essful, or the default value if mapping did not succeed. </p></td>
</tr>
</tbody>
</table></div>
</li>
<li class="listitem">
<pre class="literallayout"><span class="identifier">implementation_defined</span> <a name="idm46093045932304-bb"></a><span class="keyword">operator</span><span class="special">[</span><span class="special">]</span><span class="special">(</span><span class="identifier">attribute_value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span> key<span class="spec