Re: [Boost-users] [Units] Units in unit tests

AMDG On 05/08/2017 07:30 AM, Stephan Menzel via Boost-users wrote:

Hello Steven,

thanks for your great work on Units!

On Mon, May 8, 2017 at 3:11 PM, Steven Watanabe via Boost-users < boost-users@lists.boost.org> wrote:

...

The meaning is literally division. (1.0 * meters) / meters = 1.0 If the expression exactly cancels out all units, then the result is dimensionless and can be implicitly converted to a double.

OK, I see. From my perspective, having never used Units this was not quite intuitive if you don't mind me say so. If you would allow me to get a little off topic....

This trick wasn't really a part of the library design. We originally intended that the primary way to access the raw value would be though q.value() or quantity_cast<double>(q). Using q / meters falls out naturally from the definition of division, though.

<snip> Looks good enough. But getting the value back in centimeters was not. I tried:

double raw_value_to_send_back = q / centi*meters;

But this won't compile. So I looked into scaled units and typedefed this:

typedef boost::units::make_scaled_unit< boost::units::si::length, boost::units::scale<10, boost::units::static_rational<-2> > >::type centimeters;

And now I can convert into centimers like this:

double raw_value_to_send_back = quantity(n_value).value();

But still, I'm not sure I am using it as intended.

Yes. Casting to centimeters and then extracting the value is one way to do this.

Is this the best and easiest way to handle such things or am I mistaken and q / centi*meters really is supposed to work?

q / centi*meters is parsed as (q/centi)*meters, which is not what you intend. Ideally, static_cast<double>(q/(centi*meters)) would work, but implementing that requires C++11 (explicit conversion operators) and Units was originally written in 2006. static_cast(q/(centi*meters)) should work now. In Christ, Steven Watanabe