n class="bold"><strong><code class="computeroutput"><span class="identifier">key_of_value</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">KeyOfValueFunctionObject</span><span class="special">&gt;</span></code></strong></span>:
            A function object that will define the <code class="computeroutput"><span class="identifier">key_type</span></code>
            of the value type to be stored. This type will allow a map-like interface.
            See <a 