...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::numeric::one_or_default
// In header: <boost/accumulators/numeric/functional.hpp> template<typename T> struct one_or_default : public mpl::if_::type< is_empty< T >, default_< T >, one< T > > { };