use the <code class="docutils literal"><span class="pre">proj</span></code> adaptor to project <code class="docutils literal"><span class="pre">year_of_birth</span></code> on the comparison
operator:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="n">std</span><span class="o">::</span><span class="n">sort</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">begin</span><span class="p">(</span><span class="n">people</span><span 