Re: [boost] Boost Units library preview

From: Matthias Troyer (troyer_at_[hidden])
Date: 2006-08-24 00:13:11

On Aug 23, 2006, at 8:40 PM, Andy Little wrote:

>
> "Matthias Troyer" <troyer_at_[hidden]> wrote in message
> news:14AD1E01-8077-42A7-A25E-5B5AF8AB1EFA_at_itp.phys.ethz.ch...
>
>> I have a question here: what is the purpose of these named quantities
>>  - is it to output J instead of kgm^2s^2 for energy and Nm instead
>> of kgm^2s^2 for torque?
>
> Basically yes.  Way back someone requested that they be  
> differentiated, so I
> differentiated them and hence (BTW) the notion of an anonymous  
> quantity was born
> as a logical result of not being able to distinguish the two in  
> some cases.
>
> After all torque is a vector quantity and energy is a scalar  
> quantity, so they
> are quite different.
> One could output energy units as N.m , but the joule is the more  
> well known SI
> energy unit, so it helps to put things in a familiar context.
>
> Also there  are various non SI units which specifically mean  
> torque, such as
> (reading from the SI manual) dyne centimeter, kilogram-force meter,  
> etc.
>
> And for energy:  erg per square centimeter second, watt per square  
> inch etc..

OK, to be able to output specific unit symbols is a good thing.

>
>
>>  - is it to prevent adding energy and torque?
>
> It could be in Quan, by more querying of the unit and disallowing the
> calculation in this case. At one point Quan (in a previous  
> incarnation) did do
> this, but then maybe there are situations where adding or comparing  
> torque and
> energy is appropriate, so I opted for the freer semantics. If there  
> is a proof
> of some sort that they can never be compared or added then I will  
> change the
> current semantics, but I am not enough of an expert to say for sure.

I am strictly against any disallowing of calculations. Just consider  
the following pseudo code (I do not refer to any specific  
implementation but hope the types are self-explanatory

unit::force::N force;
unit::length::m distance;

unit::energy::J energy;
unit::torque::Nm torque;

energy = force*distance; // should this be allowed?
torque = force*distance; // should this be allowed?

I think that in both cases the implicit conversion from the  
(annonymous?) quantity force*distance to either energy or torque. I  
think that to make the library usable one needs to allow these  
conversions.

If one want to prevent the operations then because one is a vector  
scalar product (the energy) and the other a vector cross product (the  
torque) then this should be done at the level of the value_type of  
the quantity and not at the unit level.

Matthias