...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::functional::outer_product
// In header: <boost/accumulators/statistics/covariance.hpp> template<typename Left, typename Right, typename LeftTag = typename tag<Left>::type, typename RightTag = typename tag<Right>::type> struct outer_product : public boost::numeric::functional::outer_product_base< Left, Right, void > { };