n class=keyword>operator</span><span class=special>()(</span><span class=keyword>const</span> <span class=identifier>record</span><span class=special>&amp;</span> <span class=identifier>r</span><span class=special>)</span><span class=keyword>const</span> <span class=comment>// operator() must be const</span>
  <span class=special>{</span>
    <span class=keyword>return</span> <span class=identifier>r</span><span class=special>.</span><span class=identifier>d</span><span class=special>.</span><span class=ide