Question

The boost::units library provides a nice way to write quantity values to a stream, see for example.

Can I use printf with the quantity values instead of using the output stream? I have a fair amount of code which has formatted the output nicely using printf and I would like to keep the formatting. Passing the quantity into say

quantity<mass_density> rho = 1.0 * mass_density;
printf("rho %6.2e\n", rho);

throws the warning

warning: format ‘%e’ expects argument of type ‘double’, but argument 2
has type
‘boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::length_base_dimension,
boost::units::static_rational<-0x00000000000000003l> >,
boost::units::list<boost::units::dim<boost::units::mass_base_dimension,
boost::units::static_rational<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>
> >, 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> > > > > > >
> > > > >’ [-Wformat]

I could just use rho.value() instead, but I would like to output the units if possible (even if I have update the format string).

I think the answer is in here, http://www.boost.org/doc/libs/1_52_0/doc/html/boost_units/Reference.html#header.boost.units.io_hpp

Answer 1

The list of format specifiers is fixed by the language. "%e" outputs a double, "%d" outputs an int etc. There’s nothing that would output an object of type quantity<mass_density>, or any other object of a class type.

You can have this:

template <typename Unit, typename T>
string symbolic_units(const quantity<Unit, T> &)
{
    return symbol_string(Unit());
}

int main()
{
    quantity<mass_density> x;
    x = 3.72 * kilogram_per_cubic_meter;

    printf ("%g %s\n", x.value(), symbolic_units(x).c_str());
}

but not much more. Unless you are willing to use streams of course.