Re: [boost] Units of data

Subject: Re: [boost] Units of data
From: Zachary Turner (divisortheory_at_[hidden])
Date: 2009-07-01 22:08:11

On Wed, Jul 1, 2009 at 7:41 PM, Steven Watanabe<watanabesj_at_[hidden]> wrote:
> AMDG
>
> Zachary Turner wrote:
>>
>> Has anyone extended (even privately) Boost.Units to include support
>> for units of data (bits, bytes, KB, MB, etc)?  It seems like this
>> would be a useful addition to Boost.Units.
>>
>> Assuming no, what is the minimum amount of work required to do this
>> myself?  There's not a lot of info in the docs on how to define new
>> base units, although I've already begun looking over the source code
>> to try to come up with something
>
> warning untested code.
>
> #include <boost/units/base_dimension.hpp>
> #include <boost/units/base_unit.hpp>
> #include <boost/units/scale.hpp>
> #include <boost/units/scaled_base_unit.hpp>
>
> // note that the numbers here are arbitrary as long as they are unique.
> struct data_base_dimension :
> boost::units::base_dimension<data_base_dimension, 73254> {};
> typedef data_base_dimension::dimension_type data_dimension;
>
> struct bit_base_unit : boost::units::base_unit<bit_base_unit,
> data_dimension, 76389> {};
> typedef boost::units::scaled_base_unit<bit_base_unit, boost::units::scale<8>
>> byte_base_unit;
>
> namespace boost {
> namespace units {
> template<>
> struct base_unit_info< ::byte_base_unit> {
>   static const char* name() { return("byte"); }
>   static const char* symbol() { return("B"); }
> };
> }
> }
>
> typedef boost::units::scaled_base_unit<byte_base_unit,
> boost::units::scale<2, boost::units::static_rational<10> > > KB_base_unit;
> typedef boost::units::scaled_base_unit<byte_base_unit,
> boost::units::scale<2, boost::units::static_rational<20> > > MB_base_unit;
> typedef boost::units::scaled_base_unit<byte_base_unit,
> boost::units::scale<2, boost::units::static_rational<30> > > GB_base_unit;
> typedef boost::units::scaled_base_unit<byte_base_unit,
> boost::units::scale<2, boost::units::static_rational<40> > > TB_base_unit;
>
> See also
> http://www.boost.org/doc/html/boost_units/Dimensional_Analysis.html
> http://www.boost.org/doc/html/boost_units/Units.html
> I believe that these two pages show creating a minimal SI system from
> scratch.
>

Thanks for your response.  I got this working mostly, but I'm still
having difficulty seeing how to tie it all together.  I added the
following:

typedef boost::units::make_system<bit_base_unit>::type data_system;
typedef boost::units::unit<data_base_dimension::dimension_type,
data_system>	capacity;

But I'm still not sure how to create and manipulate values with these
units.  For example, I can do:

quantity<capacity> compressed_size;

but if I try to assign it to anything I get errors.  I tried using the
BOOST_UNITS_STATIC_CONSTANT macro to create aliases such as bytes,
megabytes, etc but I think this is the wrong approach, is it because
all the units above are defined as base units?

Ideally I want to be able to type:

quantity<capacity> compressed_size = 10000 * bytes;
quantity<capacity> total_size = 400000 * bytes;

float ratio = compressed_size / total_size;  //should be dimensionless

quantity<time> duration = 300 * seconds;

quantity<data_rate> = total_size / duration;  //has dimensions capacity/time