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Subject: Re: [boost] Units of data
From: Zachary Turner (divisortheory_at_[hidden])
Date: 2009-07-02 18:09:27

On Thu, Jul 2, 2009 at 3:41 PM, Steven Watanabe<watanabesj_at_[hidden]> wrote:
> Zachary Turner wrote:
>> On Thu, Jul 2, 2009 at 2:07 PM, Steven Watanabe<watanabesj_at_[hidden]>
>> Thanks.  The confusion came from the fact that the syntax I was trying
>> was being used in some of the examples.  But there is something
>> different about those examples, so that's why it wasn't working.  One
>> of the trig examples has a line:
>> quantity<plane_angle>           theta = 0.375*radians;
>> for example.   I suppose it's due to the fact that radians are the
>> "primary" unit for this dimension?
> The difference is that plane_angle is the same unit as radians.
> In your case, data_capacity is defined as bits, while you are
> assigning it a value in bytes.  Since the units change, the conversion
> is explicit.

Well what then do you think is the "correct" or "best" way to support
what I'm trying to do? It seems like a fairly simple use case but at
least to me anyway it's turning out to be rather confusing and
complicated. For example, quantity<> is defined as

template<class Unit> class quantity;

yet one of the valid values for Unit is boost::units::dimensionless.
So are we specifying a unit or a dimension? Clearly something that is
dimensionless is also unitless but it seems inconsistent for values to
be either units, or the empty dimension.

The examples have many occurences of things such as
quantity<si::length>, quantity<si::mass>, quantity<temperature>, all
of which in a physical sense are dimensions. feet,yards,meters;
kilograms, grams, ounces; fahrenheit, celsius, kelvin are all units.
So I would expect to see quantity<fahrenheit>, quantity<ounces>,
quantity<inches>, just based on the fact that the template argument is
named Unit.

However, all of the examples seem to define only 1 valid template
argument for quantity per dimension per system. So if you want si
mass then there's 1 thing you can pass to quantity<>, even though with
all the prefixes there are a multitude of different units for mass
that are still based on the si system.

I agree that the template argument to quantity should be a unit, but
then I would expect to be able to declare a quantity<> parameterized
with every possible unit of length measurement, quantity<feet>,
quantity<inches>, quantity<mm>, etc. with automatic conversions
provided between every combination.

Maybe I'm just doing it all wrong, I'm not trying to be a PITA or
anything, at this point I really just want it to work :(

Maybe units of data storage capacity could be incorporated into boost
1.40 since they're pretty common (especially for programmers),
although ultimately it would still be nice if I could understand how
to use the library.

Thanks for your patience :)

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