Re: [Boost-users] Problem with user defined Boost Units system

Subject: Re: [Boost-users] Problem with user defined Boost Units system
From: Stephen Torri (stephen.torri_at_[hidden])
Date: 2011-11-30 14:32:56

Michael,

Thanks for responding. I agree that your logical step-wise progression
is the correct way to go but it is not quite clear to me how to do it.
Starting with dimensions I see I have the following:

radii  -> length
radians -> angle
minute -> time
km -> length

If we start with radii I was thinking what I did in my attachment,
types.hpp, was correct for dimensions. Radii is a length so I used the
length physical dimension class.

Can you make changes to types.hpp for radii to show me what you mean
in a step-by-step detail? I usually do very well if I have one example
to work off when going on my own.

Stephen

On 11/30/11, Michael Powell <mwpowellnm_at_[hidden]> wrote:
> When I got exposure to boost::units, or Units of Measure in general, it is
> best to think of Dimensions first, line Length (L), Time (T), and so forth.
> Then think of your units second. After that, values and calculations are of
> a certain quantity.
>
> I might recommend you use the built-in velocity, or acceleration, as a
> template for your radii / radian dimension.
>
> You will end up with something like this in your dimension type definition,
> and forgive me the names aren't exact,
>
> //...
> typedef boost::units:dimension<radii_dimension,1,radian_dimension,-1>
> radii_radian_dimension;
> //...
> quantity<radii_t> my_radii;
> quantity<radian_t> my_radian;
> quantity<radii_radian_dimension> result = my_radii / my_radian;
> //...
>
> I think what you're missing are the quantity<>'s involved. In other words,
> you can't just go from unit (or dimension) to value; you have to run that
> through quantity<> to do anything useful with it.
>
> The kitchen sink examples are pretty good to learn from.
>
> After that, however, I find the compile-time safety to be quite attractive.
>
> HTH
>
> On Wed, Nov 30, 2011 at 9:49 AM, Stephen Torri
> <stephen.torri_at_[hidden]>wrote:
>
>> I am using Boost 1.45 and trying to use Boost Units to map the types
>> of astrodynamic equation variables. I am getting an error with the
>> following equation. The error is saying that the lvalue type for
>> k2_divded_by_semi_major is wrong.
>>
>> The questions I have are:
>>
>> Q1: Did I defined my system correctly? (System file is attached -
>> types.hpp)
>>
>> Q2: How do I define ratio in the system? For example I have some
>> constants that  define a conversion (1 earth radii = 6378 km).
>>
>> Q3: How do I define a variable type if its a fraction (e.g. radii / radian
>> )?
>>
>> ---- EQUATION ----
>>
>> sgp4::types::radii_t k2_divided_by_semi_major = m_k2 / pow<2> ( semi_major
>> );
>>
>> The types of variables are:
>>
>> m_k2         sgp4::types::radii_t
>> semi_major   sgp4::types::radian_t
>>
>> ---- ERROR ----
>>
>> 1>        [
>> 1>
>>
>> Unit=boost::units::unit<boost::units::list<boost::units::dim<boost::units::length_base_dimension,boost::units::static_rational<-1>>,boost::units::detail::merge_dimensions_impl<0,0>::apply<boost::units::dimensionless_type,boost::units::dimensionless_type>::type>,boost::units::homogeneous_system<boost::units::list<sgp4::types::radii_base_unit,boost::units::list<sgp4::types::radian_base_unit,boost::units::dimensionless_type>>>>,
>> 1>            Y=double
>> 1>        ]
>> 1>        and
>> 1>        [
>> 1>            Unit=sgp4::types::sgp4_length_t,
>> 1>            Y=double
>> 1>        ]
>> 1>        Constructor for class 'boost::units::quantity<Unit,Y>' is
>> declared 'explicit'
>> 1>        with
>> 1>        [
>> 1>            Unit=sgp4::types::sgp4_length_t,
>> 1>            Y=double
>> 1>        ]
>>
>> Regards,
>>
>> Stephen
>>
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>