an class="p">.</span><span class="n">cancel</span><span class="p">();</span>
<span class="n">std</span><span class="o">::</span><span class="n">optional</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">res</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">fu</span><span class="p">.</span><span class="n">get</span><span class="p">();</span><span class="w">  </span><span class="c1">// res 