one/quantity support

[ccorn]

While working on an adaptation of a generic vector class to support boost::units, I found boost::units::one to be quite helpful. It can act as a substitute for a unit_type when there is no quantity (and no system of units) involved, e.g. just a built-in floating-point type.

Using one got me quite far until I tried to prepare for instances of units, or one, divided by actual values. This brings divide_typeof_helper<one, T>::type into play. Which turns out to be T, even if T is a unit or a quantity type. (Such specializations are given in detail/one.hpp and quantity.hpp.) The expected result would be something with the reciprocated unit type, i.e. all base unit exponents sign-flipped.

I have looked for effects on officially documented functions, including pow<-1> (since static_int_power_sign_impl<N, true> literally uses divide_typeof_helper<one,), but I have found no misbehavior on the surface. Somehow the implementation manages to avoid this case, so I cannot label this as a bug.

Nevertheless, if you want boost::units::one() / x to work as intended for a quantity or unit constant x, you can use the attached boost-units-one-div.cc.txt as a basis for a test case.

[swatanabe]

AMDG `one` was never intended to be a public API. It exists to simplify several parts of the implementation and was only designed to work with scalars. In Christ, Steven Watanabe

[correaa]

@ccorn, I personally define a constant called "uno" (symbol "U") https://iopscience.iop.org/article/10.1088/0026-1394/35/6/3
Would that work for you?

https://gitlab.com/correaa/boost-units/blob/master/systems/si/abbreviations.hpp