n class="keyword">int</span> <span class="identifier">val</span> <span class="special">=</span> <span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">value_</span><span class="special">(</span><span class="identifier">val</span><span class="special">){}</span>

   <span class="keyword">friend</span> <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="keyword">const</span> <span class