Hi, I took much labour in defining an extra system of units (atomic units). It works very well and it also defines (optionaly) default conversion from atomic to SI units. The problem that I have now is that all the conversion has to be done explicitly. I understand that this way of doing thing is by design. But for some formulas I would like to allow implicit conversion, for example when adding energies in SI and energies in atomic units (I don't care which direction the conversion is done since I will transform to one of the two at the end of the formula). On top of that the Units Manual says: "Safety and the potential for unintended conversions leading to precision loss and hidden performance costs. Options are provided for forcing implicit conversions between specific units to be allowed. " The question is which are the "Options provided for forcing implicit conversion"? What follow is an example: What I want is some controlled implicit conversion (even better is this conversion are allowed inside a certain program scope). Note that line 4 doesn't work: quantity<si::energy> e1(3.*si::joules); quantity<atomic::energy> e2(4.*atomic::hartree); quantity<atomic::energy> e3(e1); //works e1 = e2; //doesn't work e1 = quantity<si::energy>(e2); //works cout<<e1<<'\n'<<e2<<'\n'<<e3<<endl; note that the third line works because somewhere I defined a conversion_factor plus a default_conversion between units in the different systems. And it works because the conversion is explicit. Now the fourth line doesn't work, it gives an error (see at end). comenting the third line compiles. I even tried defining a conversion for this particular units, but still doesn't work. BOOST_UNITS_DEFINE_CONVERSION_FACTOR( boost::units::atomic::energy, boost::units::si::energy, double, 4.359744e-18 ); BOOST_UNITS_DEFAULT_CONVERSION( boost::units::atomic::energy, boost::units::si::energy ); error caused by line 4: In file included from /usr/include/boost/units/io.hpp:28, from /home/alfc/prj/mag/Qbox//units/systems/ atomic.hpp:187, from eos.cpp:8: /usr/include/boost/units/quantity.hpp: In member function ‘boost::units::quantity<Unit, Y>& boost::units::quantity<Unit, Y>::operator=(const boost::units::quantity<Unit2, YY>&) [with Unit2 = boost::units::unit<boost::units::list<boost::units::dim<boost::units::length_base_dimension, boost::units::static_rational<2l, 1l> >, boost::units::list<boost::units::dim<boost::units::mass_base_dimension, boost::units::static_rational<1l, 1l> >, boost::units::list<boost::units::dim<boost::units::time_base_dimension, boost::units::static_rational<-0x00000000000000002l, 1l> >, boost::units::dimensionless_type> > >, boost::units::homogeneous_system<boost::units::list<boost::units::atomic::electron_mass_base_unit, boost::units::list<boost::units::atomic::elementary_charge_base_unit, boost::units::list<boost::units::atomic::reduced_planck_constant_base_unit, boost::units::list<boost::units::atomic::coulomb_force_constant_base_unit, boost::units::list<boost::units::atomic::boltzman_constant_base_unit, boost::units::dimensionless_type> > > > > >, void>, YY = double, Unit = boost::units::unit<boost::units::list<boost::units::dim<boost::units::length_base_dimension, boost::units::static_rational<2l, 1l> >, boost::units::list<boost::units::dim<boost::units::mass_base_dimension, boost::units::static_rational<1l, 1l> >, boost::units::list<boost::units::dim<boost::units::time_base_dimension, boost::units::static_rational<-0x00000000000000002l, 1l> >, boost::units::dimensionless_type> > >, boost::units::homogeneous_system<boost::units::list<boost::units::si::meter_base_unit, boost::units::list<boost::units::scaled_base_unit<boost::units::cgs::gram_base_unit, boost::units::scale<10l, boost::units::static_rational<3l, 1l> > >, boost::units::list<boost::units::si::second_base_unit, boost::units::list<boost::units::si::ampere_base_unit, boost::units::list<boost::units::si::kelvin_base_unit, boost::units::list<boost::units::si::mole_base_unit, boost::units::list<boost::units::si::candela_base_unit, boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<boost::units::angle::steradian_base_unit, boost::units::dimensionless_type> > > > > > > > > >, void>, Y = double]’: eos.cpp:115: instantiated from here /usr/include/boost/units/quantity.hpp:234: error: invalid application of ‘sizeof’ to incomplete type ‘boost::STATIC_ASSERTION_FAILURE<false>’ /usr/include/boost/units/quantity.hpp:234: error: invalid application of ‘sizeof’ to incomplete type ‘boost::STATIC_ASSERTION_FAILURE<false>’ Thank you, Alfredo