Re: [Boost-users] [Units] Issue with user defined quantities

Subject: Re: [Boost-users] [Units] Issue with user defined quantities
From: alfC (alfredo.correa_at_[hidden])
Date: 2012-01-21 22:53:59

On Tuesday, January 3, 2012 3:56:11 AM UTC-8, Pieter B. wrote:
>
> LS,
>
>  
>
> I have some problems working with user defined quantities/units in 
> combination with unitless quantities (such as the results of a sin() or 
> cos() operations).Taking the example of the ‘nautical mile’, which is 
> presented in the documentation:
>
>  
>
I am not sure but it maybe that your units system needs an explicit 
dimensionless unit. ???

namespace nautical{
     typedef unit<dimensionless_type, system> dimensionless;
}

Maybe not but you can try.
 

> ---START CODE ---
>
>  
>
> namespace nautical {
>
>        struct length_base_unit :
>
>               boost::units::base_unit<length_base_unit, length_dimension, 
> 1>
>
>        {
>
>               static std::string name() { return "nautical mile"; }
>
>               static std::string symbol() { return "nmi"; }
>
>        };
>
>        typedef boost::units::make_system<length_base_unit>::type system;
>
>        typedef unit<length_dimension,system> length;
>
>        static const length mile,miles;
>
> } 
>
> BOOST_UNITS_DEFINE_CONVERSION_FACTOR(nautical::length_base_unit,
>
>        boost::units::si::meter_base_unit,
>
>        double, 1.852e3);
>
>  
>
> ---END CODE ---
>
>  
>
> The following does not compile:
>
>  
>
> ---START CODE ---
>
>  
>
> typedef quantity<boost::units::si::plane_angle> TAngleInRad;
>
> typedef boost::units::quantity<nautical::length> TNauticalDistanceInNMI;
>
>  
>
> TNauticalDistanceInNMI NauticalDistanceInNMI;
>
> TNauticalDistanceInNMI SumNauticalDistanceInNMI = NauticalDistanceInNMI + 
> NauticalDistanceInNMI; // OK
>
> TNauticalDistanceInNMI ScaledSumNauticalDistanceInNMI = 
> NauticalDistanceInNMI + NauticalDistanceInNMI*sin(AngleInRad); 
>
> // Last line does not compile due to lack of a suitable operator+
>
>  
>
> ---END CODE ---
>
>  
>
> It *does* work correctly when using the ‘nautical mile’ as it has been 
> defined in the library itself in “nautical_mile.hpp” (as a scaled base unit 
> related to the si::meter):
>
>  
>
> ---START CODE ---
>
>  
>
> namespace metric {
>
> typedef scaled_base_unit<boost::units::si::meter_base_unit, scale<1852, 
> static_rational<1> > > nautical_mile_base_unit;
>
> } // definition of the nautical mile from “nautical_mile.hpp”
>
>  
>
> typedef boost::units::metric::nautical_mile_base_unit::unit_type 
> nautical_mile_unit;
>
> typedef quantity<nautical_mile_unit> TNauticalDistanceInNMI;
>
>  
>
> ---END CODE ---
>
>  
>
> However, I don’t think I can specify my user-defined units/quantities in 
> this manner as they use a scale factor represented by an arbitrary floating 
> point value. 
>
>  
>
> My perception is that the example of the documentation does not work due 
> to the mixing of quantities from the si system (the sin() returns a 
> dimensionless quantity in that system) with user-defined quantities in 
> another (‘nautical’) system, while the ‘nautical mile’ from 
> ‘nautical_mile.hpp’ is a scaled version of the si::meter and thus defined 
> within the realms of the si::system – but please correct me if I am wrong 
> here.
>
>  
>
> My problem can thus be summarized as follows:
>
>  
>
> -          the solution from the documentation does not compile
>
> -          the solution from ‘nautical_mile.hpp’ uses a scaled base unit 
> which is limited to scaling factors of the form scale<Base,Exponent>
>
>  
>
> and my question is therefore: how can I define my user-defined 
> units/quantities in such a way that they allow the required operations 
> while being related to base units by an arbitrary scale factor?
>
>  
>
> Help would be highly appreciated,
>
>  
>
> Pieter
>
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> -------------------
>
> Nautical mile version #1 (from the included nautical_mile.hpp):
>
>  
>
> namespace metric {
>
> typedef scaled_base_unit<boost::units::si::meter_base_unit, scale<1852, 
> static_rational<1> > > nautical_mile_base_unit;
>
> }
>
>  
>
> template<>
>
> struct base_unit_info<metric::nautical_mile_base_unit> 
>
> {
>
>     static const char* name()   { return("nautical mile"); }
>
>     static const char* symbol() { return("nmi"); }
>
> };
>
>  
>
>  
>
>  
>
>  
>
> typedef quantity<boost::units::si::plane_angle> TAngleInRad;
>
> typedef quantity<boost::units::si::length> TDistanceInM;
>
>  
>
> typedef boost::units::metric::nautical_mile_base_unit::unit_type 
> nautical_mile_unit;
>
> typedef quantity<nautical_mile_unit> TDistanceInNMI;
>
>  
>
> typedef boost::units::quantity<nautical::length> TNauticalDistanceInNMI;
>
>  
>
>  
>
> # this is OK:
>
>  
>
> TAngleInRad Angle(0.5*radians);
>
> TDistanceInNMI DistanceInNMI;
>
> TDistanceInNMI SumDistanceInNMI = DistanceInNMI + DistanceInNMI;
>
> TDistanceInNMI ScaledSumDistanceInNMI = DistanceInNMI + 
> DistanceInNMI*sin(AngleInRad);
>
>  
>
>  
>
> TNauticalDistanceInNMI NauticalDistanceInNMI;
>
> TNauticalDistanceInNMI SumNauticalDistanceInNMI = NauticalDistanceInNMI + 
> NauticalDistanceInNMI;
>
> TNauticalDistanceInNMI ScaledSum NauticalDistanceInNMI = 
> NauticalDistanceInNMI + NauticalDistanceInNMI*sin(AngleInRad); // DOES NOT 
> COMPILE for lack of a suitable operator+
>
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