Re: [boost] [review][constrained_value] Review of Constrained Value Library begins today

Subject: Re: [boost] [review][constrained_value] Review of Constrained Value Library begins today
From: Neal Becker (ndbecker2_at_[hidden])
Date: 2008-12-05 09:25:06

>> 
>> "Why C++'s floating point types shouldn't be used with
>> bounded objects?
>> [...] there exist a number of myths of what IEEE-compliance really
>> entails from the point of view of program semantics. We shall discuss
>> the following myths, among others: [...] "If x < 1 tests true at one
>> point, then x < 1 stays true later if I never modify x."
>> 
>> I don't fully understand this...AFAIK, this is not allowed
>> for an IEEE
>> complient implementation. How could we ever implement *anything* with
>> respect to floats then?
> 
> I also don't like this, but this is the reality (see
> http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.18 too). It's
> very important to realise that floating-point operations may be not
> repeatable. Unfortunately, to be able to have a reliable constrained
> object, the comparison of two values *must* be repeatable.
> 
> I've heard of libraries for "reliable floating-point computations" and of
> compiler switches that turn some surprising floating-point optimisations
> off, but I still don't feel competent enough in this respect to find a way
> that *guarantees* proper working of constrained objects. If somebody does
> -- you are welcome to help. ;-)
> 

How about, floating point is allowed, but the results may sometimes be surprising?  We already accept this sort of floating point behavior in many other situations.  So, bounded_float should be allowed, but buyer should beware.