Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Function template optional

boost::xpressive::optional — Make a sub-expression optional. Equivalent to !as_xpr(xpr).

template<typename Xpr> unspecified optional(Xpr const & xpr);

Parameters:

xpr

The sub-expression to make optional.