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

I am trying to understand the dimensionality of the following two equations. --- first equation --- example::types::radii_t val1 = 5 * example::types::radii; example::types::radii_t val2 = 4 * example::types::radii; example::types::radii_t L1 = 1 / ( val1 - va2 ); If I was doing the dimensional analysis by hand I would see 1 / ( radii - radii ) Such that the end type would be radii^-1 Given that end type here is the second equation: example::types::radii_t val3 = 5 * example::types::radii; example::types::dimensionless_t val4 = 10; example::types::dimensionless_t L2 = L1 * val4 * val3; Doing the dimensional analysis by hand I think I should see: L2 = radii * (nothing) * radii^-1 = (nothing) or dimensionless What Boost Units is reporting is that L2 is actually units in radii_t. --------- QUESTIONS -------------- Q1: How do you print out the types at compile time? Q2: How can you confirm you have the correc power for a type? For example radii is indeed raised to the -1 power. Stephen On 11/30/11, Michael Powell wrote:

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 wrote:

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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...

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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 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.

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Q3: How do I define a variable type if its a fraction (e.g. radii / radian )?

typedef derived_dimension::type length_over_angle_dimension; typedef unit length_over_angle;

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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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