<span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span>    <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">convert</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span><span class="special">(</span><span class="string">"12"</span><span class="special">)</span><span cl