/span> EnableIf<span class="special">&gt;</span> 
<span class="keyword">struct</span> <a class="link" href="promote_base.html" title="Struct template promote_base">promote_base</a> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">unary_function</span><span class="special">&lt;</span> <span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span> <span class="spe