Change unit of a variable

[bansan85]

Dear,

I try to assign a variable to another variable:

  auto v1 = 100.f * (m);
  auto v2 = 100. * (km);
  std::cout << "A" << v1.number() << "m, " << v2.number() << "km\n";
  v1 = v2;
  std::cout << "B" << v1.number() << "m, " << v2.number() << "km\n";

I have:

A100m, 100km
B100000m, 100km

I can clearly see that the number (quantity) has been converted from km to m during the assignment v1 = v2.

I was wondering if it's possible to do the opposite: change the unit and the quantity of v1 according to the value of v2.

I don't think it's possible because v1 is units::quantity<units::isq::si::dim_length, units::isq::si::metre, float> and v2 is units::quantity<units::isq::si::dim_length, units::isq::si::kilometre, double>. So unit is hardcoded but not quantity.

Am I right ? Do you think this point should be added in the FAQ of mp-units ? I didn't read anything about this point in the documentation.

I want to create a GUI with a user-defined value. The user may be able to select the unit.
So if I can't change the unit of the field value in the class, do you have a trick/pattern to be able to do it ? A field with the unit selected from the user + a field with the quantity entered by the user + a function that returns the quantity in the SI unit ?

Thanks for you advise.

[JohelEGP]

For quantity types of the same dimension q0, q1, ..., qn, two possible options include std::variant<q0, q1, ..., qn> and std::common_type<q0, q1, ..., qn>.

[bansan85]

Thanks for the answer. I wanted to avoid std::variant but I just tested, std::variant works well.

template <class... Ts> struct overload : Ts... { using Ts::operator()...; };

using all_length = std::variant<
    units::quantity<units::isq::si::dim_length, units::isq::si::metre, double>,
    units::quantity<units::isq::si::dim_length, units::isq::si::kilometre,
                    double>>;

static std::string visite(const all_length &x) {
  return std::visit(
      overload{[](const auto &v) { return std::to_string(v.number()); }}, x);
}

int main() {
  using namespace units::isq::si::literals;
  using namespace units::isq::si::references;
  using namespace units::aliases::isq::si::international;

  all_length v1, v2, v3;

  v1 = 10. * (m);
  v2 = 100. * (km);

  std::cout << "A" << visite(v1) << "km, " << visite(v2) << "km\n";

  v1 = v2;

  std::cout << "B" << visite(v1) << "km, " << visite(v2) << "km\n";

I didn't knew about std::common_type<...>::type. It's interesting :) but I think that std::variant is better because with std::common_type, everything will be convert to meter and I want to keep the unit in the object.