span class="keyword">struct</span>  <span class="identifier">test1</span> <span class="special">{</span> <span class="keyword">int</span>  <span class="keyword">operator</span><span class="special">()(</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="number">0</span><span c