Although the <a href="#predefined_key_extractors">predefined key extractors</a>
provided by Boost.MultiIndex are intended to serve most cases,
the user can also provide her own key extractors in more exotic situations,
as long as these conform to the
<a href="../reference/key_extraction.html#key_extractors"><code>Key
Extractor</code></a> concept.
</p>

<blockquote><pre>
<span class=comment>// some record class</span>
<span class=keyword>struct</span> <span class=identifier>record</span>
<span class=special>