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

Subject: Re: [Boost-users] [Units] Units in unit tests
From: Stephan Menzel (stephan.menzel_at_[hidden])
Date: 2017-05-08 13:30:31

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_at_[hidden]> 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....

My reason behind switching to units was that I wanted to get rid of
implicit assumptions about what unit a quantity is in. I am in a client /
server environment and store such vectors coming from a client that uses
centimeters as all units. Now I expect further clients and want to be
prepared for them to use other units, say millimeters. By using Units I
intend to push that translation to the foremost interface of the system and
not translate anything behind that.

I have much appreciated the constructor of quantity and the fact that it
fails to compile without a unit to give meaning to a scalar. This is an
excellent design paradigm and will help eliminate bugs!

qlength q(42.0 * meters);

Nice.

My problem that led to the above misconception was, that I needed to
translate that back and forth to the client. From the client was quite
easy:

qlength q(4200.0 * centi*meters);

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<centimeters,
double>(n_value).value();

But still, I'm not sure I am using it as intended. Is this the best and
easiest way to handle such things or am I mistaken and q / centi*meters
really is supposed to work?

Best regards...

Stephan