title: Why C++ allow converting pointer to base object into pointer of derived class
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/6322949/downcasting-using-the-static-cast-in-c">Downcasting using the Static_cast in C++</a>  </p>
  </blockquote>
  
  
  
  <p>I'm very confused by when do we need to convert pointer to base object into pointer of derived class? 
  Can anyone do me a favor to give me some example?</p>