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

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...

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. 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. Matthias