First, ignore the value problems and such...I know the tests will fail. Just trying to understand typing for now. In one unit test this compiles and succeeds: CPPUNIT_ASSERT(abs(sr.get<0>() - (773.0 * rpm)) < (1.0 * rpm)); CPPUNIT_ASSERT(abs(sr.get<1>() - (.11666667 * hp)) < (.002 * hp)); That is a custom system (actually a set of independent systems since I was working in units that aren't systemic). "sr" is a tuple containing quantities of the corresponding units. In another unit test trying to work with the SI system this won't compile: vflow_qty Q = 10.0 * gpm; // 0.000630901967 m^3s^-1 vflow_qty x = .6 * m3ps; CPPUNIT_ASSERT( abs(Q - x) < (.00006 * m3ps)); // less than +/- 1% Right now gpm is just another unit instance just like m3ps - I never got past this stage in my unit testing. This DOES compile: CPPUNIT_ASSERT( abs(Q - x) < (.00006)); // less than +/- 1% The compile failure shows that abs() is returning a dimensionless type; this seems to coincide better with the documentation. If this is the case, how did the previous versions work just fine? The rpm and hp units are not dimensionless. Looking at it, the first version doesn't use my own creation of composite dimensions - maybe I don't understand how to do that correctly. Full code of the two setups follows, first is the one that works, second is the one that does not: FIRST---------------- namespace gearpump_system { /// \cond struct gp_sys_press_tag : boost::units::ordinal<101> {}; struct gp_sys_speed_tag : boost::units::ordinal<102> {}; struct gp_sys_flow_tag : boost::units::ordinal<103> {}; struct gp_sys_visc_tag : boost::units::ordinal<104> {}; struct gp_sys_power_tag : boost::units::ordinal<105> {}; /// \endcond /// System for pressure quantities (psi) typedef boost::units::heterogeneous_system<gp_sys_press_tag> pressure_system; /// System for frequency quantities (rpm) typedef boost::units::heterogeneous_system<gp_sys_speed_tag> speed_system; /// System for volumetric flow quantities (gpm) typedef boost::units::heterogeneous_system<gp_sys_flow_tag> flow_system; /// System for kinematic viscosity quantities (ssu) typedef boost::units::heterogeneous_system<gp_sys_visc_tag> visc_system; /// System for power quantities (hp) typedef boost::units::heterogeneous_system<gp_sys_power_tag> power_system; /// \cond typedef boost::units::composite_dimension<boost::units::length_tag, 3, boost::units::time_tag, -1> flow_type; typedef boost::units::composite_dimension < boost::units::length_tag, 2 , boost::units::time_tag, -1 > kinematic_viscosity_type; /// \endcond /// Unit type for pressure quantitien typedef boost::units::unit<boost::units::pressure_type, pressure_system> pressure; /// Unit type for volumetric flow quantities typedef boost::units::unit<flow_type, flow_system> flow_rate; /// Unit for frequency quantities typedef boost::units::unit<boost::units::frequency_type, speed_system> frequency; /// Unit for power quantities typedef boost::units::unit<boost::units::power_type, power_system> power; /// Unit for kinematic viscosity quantities typedef boost::units::unit<kinematic_viscosity_type, visc_system> kinematic_viscosity; // This doesn't work for doxygen. Figure out later maybe... /*! ** \var static pressure psi ** \brief Pressure unit instance */ BOOST_UNITS_STATIC_CONSTANT(psi, pressure); BOOST_UNITS_STATIC_CONSTANT(gpm, flow_rate); BOOST_UNITS_STATIC_CONSTANT(rpm, frequency); BOOST_UNITS_STATIC_CONSTANT(hp, power); BOOST_UNITS_STATIC_CONSTANT(ssu, kinematic_viscosity); /// Pressure values typedef boost::units::quantity<pressure> pressure_qty; /// Volumetric flow values typedef boost::units::quantity<flow_rate> flow_qty; /// Frequency values typedef boost::units::quantity<frequency> frequency_qty; /// Power values typedef boost::units::quantity<power> power_qty; /// Kinematic viscosity values typedef boost::units::quantity<kinematic_viscosity> kinematic_viscosity_qty; ----------------------- SECOND------------ namespace units { using namespace boost::units; namespace gearpump_system { using namespace boost::units::SI; typedef composite_dimension<boost::units::length_tag, 3, boost::units::time_tag, -1> vflow_type; typedef unit<vflow_type, SI::system> vflow; typedef quantity<vflow> vflow_qty; BOOST_UNITS_STATIC_CONSTANT(gpm, vflow_qty); BOOST_UNITS_STATIC_CONSTANT(m3ps, vflow_qty); } }