lass="special">)</span><span class="special">;</span>
  <span class="keyword">auto</span> <a class="link" href="category.html#idm14169-bb"><span class="identifier">value</span></a><span class="special">(</span><span class="identifier">index_type</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
  <span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <a class="link" href="category.