...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::accumulators::feature_of<tag::external< Feature, Tag, AccumulatorSet >>
// In header: <boost/accumulators/framework/accumulators/external_accumulator.hpp> template<typename Feature, typename Tag, typename AccumulatorSet> struct feature_of<tag::external< Feature, Tag, AccumulatorSet >> : public boost::accumulators::feature_of< Feature > { };