Re: [boost] Boost Units library preview

From: Eric Lemings (lemings_at_[hidden])
Date: 2006-08-23 12:23:56

> -----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.

Does that make sense?

> >  - 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.

> >  - 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.

Eric.