Re: [Boost-users] [Units] Temperature conversion problem

Subject: Re: [Boost-users] [Units] Temperature conversion problem
From: Matthias Schabel (boost_at_[hidden])
Date: 2011-11-30 18:00:28

> 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_at_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_at_wsblade001 ~]$ ./a.out
> 212 F
> 117.778 C
> 
> If 'f' is 212ºF, why, when converting it to ºC, 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.

Because a temperature change of 212ºF is equal to a temperature change of 117.778ºC = (5/9)*212. What would you expect the following to do:

int main(int, char **)
{
 quantity<fahrenheit::temperature> f(32*fahrenheit::temperature());
 quantity<celsius::temperature> c(0*celsius::temperature());

 std::cout << 2.*f << std::endl;
 std::cout << 2.*c << std::endl;

std::cout << f+quantity<fahrenheit::temperature>(c) << std::endl;
std::cout << quantity<celsius::temperature>(f)+c << std::endl;

 return 0;
}

? 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;
}

gives

10 C
10 F
10 K
0 absolute C
32 absolute F
0 absolute K
0 absolute C
-273.15 absolute C
32 absolute F
-459.67 absolute F
5.55556 absolute C
50 absolute F

Essentially the design of Boost.Units emphasizes safety over convenience so, generally, if you can't do something it is a problem with your abstraction.