Oh, okay, missed that. So I probably don't fully understand what the units library is doing but I find this behavior a bit worrisome.

[kbelco@wsblade001 ~]$ cat test3.cpp

int main(int, char **) {
quantity<fahrenheit::temperature> f(212 * fahrenheit::temperature());
quantity<celsius::temperature> c(f);
std::cout << f << std::endl;
std::cout << c << std::endl;
return 0;
}

[kbelco@wsblade001 ~]$ ./a.out
212 F
117.778 C

If 'f' is 212慚, why, when converting it to 慢, do I end up at 117 and change? Is this really what we should expect? It seems that this very simple use case produces unexpected, and to a casual user wrong, results. I understand the need for complexity but it seems to show up very early in the user experience.

? What does multiplying an absolute temperature by 2 mean? Should the last two lines give me 32+32 = 64 and 0+0 = 0? Why would the order of conversion matter? I understand the desire for temperature to equate to the common interpretation, but this approach quickly becomes problematic when you allow arithmetic operations. The same issue arises in std::chrono in differentiating between specific time points and durations. If you want a temperature point, use quantity< absolute<fahrenheit::temperature> >. This will have the added benefit of allowing conversions but disallowing operations that don't make sense... Some code:

#include <iostream>

#include <boost/units/absolute.hpp>

#include <boost/units/systems/temperature/celsius.hpp>

#include <boost/units/systems/temperature/fahrenheit.hpp>

#include <boost/units/systems/si/temperature.hpp>

#include <boost/units/base_units/temperature/conversions.hpp>

using namespace boost::units;

int main()

{

quantity<celsius::temperature> dTC(10.*celsius::temperature());

quantity<fahrenheit::temperature> dTF(10.*fahrenheit::temperature());

quantity<si::temperature> dTK(10.*si::kelvin);

std::cout << dTC << std::endl;

std::cout << dTF << std::endl;

std::cout << dTK << std::endl;

quantity<absolute<celsius::temperature> > TC(0*absolute<celsius::temperature>());

quantity<absolute<fahrenheit::temperature> > TF(32.*absolute<fahrenheit::temperature>());

quantity<absolute<si::temperature> > TK(0.*absolute<si::temperature>());

std::cout << TC << std::endl;

std::cout << TF << std::endl;

std::cout << TK << std::endl;

std::cout << quantity<absolute<celsius::temperature> >(TF) << std::endl;

std::cout << quantity<absolute<celsius::temperature> >(TK) << std::endl;

std::cout << quantity<absolute<fahrenheit::temperature> >(TC) << std::endl;

std::cout << quantity<absolute<fahrenheit::temperature> >(TK) << std::endl;

// std::cout << TC+quantity<absolute<celsius::temperature> >(TF) << std::endl; // fails because you cannot add two absolute temperatures

std::cout << TC+quantity<celsius::temperature>(dTF) << std::endl;

std::cout << TF+quantity<fahrenheit::temperature>(dTC) << std::endl;

return 0;

}