y evaluated if its first argument is <tt class="docutils literal">false</tt>.
Similarly, in <tt class="docutils literal"><span class="pre">color_map[param||f]</span></tt>, <tt class="docutils literal">f</tt> is only invoked if
no <tt class="docutils literal">color_map</tt> argument was supplied.</p>
</div>
<p>The expression <tt class="docutils literal"><span class="pre">bind(std::plus&lt;std::string&gt;(),</span> ref(s1), ref(s2))</tt> yields
a <em>function object</em> that, when invoked, adds the two strin