Re: [boost] [units] Crazy idea/question

From: Michael Fawcett (michael.fawcett_at_[hidden])
Date: 2007-02-16 18:15:53

On 2/16/07, Matthias Schabel <boost_at_[hidden]> wrote:
> > if (aircraft.altitude > ground_elevation + min_agl_offset)
> >
> > as
> >
> > if (unit_cast<elevation_type>(aircraft.altitude) > ground_elevation +
> > min_agl_offset)
>
> This is actually a very good example of the problem; here there are
> two steps,
> the addition operation and the comparison. Without specifying a priori a
> particular common base unit system through which all conversions are
> mediated (which is basically the same as requiring everyone use one unit
> system), this is just fundamentally ambiguous - it has nothing to do
> with
> implementation. If I ask :
>
> Is 3 feet plus 1 meter less than one fathom?
>
> How would you answer it? Naturally, you would want to convert to some
> common unit of length, but which one? There are three choices, and all
> are roughly equivalent. Now, in this case, it doesn't matter since
> all three
> systems would give you the same answer, but since there is no good way
> for the compiler to make this decision automatically,the reasonable
> thing
> for it to do is issue an error. this problem doesn't go away if you have
> runtime units either - it is just fundamentally bad practice to have the
> compiler make arbitrary decisions in arithmetic expressions.

I see.  I find this area interesting, although I am hoping you stick
to compile-time only for now.

I think the only hard part is the arithmetic portion.  A comparison
only deals with two types, and it doesn't matter which way (precision
aside, maybe there could be some smarts here) the conversion happens,
the result is the same.

3 feet + 1 meter, what is the result?  What are the pros/cons to
always using the left-hand-side type?  If a decision was made,
arithmetic would be defined, and comparisons would be easy.

I realize this should not be a focus of the current library, and
again, I'm not asking for it.  I do not know what the ramifications in
terms of implementation and usage are.  It just interested me.

--Michael Fawcett