ive::key_of_value</a></code>.
    </p>
<p>
      If a user specifies that option when defining a <code class="computeroutput"><span class="identifier">set</span><span class="special">/</span><span class="identifier">multiset</span></code>
      intrusive container, it specifies a function object that will tell the container
      which is the type of the <span class="emphasis"><em>key</em></span> that <code class="computeroutput"><span class="identifier">value_type</span></code>
      holds and how to obtai