How to add your own base unit and conversions using boost::units

Question

I am currently using boost::units to represent torque in si units, however I am given the torque in pound feet. I am attempting thus to create a pound_foot unit of torque and a conversion to support this. My lazy attempt was to simply define:

BOOST_STATIC_CONST(boost::si::torque, pound_feet = 1.3558179483314 * si::newton_meters);

And then do:

boost::si::torque torque = some_value * pound_feet;

But this feels unsatisfactory. My second attempt was to attempt to define a new base unit called a pound_foot (see below). But when I attempt to use it in a way similar to the above (with a cast to the si unit) I get a page full of errors. Any suggestions on the correct approach?

namespace us {
  struct pound_foot_base_unit : base_unit<pound_foot_base_unit, torque_dimension> { };
    typedef units::make_system<
            pound_foot_base_unit>::type us_system;
    typedef unit<torque_dimension, us_system> torque;
    BOOST_UNITS_STATIC_CONSTANT(pound_foot, torque);
    BOOST_UNITS_STATIC_CONSTANT(pound_feet, torque);        
}
BOOST_UNITS_DEFINE_CONVERSION_FACTOR(us::torque, 
                                     boost::units::si::torque, 
                                     double, 1.3558179483314);

Beeswing

Answer 1

Pound foot isn't really a base unit, so better go with the clean way and define it in terms of the base units, which are feet and pounds:

#include <boost/units/base_units/us/pound_force.hpp>
#include <boost/units/base_units/us/foot.hpp>
#include <boost/units/systems/si/torque.hpp>
#include <boost/units/quantity.hpp>
#include <boost/units/io.hpp>
#include <iostream>

namespace boost {
namespace units {
namespace us {

typedef make_system< foot_base_unit, pound_force_base_unit >::type system;
typedef unit< torque_dimension, system > torque;

BOOST_UNITS_STATIC_CONSTANT(pound_feet,torque);

}
}
}

using namespace boost::units;

int main() {
    quantity< us::torque > colonial_measurement( 1.0 * us::pound_feet );
    std::cerr << quantity< si::torque >(colonial_measurement) << std::endl;
    return 0;
}

This program computes the conversion factor from the known values of foot and pound, the output is 1.35582 m^2 kg s^-2 rad^-1. Please allow me nonetheless to sneer at the inferiority of the imperial system.

Liaoning

Why would you sneer at the imperial system? It makes so much sense, sometimes you have 12 units per sub-unit, sometimes 60, sometimes 3. and the names for prefixes are different between types of things. Its a great system and your sneering is not appreciated. – Yingling