m>C++ reference</em>, bound either to an actual argument passed by
the caller or to the result of evaluating a default expression.
In most cases, parameter types are of the form <tt class="docutils literal">T const&amp;</tt> for
some <tt class="docutils literal">T</tt>.  Parameters whose values are expected to be modified,
however, must be passed by reference to <em>non</em>-<tt class="docutils literal">const</tt>.  To
indicate that <tt class="docutils literal">color_map</tt> is both read and written, we wr