"Geoffrey Romer" <geoff.romer@gmail.com> wrote
Would there be any interest in a library for attaching units (physical or otherwise) to numeric variables, so that the type system could enforce dimension- and unit-consistency in all calculations, and convert semi-automatically between compatible units?
To make this a little more concrete, here's a sketch of the kind of code such a library could enable:
--- quantity<meters, double> distance(1.0); quantity<newtons, double> force(5.0);
quantity<joules, double> work = distance * force; quantity<calories, double> work_cal = unit_cast<calories>(work); work_cal = work; //Compile error or implicit conversion, // possibly depending on compiler flag quantity<watts, double> power = work; //Compile error- incompatible types
In pqs http://www.servocomm.freeserve.co.uk/Cpp/physical_quantity/index.html it would look like this: pqs::length::m distance(1.); pqs::force::N force(5.); pqs::energy::J work = distance * force; // note: you would need to add calories of which according to the SI there are I // think 6 varieties //I opted to make automatic conversions between units... pqs::energy::cal work_cal = work; //pqs::power::W power = work ; //Compile error- incompatible types pqs::power::W power = work / pqs::time::s(1); //ok
I know there's been a lot of discussion of units for a GUI library, so I'm especially interested in what people involved in that thread think of this idea- I want to make something sufficiently powerful to be useful in a context like that.
The problem with using physical units for a GUI is that they can make the code rather inflexible. It is possible to template functions on the unit parameter but this will cause code bloat. Far better to set a notional unit somewhere and then have the ability to change the unit easily at runtime. That said some applications can work only with one set of units because the domain is fixed. e.g PCB design . In these cases it is helpful to users to provide a set of user units(micrometers, millimeters, centimeters, inches) that can be converted to-from a standard unit for use by the application( probably millimeters in the case of PCBs.) regards Andy Little