QString &amp;&amp;s4=s3;
  </code></pre>
  <p>It complains &quot;You cannot bind an lvalue to an rvalue reference&quot;</p>