[boost] speed of light from universal_constants Boost.Units

Hi, I've tried and tried and just not understanding or can't muddle my way through the boost units(looks powerful though). For a start, just trying to get at the speed of light in boost/units/systems/si/codata/universal_constants.hpp boost::units::si::constants::codata::c boost::unitsboost::units::si::constants::codata::c.value() and massive errors; especially if I try to multiplying with something. I use boost::math::constants::pi<double>() from #include "boost/math/constants/constants.hpp" But the universals allude me; and example? where is my brain stuck?