[boost] Re: Proposal: rational numbers class

From: Kevlin Henney (Kevlin.Henney_at_[hidden])
Date: 1999-12-06 08:53:37

>    template<typename Int, bool normalize = true>
>    class rational
>    {
>        ...
>    };

The difficulty with this is two-fold. First (a minor one),
rational<int,false> doesn't seem to me to be an intuitive way of getting
denormatised fractions of ints.

     Yup, I admit the naming's not so hot :->

     A better proposal would be:

          enum normalization { normalized, denormalized };
          template<typename Int, normalization Normalization = normalized>
          class rational;

The more important issue is that we simply
have two largely disjoint implementations (the guts of the implementation
is
so small that it would be hard for *any* implementation variation not to
affect everything!), and triggering the distinction via a template
parameter
seems (to me) to be using templates inappropriately. What's wrong with
denormalised_rational<int>? Or just making a decision on behalf of the
user?
The standard library doesn't provide every possible implementation strategy
for map<> or set<> or list<>... For that matter, std::complex<> doesn't let
you choose whether or not you want polar or cartesian representation.

     I'm not sure that it overcomplicates the implementation in any
     profound way, or is indeed a misuse of template parameters. Provide a
     private member function:

          void dependent_normalize()
          {
              if(Normalization == normalized)
                  normalize();
          }

     And in each member function that currently calls normalize(), call
     dependent_normalize() instead. This would be compiled in at the same
     cost as before for normalized and at no cost for denormalized. Anyway,
     just a thought.

     I'm not sure that comparisons with the existing standard library hold
     up all that well -- consider the joy that is allocators ;->

     Kevlin