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

Subject: Re: [Boost-users] Problem with user defined Boost Units system
From: Michael Powell (mwpowellnm_at_[hidden])
Date: 2011-11-30 15:29:13

Thanks, Matthias. IMHO, this is one of the greatest things about
boost::units, compile time unit safety. If your dimensions aren't correct
then, there's no point in proceeding until they are. Runtimes are another
question, though; but, for compile time, I like that about it.

On Wed, Nov 30, 2011 at 1:08 PM, Matthias Schabel
<boost_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.
>
> This is the library doing what it is supposed to do : your equation is
> dimensionally incorrect...
>
> > 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).
>
> Look at <boost/units/base_units/us/yard.hpp> for an example defining a new
> base unit of length with conversion to metric. You can look at the macro
> details to see how it is implemented.
>
> > Q3: How do I define a variable type if its a fraction (e.g. radii /
> radian )?
>
> typedef
> derived_dimension<length_base_dimension,1,plane_angle_base_dimension,-1>::type
> length_over_angle_dimension;
> typedef unit<length_over_angle_dimension,my_system>     length_over_angle;
>
>
> > 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
>
> You are trying to assign a [length]/[plane_angle^2] to a [length] which is
> clearly dimensionally incorrect. Depending on your perspective, one of the
> upsides/downsides of Boost.Units is that it requires that you be precise in
> formulation of equations...
>
> Matthias
>
>
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