different lookup semantics. For this reason,
Boost.MultiIndex formalizes the concept of a 
<a href="../reference/key_extraction.html#key_extractors"><code>Key
Extractor</code></a> in order to make it explicit and controllable
in the definition of key-based indices.
</p>

<p>
Intuitively speaking, a key extractor is a function object that accepts
a reference to an element and returns its associated key. The formal
concept also imposes some reasonable constraints about the stability
of the process, in the sen