Re: [Boost-users] [mpl] compile time integer power

From: Sami (randomaccessiterator_at_[hidden])
Date: 2007-01-23 16:19:21

On 1/23/07, Cromwell Enage <sponage_at_[hidden]> wrote:
>
> --- Sami wrote:
> > On 1/22/07, Sami wrote:
> > >
> > > Hi Tony,
> > >
> > > Thx for your reply first of all.
> > >
> > > On 1/22/07, Gottlob Frege wrote:
> > > >
> > > >
> > > >
> > > > On 1/22/07, Sami wrote:
> > > > >
> > > > > Hi All,
> > > > >
> > > > >
> > > > >
> > > >     if (Pow == 0) return 1; // to do argue about
> > the value of '0 to the
> > > > 0'.
> > > >     if (Pow == 1) return r;
> > > >     return pow<Pow - 1>(r) * r;
> > > >
> > >
> > >  }
> > > >
> > > >
> > > Now, why do you want it - I doubt that it will buy
> > you anything in terms
> > > of performance.
> > > The issue is not particularly about performance,
> > it is about being able
> > > to do this at compile time; other than, well,
> > trying out MPL. Your
> > > resolution to the problem is quite nice, though
> > that's not what I am asking.
> > > My question is about MPL, not "how to do this with
> > C++"...
> > >
> > > There should be a way of doing this using MPL with
> > a single line of code.
> > > Or, even compute integer power of integers and get
> > a compile time result.
> > > This is useful for many metaprograms and again,
> > not a performance concern.
> > >
> > > Any ideas?
> > >
> > >
> > >
> >
> > One more note. In my original post, I mentioned this
> > constant sequence
> > wrapper. If there were such a thing (is there?) then
> > one could evaluate
> > integer power of an integer at compile time. For
> > instance:
> >
> > const unsigned SIZE = 3;
> > const unsigned EIGHT  = fold ( constant_sequence<
> > int_<2>, SIZE >,
> > int_<1>, times< _ , _ > :: type :: value ;
> >
> > This is only a thought. Maybe there is a better way
> > of computing powers,
> > without using fold or that constant_sequence
> > thingy..
> >
> > As for the real powers, maybe there is a
> > Boost.Fusion algorithm that can do
> > it..
>
> Actually, there are two MPL algorithms in the Boost
> Vault < http://boost-consulting.com/vault/ >.  Look
> inside the Template Metaprogramming folder for
> mpl_math.zip; the algorithms are integral_power and
> power, IIRC.
>
>                              Cromwell D. Enage

wouv! This library blew my head off. I was only asking for a silly single
line statement, look what you have done in there!  I am so surprised that
this library is buried down in the vault and didn't make it to Boost
collection yet..

thanks Cromwell.

sami