Re: [boost] [review] pqs

From: Andy Little (andy_at_[hidden])
Date: 2006-06-08 03:49:41

"David Greene" <greened_at_[hidden]> wrote in message 
news:44873FB6.6020709_at_obbligato.org...
> Andy Little wrote:
>
>>>>The kilogram rather than the gramme  is a base unit in the SI. I think that
>>>>the
>>>>history has to do with the emergence of the SI system from the C.G.S system.
>>>>Presumably they decided that the gramme from the C.G.S system was too small 
>>>>a
>>>>unit, the mystery then being why C.G.S used centimeters in the first place.
>>>>Whatever. I agree the whole thing is a logicians nightmare. I'm just the
>>>>messenger...
>>>
>>>But I don't understand why the SI "interface" commands a particular
>>>pqs implementation.  From the user perspective, they don't really care.
>>>But as soon as a programmer tries to extend the framework, nightmares
>>>ensue.
>>
>>
>> I dont think electrotechnology is an extension to the SI. IMO its a different
>> system in which bytes and time exists, but not mass, length, current etc etc.
>
> We're not talking about Mibibytes here.  My question is, why does
> someone extending the "mass" abstract quantity (is that the correct
> term?) need to worry about prefix_offset?  Why isn't "kilo" specified
> with "3" like it is everywhere else?

kilogram is an si base_unit.

In pqs base units are represented in compile time code by the 
boost:pqs::meta::unit
in header <boost/pqs/meta/unit.hpp>

The unit consists in two parts used for calculation, and an id whose purpose is 
irrelevent here.

The two parts of the unit used in unit conversions are the exponent and the 
multiplier, both represeneted as compile time constants

For coherent units the multiplier is one. The kilogram is a coherent unit so we 
can also ignore that.

The exponent is the  power of 10 by which the base unit is multiplied to get the 
unit.

If kilograms played by the rules its exponent would be 3 , but for whatever 
historical reasons its 0.

The output for named quantities with coherent units is computed at compile time 
from the class template signature.
For those interested this is in:
<boost/pqs/meta/components/adjusted_coherent_prefix.hpp>

 Without the prefix_offset, because kilograms has an exponent of 0, then its 
unit output would be worked out as 'g', which is plainly wrong.

Does this help?

> AFAICT this has no impact on the user interface.  The user will
> just specify pqs::mass::kg and doesn't care what the underlying
> "prefix number" is.  But this change makes the framework extender's
> job much easier.

Complain to the SI people. It aint my fault guvnor!

regards
Andy Little