[boost] Re: Re: Re: Dimensional Analysis

From: Andy Little (andy_at_[hidden])
Date: 2005-02-02 05:10:08

"Matt Austern" <austern_at_[hidden]> wrote

> There's actually something even lower level than a dimension library.
> In essence, what we've got is:
> - We're generalizing the notion of an arithmetic type T to a type of
> the form <T,X>, where X is some sort of tag.
> - For any X1 and X2 and any arithmetic operation 'op', we have rules
> for whether <T,X1> op <T,X2> is well defined.  If it is then the
> result is tagged as <T,X3>, and we have rules to determine X3 in terms
> of X1, X2, and op.

I dont  understand the tag part. (This must work seamlessly for inbuilt
types too.)

 "for any T1 and T2" ... "rules to determine TR ( the result_type) in terms
of T1, T2 and op".  ..?

I am not sure if this is exactly what you mean but I have tried to raise
this on comp.std.c++:

http://groups-beta.google.com/group/comp.std.c++/msg/a921d4d5bdb362e2

(note the above code inspired by Doug Gregors code in a boost discussion )

I think something like that would still be required for implementation even
when decltype and auto are in place.

> All of the knowledge about how to represent dimensional systems (mpl
> vectors, compile-time fractions, etc.) is higher level than this.

As a detail, I think dimensions  is orthogonal,
eg
mpl::plus<int_<A>,int_<B> >::type
mpl::plus<int,int>::type

The first is a compiletime operation, whereas the second is misuse of
mpl::plus for result_type deduction. dimensional analysis occurs as the
first, whereas we are discussing result_type deduction above.
I Think !

regards
Andy Little