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

Struct template reduce_unit

boost::units::reduce_unit — Returns a unique type for every unit.

// In header: <boost/units/reduce_unit.hpp>

template<typename Unit> 
struct reduce_unit {
  // types
  typedef unspecified type;
};