Re: [Boost-users] [Units] unary minus on units is counter intuitive (or a bug)

2010/11/30 Matthias Schabel :

I found this counterintuitive result of applying the unary minus operator to a unit

"- si::meter" is the same as "si::meter" (or any other unit).

or worst

std::cout <<  - si::meter*1. << std::endl; // prints 1*m

i.e. the minus is explicitly ignored. The root of the problem is this curious line in boost/units/unit.hpp

I agree this is counterintuitive - the intent was that the unary operators should not affect units.

a more intuitive result (but not completely elegant) could be attained by defining instead:

template quantity > operator-(unit const& u){ return -1.*u; }

This would be inconsistent with library architecture...

it seems so, but why exactly? because it commits to a certain internal representation (i.e. double)? (after all it is the default representation chosen by the library). Arguably, I think it is not that bad to define operator- or operator+ on units to return a quantity. In any case at least with the changes one is free to define such a function. On the other hand being forced to type -1.*si::meter or 1.*si::meter is not that bad either.

Note aside: Why would I need "minus units" in the first place? The original code that produced the confusion was

- kB*T

which appears commonly in physics. kB is the "atomic" unit of entropy/ specific heat (atomic::heat_capacity) and T is a temperature quantity. This was parsed as

(-kB)*T and then as kB*T

i.e. the minus sign was ignored all together.

In my opinion unary operator- should not be defined for units in the first place. The current definition only makes things worst, by creating a counter intuitive result AND making the fix conflict with the definition.

I think you're right that it is more correct for those operators to be undefined in this case.

yes, I think unary minus shuould be undefined for units. After all units, as mathematical objects, are quite weird. (see http://arxiv.org/abs/0710.1313).

Try the following and see if it works for you : in unit.hpp make the following change to lines 102-116 : /// unit unary plus typeof helper /// INTERNAL ONLY //template //struct unary_plus_typeof_helper< unit > //{ //    typedef unit    type; //}; /// unit unary minus typeof helper /// INTERNAL ONLY //template //struct unary_minus_typeof_helper< unit > //{ //    typedef unit    type; //}; In quantity.hpp make the following change to lines 559-579 : /// specialize unary plus typeof helper /// INTERNAL ONLY template struct unary_plus_typeof_helper< quantity > {     typedef typename unary_plus_typeof_helper<Y>::type      value_type; //    typedef typename unary_plus_typeof_helper<Unit>::type   unit_type;     typedef Unit                                            unit_type;     typedef quantity                  type; }; /// specialize unary minus typeof helper /// INTERNAL ONLY template struct unary_minus_typeof_helper< quantity > {     typedef typename unary_minus_typeof_helper<Y>::type     value_type; //    typedef typename unary_minus_typeof_helper<Unit>::type  unit_type;     typedef Unit                                            unit_type;     typedef quantity                  type; }; This should result in a compile error in your code at -kB*T which is certainly better than a silent error. If that works, we can commit the changes to trunk.

thank you, I made the changes and it works, that is, this program doesn't compile: #include int main(){ using namespace boost::units; std::clog << -2.*si::meter << std::endl; //compiles std::clog << -si::meter*2. << std::endl; //compile error std::clog << +2.*si::meter << std::endl; //compiles std::clog << +si::meter*2. << std::endl; //compile error return 0; } Therefore, I believe you can commit the changes. Thank you, Alfredo