Using physical constants with boost

Question

I'd like to define some constants in a c++ program. The excellent boost library has them predefined, and I like to assign my constants these values, e.g. something like

#include <iostream>
#include <boost/units/systems/si/codata/electromagnetic_constants.hpp>

int main(int argc, const char * argv[]) {
    double electron_charge = boost::units::si::constants::codata::e;

    std::cout << electron_charge << std::endl;
    return 0;
}

But this does not work since boost's e is not of type double but of type value_type. How can I access the value in double precision?

You probably need to use `boost::units::quantity<>` rather than `double`. All of the purpose is to make units type safe. — πάντα ῥεῖ, Nov 22 '15 at 11:06
But what if all you want is the constant and don't need the type safety? — Karoly Horvath, Nov 22 '15 at 11:07
@KarolyHorvath I think that's what `value()` function of this class is for. — πάντα ῥεῖ, Nov 22 '15 at 11:14

score 6 · Accepted Answer · answered Nov 22 '15 at 11:14

6

You need to divide the electric charge of the electron by the electric charge unit: Coulomb:

double electron_charge = boost::units::si::constants::codata::e / boost::units::si::coulomb;

answered Nov 22 '15 at 11:14

Christophe Vu-Brugier

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Nice. It makes it unitless and in the same time documents the unit. – Karoly Horvath Nov 22 '15 at 11:19

Using physical constants with boost

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