C++ keyword mutable
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/105014/c-mutable-keyword">C++ &lsquo;mutable&rsquo; keyword</a>  </p>
</blockquote>



<p>What does the keyword <code>mutable</code> do in the following C++ code:</p>

<pre><code>class X {
   mutable int x;
...
};
</code></pre>
