Re: [Boost-users] boost::units - converting from one derived_dimension to another across systems (imperial to metric)

Subject: Re: [Boost-users] boost::units - converting from one derived_dimension to another across systems (imperial to metric)
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2018-11-21 01:55:19

On Tue, Nov 20, 2018 at 8:47 PM Matt Vinson via Boost-users
<boost-users_at_[hidden]> wrote:
>
> degski: I do not think am making the "nauseam" mistake.  My issue is using Boost.Units correctly which I concede is difficult for me but I am reading the docs and trying to get it.
> In our field of engineering, we use Kg as a force; not a mass.  (We also use pound as a force; not a mass - for your "UK's Weights and Measures Act 1963" counterpart).  That said, I should not have made the mistake of using Kg as a force in Boost.Units, it's true.  Shame on me.
> My task is to convert between lineal forces: k/ft, k/in, lb/ft, lb/in, kN/m, kN/cm, kN/m, N/m, N/cm, N/m, Metric ton/m, Metric ton/cm, Metric ton/mm, Kg/m, Kg/cm, Kg/mm.  These are all force/length.

Coming from an energy oil and gas background applying Boost.Units,
that's a difficult conversation to have especially when "units" are
used, but in fact are masking certain details, magic numbers, etc,
confounding their dimensions.

> Steven Watanabe: Thank you, again.
> #1: I was not trying to say it's wrong; sorry.  Just that I did not see 'length' and opted for the one that did.  The unit's are correct, as you say.
> #2: This is an error to start with Kg(force)/meter.  I can start with N/meter.  I've changed this variable name to newton_per_meter and the conversion factors.

I can tell you this much; once you get reasonably proficient with
Boost.Units, there is a strong compiler confidence in correctness once
you have your dimensions, units, etc, meshing well together. By that I
mean, in truth, compile time confidence. The compiler / Boost.Units
will tell you when you have it wrong, and it will compile when you
have it correct. That's a major plus in my book.

Cheers, best of luck!

> *.h-----------------------------------------------------
>
> namespace dimensional_analysis {
>
>     typedef boost::units::length_base_dimension::dimension_type length_dimension;
>
>     typedef boost::units::mass_base_dimension::dimension_type mass_dimension;
>
> typedef boost::units::make_system<boost::units::us::inch_base_unit,boost::units::us::pound_base_unit>::type ip_system;
>
>        namespace lineal_force {
>
>              typedef boost::mpl::divides<boost::units::force_dimension,boost::units::length_dimension>::type lineal_force_dimension;
>
>              namespace imperial {
>
>                      ////////////////
>
>                      ////lb/in, right?
>
> ////boost::units::force_dimension = pound
>
> ////boost::units::length_dimension = in
>
>                      ////////////////
>
>                      typedef boost::units::unit<lineal_force_dimension,lengths::ip_system> lineal_force_unit;
>
>               }
>
>               namespace si {
>
>                      ////////////////
>
>                      ////N/m, right?
>
> ////boost::units::force_dimension = newton
>
> ////boost::units::length_dimension = meter
>
>                      ////////////////
>
>                      typedef boost::units::unit<lineal_force_dimension,boost::units::si::system> lineal_force_unit;
>
>               }
>
>        }//lineal_force
>
> }//dimensional_analysis
>
> *.cpp-----------------------------------------------------
>
> //Imperial lb/in to SI N/m
>
> BOOST_UNITS_DEFINE_CONVERSION_FACTOR(
>
> dimensional_analysis::lineal_force::imperial::lineal_force_unit,
>
> dimensional_analysis::lineal_force::si::lineal_force_unit,
>
> double,
>
> 175.1268369864); // exact conversion
>
> BOOST_UNITS_DEFAULT_CONVERSION(
>
> dimensional_analysis::lineal_force::imperial::lineal_force_unit,
>
> dimensional_analysis::lineal_force::si::lineal_force_unit);
>
> //SI N/m to Imperial lb/in
>
> BOOST_UNITS_DEFINE_CONVERSION_FACTOR(
>
> dimensional_analysis::lineal_force::si::lineal_force_unit,
>
> dimensional_analysis::lineal_force::imperial::lineal_force_unit,
>
> double,
>
> 0.1837185501); // exact conversion
>
> BOOST_UNITS_DEFAULT_CONVERSION(
>
> dimensional_analysis::lineal_force::si::lineal_force_unit,
>
> dimensional_analysis::lineal_force::imperial::lineal_force_unit);
>
> //Imperial to SI.  This should convert lb/in to N/m and return 175.1268369864 (N/m)
>
> //But it does not.  Its converting lb (mass) to Kg (mass), 0.454
>
> const auto t2 = static_cast<boost::units::quantity<dimensional_analysis::lineal_force::si::lineal_force_unit>>(
>
> 1.0 * dimensional_analysis::lineal_force::imperial::lineal_force_unit::unit_type());
>
> //Again, Imperial to SI.  This should convert lb/in to N/m and return 175.1268369864 (N/m)
>
> //But it does not.  It returns lb (mass) to Kg (mass)
>
> const auto t3 = boost::units::conversion_factor(
>
> dimensional_analysis::lineal_force::imperial::lineal_force_unit::unit_type(),
>
> dimensional_analysis::lineal_force::si::lineal_force_unit::unit_type());
>
> //SI to Imperial.  This should convert N/m to lb/in and return 0.1837185501 (lb/in)
>
> //But it does not.  Its converting Kg (mass) to lb (mass), 2.205
>
> const auto t1 = static_cast<boost::units::quantity<dimensional_analysis::lineal_force::imperial::lineal_force_unit>>(
>
> 1.0 * dimensional_analysis::lineal_force::si::lineal_force_unit::unit_type());
>
> //So, I don’t know why the library is converting mass; I’ve tuned it to use boost::units::force_dimension/boost::units::length_dimension
>
> //I am doing something wrong where the library is not using force/length.
>
> //Below does work, mainly.
>
> //This is convert Si to SI and return value of 1.0 is correct.
>
> const auto t4 = static_cast<boost::units::quantity<dimensional_analysis::lineal_force::si::lineal_force_unit>>(
>
> 1.0 * boost::units::si::newton / boost::units::si::meter);
>
> //This convert lb/in to SI and return value is 175.1268369864 is correct.
>
> const auto t5 = static_cast<boost::units::quantity<dimensional_analysis::lineal_force::si::lineal_force_unit>>(
>
> 1.0 * boost::units::us::pound_force_base_unit::unit_type() / boost::units::us::inch_base_unit::unit_type());
>
> //This converts lb/in to lb/in and return value is of 1.0 is correct.
>
> const auto t6 = static_cast<boost::units::quantity<dimensional_analysis::lineal_force::imperial::lineal_force_unit>>(
>
> 1.0 * boost::units::us::pound_force_base_unit::unit_type() / boost::units::us::inch_base_unit::unit_type());
>
> //This is 2.205 and not correct.
>
> //It’s not converting SI-force/length to Imperial-force/length
>
> //Its converting Kg (mass) to lb (mass), 2.205
>
> const auto t7 = static_cast<boost::units::quantity<dimensional_analysis::lineal_force::imperial::lineal_force_unit>>(
>
> 1.0 * boost::units::si::newton / boost::units::si::meter);
>
> //What have I done wrong?
>
> //Any help is appreciated.
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