Re: Expressing physical units

[Hyman Rosen <hyrosen@mail.com>]

Jacob Sparre Andersen wrote:
> I don't understand C++ templates well enough to be able to see if it
> is realistic to copy enough of it to Ada, to allow the easier handling
> of compile-time unit checking available in C++.  I am afraid it would
> give too many unwanted possibilities in Ada, but it is very tempting.

The main problem with adopting this feature in Ada is Ada's lack of
automatic instantiation of generics. This arises in the multiplicative
operators. If you look at how I wrote operator*, you'll see that it
synthesizes a return type by adding the corresponding template parameters
of the operands. In use, you simply multiply a pair of units together and
the result has the correct type and can participate in further operations.

The MACKS package that people talk about here uses the same techniques to
represent units without overhead, but it does a whole bunch of explicit
instantiations to make them ready for use. If it happens to miss some needed
versions, it needs to be reconfigured to generate them.