html" title="Struct null_type">null_type</a><span class="special">;</span>

      <span class="keyword">typedef</span> <span class="keyword">int</span> <a name="boost.histogram.axis.index_type"></a><span class="identifier">index_type</span><span class="special">;</span>  <span class="comment">// Integral type for axis indices. </span>
      <span class="keyword">typedef</span> <span class="keyword">double</span> <a name="boost.histogram.axis.real_index_type"></a><span class="identifier">real_index_type</spa