2.1 KiB
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.
!!! note
It is always assumed that one can convert a quantity into another one with a unit of a higher
resolution. There is no protection against overflow of the representation type. In case the target
quantity ends up with a value bigger than the representation type can handle, you will be facing
Undefined Behavior.
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;
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 = value_cast<km>(q1);
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.
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.