"special">}</span>

<span class="keyword">private</span><span class="special">:</span>
    <span class="identifier">T</span> <span class="keyword">const</span> <span class="identifier">old_</span><span class="special">;</span> <span class="comment">// The old value copy.</span>
<span class="special">}</span><span class="special">;</span>
</pre>
<p>This library will instantiate and use this trait only on old value types <code class="computeroutput">T</code> that are copyable (i.e., for which <code class="com