Why is Mutable keyword used
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/105014/c-mutable-keyword">C++ &#39;mutable&#39; keyword</a>  </p>
</blockquote>



<pre><code>class student {

   mutable int rno;

   public:
     student(int r) {
         rno = r;
     }
     void getdata() const {
         rno = 90;
     } 
}; 
</code></pre>
