 class="identifier">category</span></a><span class="special">(</span><span class="keyword">const</span> <span class="identifier">C</span> <span class="special">&amp;</span><span class="special">,</span> <span class="identifier">metadata_type</span> <span class="special">=</span> <span class="special">{</span><span class="special">}</span><span class="special">,</span> <span class="identifier">allocator_type</span> <span class="special">=</span> <span class="special">{</span><span class="special">}</span><sp