but rather it is a value returned by a particular member function.
This resembles the notion of <i>calculated indices</i> supported by some
relational databases. Boost.MultiIndex supports this
kind of key extraction through
<a href="../reference/key_extraction.html#const_mem_fun"><code>const_mem_fun</code></a>.
Consider the following container where sorting on the third index
is based upon the length of the name field:
</p>

<blockquote><pre>
<span class=preprocessor>#include</span> <span class=special>&lt;