Re: [boost] Boost Units library preview

From: Matthias Troyer (troyer_at_[hidden])
Date: 2006-08-24 00:17:09

On Aug 24, 2006, at 1:23 AM, Eric Lemings wrote:

>
>
>> -----Original Message-----
>> From: boost-bounces_at_[hidden]
>> [mailto:boost-bounces_at_[hidden]] On Behalf Of Phil Richards
>> Sent: Wednesday, August 23, 2006 8:58 AM
>> To: boost_at_[hidden]
>> Subject: Re: [boost] Boost Units library preview
>>
>> On 2006-08-23, Matthias Troyer <troyer_at_[hidden]> wrote:
>>>  - Nm would be a "dimension"
>>
>> Not really.  It is the unit.
>
> Correct.  A newton is a derived unit composed of base units,
> specifically kilogram, meter, and second.  Meter is of course a
> base unit.
>
>> The dimension is: MASS * LENGTH^2 * TIME^2
>
> Also correct.  Every unit, whether a base unit or derived, has a
> corresponding dimension.
>
>>>  - 1 Nm would be a "quantity"
>
> No.  This is a measure.  It has a specific unit and a specific
> value.
>
> Here's an example of a quantity.  The distance from
> here->|                        |<-to here is a quantity with a
> dimension of length.  You don't know what its value is exactly
> but you do know that it has a value and can be measured.  Until
> you actually measure this length with a particular unit does the
> "quantity" become a "measure".  Converting a measure into a
> quantity is essentially the same as stripping away the unit but
> keeping the dimension.

OK, but then a quantity can never be represented in the computer,  
since to assign a numerical value to above distance one needs a unit.

>>>  - a torque measured in Nm or an energy measured in Nm
>> would be two distinct units
>
> Dimensions and units are C++ types.  They have no "value" in
> the conventional sense.

OK, so would the type of "Nm torque" or "Nm energy" be the same or  
different?
>>>

>>>  - 1 Nm as torque, 1 Nm as energy or 1 J as energy would be
>>> three distinct measures
>>
>> I think as far as a dimensionality checking goes, they are
>> all the same.  As far as a units checking goes, Nm and J
>> *could* be distinguished.  It's an interesting question as to
>> what, exactly, "energy" and "torque" are once you try to
>> separate away the dimensionality and units bits... :-)
>
> Yep, if the units are the same and the value is the same then
> they are (or should be) considered the same measure.  I think
> it should be left up to users to interpret these measures and
> how they are used.

Well, as a physicist for me 1 J and 1 Nm are the same. I would reject  
and never use a library that forces me to distinguish physically  
identical quantities just because I use a different name for the unit.

Matthias