Question

I am currently designing an API where I want that the user to be able to write code like this:

PowerMeter.forceVoltage(1 mV);
PowerMeter.settlingTime(1 ms);

Currently we do this using defines like:

#define mV *1.0e-03

This makes it very convenient for the user to write their code and it is also very readable, but of course has also drawbacks:

int ms;

Will throw some compiler errors which are hard to understand. So I am looking for a better solution.

I tried the new C++11 literals, but with this all I could achieve is:

long double operator "" _mV(long double value) {
  return value * 1e-3;
}
PowerMeter.forceVoltage(1_mV);

In the end the API does not care about the unit like Volt or second but only takes the number, so I don’t want to do any checking if you really input Volts in forceVoltage or not. So this should also be possible:

PowerMeter.forceVoltage(2 ms);

Any idea besides staying with the defines?

Answer 1

I prefer avoiding macros where ever I can, and this is an example where it should be possible.
One lightweight solution that gives you correct dimensions would be:

static double m = 1;
static double cm = 0.1;
static double mV = 0.001;

double distance = 10*m + 10*cm;

This also reflects the physical concept that units are something that’s multiplied with the value.