<span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">old_value_copy</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">old</span><span class="special">)</span> <span class="special">:</span>
        <span class="identifier">old_</span><span class="special">(</span><span class="identifier">old</span><span class="special">)</span> <span class="comment">/