Re: [Boost-users] Boost.Units - Can't figure out how to use it

23 Dec 2009

      On 2009-12-23, Steven Watanabe wrote:
...
AMDG
Brandon Olivares wrote:
...
I have a few other questions now though.
It is able to do scaling of units I know, but let's say you want to
use kilometer for instance. It appears you cannot have two length
units under the same system (correct?), so would you have to create
a new system just for that unit?
More practically, I need to use most of the astronomical units, or
at least light years, parsecs, and astronomical units. It seems I
can't put them under the same system, so must I have a separate
system for each of them, or am I missing something?
You can just use the ::unit_type typedef in each base unit, if you
don't want to worry about systems.
I have one more question about unit_type vs. system.

If I want a unit composed of several base units, how would I do that if I
just use unit_type as you were suggesting?

For instance, right now I have something like the following, just for
testing.

#ifndef BOOST_UNITS_ASTRONOMICAL_LIGHT_SECOND_HPP
#define BOOST_UNITS_ASTRONOMICAL_LIGHT_SECOND_HPP

#include 
#include 
#include 
#include 
#include 

namespace boost
{
namespace units
{
namespace light_second
{

typedef make_system<
boost::units::astronomical::light_second_base_unit,
boost::units::si::second_base_unit>::type system;

typedef unit dimensionless;
typedef unit length;
typedef unit velocity;

BOOST_UNITS_STATIC_CONSTANT(light_second, length);
BOOST_UNITS_STATIC_CONSTANT(light_seconds, length);
BOOST_UNITS_STATIC_CONSTANT(light_second_per_second, velocity);
BOOST_UNITS_STATIC_CONSTANT(light_seconds_per_second, velocity);

} // namespace light_second
} // namespace units
} // namespace boost

#endif

Let's just say for instance that I wanted also light_year as a unit, and
perhaps light_speed (light_year/year) or something like that. Using a system
as above, it'd be impossible to define them in the same way, so I'm curious
how you'd use unit_type as you mentioned to do this. I think you can do
something like:

BOOST_UNITS_STATIC_CONSTANT(light_year,boost::units::astronomical::light_yea
r_base_unit::unit_type);

But I don't know how you can combine two units to get velocity.

Brandon
...
In Christ,
Steven Watanabe
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