may be std::nullopt or 1</span>

<span class="c1">// now - use std::future instead</span>
<span class="n">std</span><span class="o">::</span><span class="n">future</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">fu</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">async</span><span class="p">([](){</span>
<span class="w">  </span><s