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: Steven Watanabe (watanabesj_at_[hidden])
Date: 2018-11-21 02:58:27

AMDG

On 11/20/2018 06:46 PM, Matt Vinson via Boost-users wrote:
> <snip>
> <some reformatting to reduce line wrapping>
> *.h-----------------------------------------------------
> 
> namespace dimensional_analysis {
> 
> Â Â Â Â typedef length_base_dimension::dimension_type length_dimension;
> Â Â Â Â typedef mass_base_dimension::dimension_type mass_dimension;
> 
> typedef make_system<inch_base_unit,pound_base_unit>::type ip_system;
> 

This should be pound_force_base_unit.

> Â  namespace lineal_force {
> Â Â Â  typedef mpl::divides<force_dimension,length_dimension>::type lineal_force_dimension;Â Â Â Â Â Â Â Â Â Â  
> 
> Â Â Â  namespace imperial {
> 
> Â Â Â Â Â  ////////////////
> Â Â Â Â Â  ////lb/in, right?
> 
> ////boost::units::force_dimension =pound
> ////boost::units::length_dimension = in
> Â Â Â Â Â  ////////////////
> Â Â Â Â Â  typedef unit<lineal_force_dimension,lengths::ip_system> lineal_force_unit;

This unit is nonsensical because the ip_system
as written is unable to represent the lineal_force_dimension.
That this doesn't give a hard compiler error is
a bug in the library.  Once you fix ip_system,
the conversions should be right.

> Â Â Â  }
> Â Â Â  namespace si {
> Â Â Â  ////////////////
> Â Â Â  ////N/m, right?
> 

Yep.

> ////boost::units::force_dimension = newton
> ////boost::units::length_dimension = meter
> Â Â Â Â Â  ////////////////
> Â Â Â Â Â  typedef unit<lineal_force_dimension,si::system> lineal_force_unit;
> Â Â Â  }
> Â  }//lineal_force
> }//dimensional_analysis
> 
> *.cpp-----------------------------------------------------
> 
> //Imperiallb/in to SI N/m
> 

The following conversion definitions are unnecessary and have no effect:

> 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/mto 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);
> 
> <snip>
> 

In Christ,
Steven Watanabe