an class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  <span class="identifier">implementation_defined</span> <a class="link" href="basic_custom_mapping.html#idm31919-bb"><span class="keyword">operator</span><span class="special">[</span><span class="special">]</span></a><span class="special">(</span><span class="identifier">attribute_value_type</span> <span class="keyword">const</span> <span class="special">&amp;</span><span class="special">)</span><span class="speci