Have you used the Boost.Optional library? There is one thing you might need to keep an eye on as explained by Andrzej.

  A gotcha with Optional

  by Andrzej Krzemieński

From the article:

This post is about one gotcha in Boost.Optional library. When starting to use it, you might get the impression that when you try to put optional<T> where T is expected, you will get a compile-time error. In most of the cases it is exactly so, but sometimes you may get really surprised...