 approach ensures that not
      only is a non-zero pointer returned, but also that it is correct in the presence
      of multiple inheritance. Attempts to crosscast using <code class="computeroutput">polymorphic_downcast</code>
      will fail to compile.
    </p>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../doc/src/images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" v