fier">first</span><span class="special">;</span>
   <span class="keyword">explicit</span> <span class="identifier">MyClass</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">first</span><span class="special">(</span><span class="identifier">i</span><span class="special">){}</span>
<span class="special">};</span>

<span class="comment">//key_of_value function object, must: