Value Conversions

Value-preserving conversions

auto q1 = 5 * km;
std::cout << q1.in(m) << '\n';
quantity<si::metre, int> q2 = q1;

The second line above converts the current quantity to the one expressed in metres and prints its contents. The third line converts the quantity expressed in kilometres into the one measured in metres.

In case a user would like to perform an opposite transformation:

auto q1 = 5 * m;
std::cout << q1.in(km) << '\n';
quantity<si::kilo<si::metre>, int> q2 = q1;

Both conversions will fail to compile.

There are two ways to make the above work. The first solution is to use a floating-point representation type:

auto q1 = 5. * m;
std::cout << q1.in(km) << '\n';
quantity<si::kilo<si::metre>> q2 = q1;

or

auto q1 = 5 * m;
std::cout << value_cast<double>(q1).in(km) << '\n';
quantity<si::kilo<si::metre>> q2 = q1;  // double by default

Important

The mp-units library follows std::chrono::duration logic and treats floating-point types as value-preserving.

Value-truncating conversions

The second solution is to force a truncating conversion:

auto q1 = 5 * m;
std::cout << value_cast<km>(q1) << '\n';
quantity<si::kilo<si::metre>, int> q2 = q1.force_in(km);

This explicit cast makes it clear that something unsafe is going on. It is easy to spot in code reviews or while chasing a bug in the source code.

Note

q.force_in(U) is just a shortcut to run value_cast<U>(q). There is no difference in behavior between those two interfaces. q.force_in(U) was added for consistency with q.in(U) and q.force_numerical_value_in(U).

Another place where this cast is useful is when a user wants to convert a quantity with a floating-point representation to the one using an integral one. Again this is a truncating conversion, so an explicit cast is needed:

quantity<si::metre, int> q3 = value_cast<int>(3.14 * m);

Info

It is often fine to use an integral as a representation type, but in general, floating-point types provide better precision and are privileged in the library as they are considered to be value-preserving.