Re: Expressing physical units (Was: Reprise: 'in out' parameters for functions)

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

Hyman Rosen wrote:

> Dmitry A. Kazakov wrote:
>> 1. Shifted units, like Celsius degree.
> 
> This can be done by adding an extra shift prameter.

But C++ does not allow double as a template parameter.

> You would also need
> to describe the units system more thoroughly. The standard units can
> appear in positive or negative powers, but you obviously cannot do this
> with shifted units.

Why? It is no matter. Shifted units represent linear scales:

x = a * y + b

The dimension of a and b may have any powers.

>> 2. Dealing with units unknown at compile time. How would you communicate
>> with a data base, or develop a gauge widget, or write a unit calculator?
> 
> This requires a different solution. But that's OK. If many applications
> do not need this dynamic behavior, then they stand to benefit from the
> zero-overhead of the given solution.

Because there can be a universal solution with zero-overhead in static case.

>> How to write a unit I/O package?
> 
> I demonstrated that in another post. You write a bunch of 'if' statements
> whose conditions depend on the template parameters, and they are all known
> at compile-time for a given unit.

(:-))

>> 3. Dealing with multiple unit systems.
> 
> You can add a scale factor template parameter for each dimension, and then
> they will be different types, so the compiler will prevent you from adding
> mm to cm, say.

Not that is the problem. One can add 1 [mm] and 1 [cm], it is legal. The
question is in which unit system the arguments are and the result will be.
There could be a need to have both m,s,A,... and mm,h,kA,..., for reasons I
explained before.

>> 5. How to handle logarithmic units and other non-linear scales?
> 
> By specializing the templates for these units to do the appropriate
> operations. Ada does not have the concept of specialized templates, but
> C++ does.

I do not see how you could add x in meters of SI with y in ft of logarithmic
scale using specialized templates. To represent scale you will probably
need some pointer to the scale descriptor in your template.

>> 7. Any generic (template) solution would make everything based upon it
>> also generic.
> 
> Modern C++ programmers see that as an opportunity, not a problem!

I do not see any way of making a GUI or DB interface based on templates.
Consider a distributed system with should have a communication protocol to
exchange physical measures. How would you do it using templates? Consider a
large finite element library. If that has to be used for physical values,
then the whole library has to be generic.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de