Re: [boost] [mpl::math Rational] type member

From: Andy Little (andy_at_[hidden])
Date: 2005-10-24 16:44:14

"David Abrahams" wrote

>> As it stands these two typedefs are normalized versions of the Numerator and
>> Denominator parameters.
>
> If you're trying to save cycles at compilation time by normalizing
> lazily, then of course the numerator and denominator typedefs can't be
> normalized.  So, sure, typedefs for the template arguments sound fine
> to me.

I wouldnt mind this from a practical point of view. I believe that Cromwells
implementation of gcd is overkill for my needs, so I guess that would make it
easier to customise.

Overall though I am opposed to it because I  believe the current interface is
superior from a users point of view and its a classic case of optimisation
ruling the interface.

In the case of  dimensional analysis in a quantity, it seems to be perfectly
possible to mix rationals and integral constants in a mpl container, though some
care needs to be taken with the maths ( It may be worth using a custom division
operator). In use for dimensional analysis rationals are actually pretty rare
(integral constants can be substituted in the vast majority of cases) and all
rationals used have a very small range of values, so I hope that  any poor
compile time performance of rationals will not  have such a great effect.  In
fact I think that division is the only operation where extra care is required
though any operation involving a rational requires normalisation if its involved
in math and comparisons etc.

I guess it might be worth getting Cromwell Enage's  view as to whether he feels
changing  the current interface is worthwhile.

regards
Andy Little