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

Subject: Re: [Boost-users] Problem with user defined Boost Units system
From: Matthias Schabel (boost_at_[hidden])
Date: 2011-11-30 15:08:46

> 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