r to read into. This must be move-only
</span><span class="c1"></span>  <span class="c1">// so it can be attached to the completion handler, hence the unique_ptr.
</span><span class="c1"></span>  <span class="k">auto</span> <span class="n">buffer</span> <span class="o">=</span> <span class="n">std</span><span class="o">::</span><span class="n">make_unique</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n