How to handle units in c++ interface

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);


        

Answer 1

Here's what I came up with... pretty much the same idea as Anders K, but since I wrote the code, I'll post it:

#include <iostream>

using namespace std;

class MilliVoltsValue;
class VoltsValue;

class VoltsValue
{
public:
   explicit VoltsValue(float v = 0.0f) : _volts(v) {/* empty */}
   VoltsValue(const MilliVoltsValue & mV);

   operator float() const {return _volts;}

private:
   float _volts;
};

class MilliVoltsValue
{
public:
   explicit MilliVoltsValue(float mV = 0.0f) : _milliVolts(mV) {/* empty */}
   MilliVoltsValue(const VoltsValue & v) : _milliVolts(v*1000.0f) {/* empty */}

   operator float() const {return _milliVolts;}

private:
   float _milliVolts;
};

VoltsValue :: VoltsValue(const MilliVoltsValue & mV) : _volts(mV/1000.0f) {/* empty */}

class PowerMeter
{
public:
   PowerMeter() {/* empty */}

   void forceVoltage(const VoltsValue & v) {_voltsValue = v;}
   VoltsValue getVoltage() const {return _voltsValue;}

private:
   VoltsValue _voltsValue;
};

int main(int argc, char ** argv)
{
   PowerMeter meter;

   meter.forceVoltage(VoltsValue(5.0f));
   cout << "Current PowerMeter voltage is " << meter.getVoltage() << " volts!" << endl;

   meter.forceVoltage(MilliVoltsValue(2500.0f));
   cout << "Now PowerMeter voltage is " << meter.getVoltage() << " volts!" << endl;

   // The line below will give a compile error, because units aren't specified
   meter.forceVoltage(3.0f);   // error!

   return 0;
}

Answer 2

how about instead turning it around a bit by creating classes (ms,mV) for the different currents

e.g.

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

It is pretty clear to the user and arguably not hard to read plus you would get type checking for free. having a common base class for the different units would make it easier to implement.

Answer 3

You can see the library "C++ Units" from Calum Grant as a good example of how to implement this. The library is a bit outdated, but still worth to see or may be to use.

Also, i think it might be interesting to read: "Applied Template Metaprogramming in SI UNITS: the Library of Unit-Based Computation"

There is one more good library: UDUNITS-2 which:

contains a C library for units of physical quantities and a unit-definition and value-conversion utility.