Re: [Boost-users] extending boost::rational to represent infinities

From: Jeff Garland (jeff_at_[hidden])
Date: 2005-04-03 13:28:37

On Sun, 3 Apr 2005 12:33:22 -0500, me22 wrote
> On Apr 3, 2005 5:08 AM, Jedediah Smith <jedediah_at_[hidden]> wrote:
> > I need a type equivelant to rational<int> but supporting two special
> > values, +inf and -inf which compare greater and less (respectively) to
> > all other values besides themselves. I don't need to perform any
> > arithmetic on the inf values so those operations can be undefined.
> > 
> > What is the best strategy for creating such a type? Would it be better
> > to subclass rational or create an underlying integer type with the inf
> > values and use rational based on that underlying type? If the former
> > option, how exactly would I do it?
> 
> I expect that subclassing wouldn't be a good idea since none of the
> critical functions are virtual ( for good -- speed -- reason ).  The
> way that immediatly jumps to mind is copying rational<> and changing
> it to allow a zero denominator to represent infinities, although this
> violates OnceAndOnlyOnce, so perhaps a class that has a rational as a
> member would be a more maintainable option, but I expect it would be
> to run and more annoying to code.
> 
> A with_inf<> wrapper for a numeric type seems to me to be the best
> option as it would be straight-foreward and usable in other 
> contexts. The only problems I see with that is your denominator 
> would also get overhead from checking for infs ( when it should 
> never have them ) and that there's no simple way for a rational< 
> with_inf<> > divide-by-zero to return the appropriate +/- inf value.

FYI, Boost date-time has an int_adpater class that provides infinities and NAN
values to a pure integer type.  It's not perfectly suited for rational as-is
since it has a few date-time specific things, but they could be easily
factored out. And yes, it adds overhead to operators since the special values
need to be checked.

http://www.boost.org/boost/date_time/int_adapter.hpp

Jeff