...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::units::subtract_typeof_helper
// In header: <boost/units/operators.hpp> template<typename X, typename Y> struct subtract_typeof_helper { // public member functions typedef typeof((typeof_::make< X >()-typeof_::make< Y >())); };