The type of optional used by the library.
Defined in header
template< class T> using optional = boost::optional< T >;
This alias is no longer supported and should not be used in new code. Please
boost::optional instead. This alias is included
for backwards compatibility with earlier versions of the library. However,
it will be removed in future releases, and using it in new code is not recommended.
Please use the updated version instead to ensure compatibility with future
versions of the library.