"David Greene" <greened@obbligato.org> wrote in message news:44873FB6.6020709@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