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

[Hyman Rosen <hyrosen@mail.com>]

Dmitry A. Kazakov wrote:
> 1. Shifted units, like Celsius degree.

This can be done by adding an extra shift prameter. 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.

> 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.

> 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. You can also code the conversions as templates. I suspect
it's unwieldy and people would be just as happy to pick one unit system, but
it could be done.

> 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.

> 6. Container problem. Surely a vector, matrix etc of dimensioned values
> should have a dimension.

I don't see why, but that's certainly not a problem for C++. All of the
standard container templates export the type of the object they hold as a
named type called "value_type" so the units of the contained objects are
available if needed.

> 7. Any generic (template) solution would make everything based upon it also
> generic.

Modern C++ programmers see that as an opportunity, not a problem!