Re: [boost] meta-double

From: David Abrahams (dave_at_[hidden])
Date: 2005-06-12 11:32:23

Peder Holt <peder.holt_at_[hidden]> writes:

> On 6/11/05, David Abrahams <dave_at_[hidden]> wrote:
>> Peder Holt <peder.holt_at_[hidden]> writes:
>> 
>> > On 6/10/05, David Abrahams <dave_at_[hidden]> wrote:
>> >> Peder Holt <peder.holt_at_[hidden]> writes:
>> >>
>> >> >> c.f. http://gcc.gnu.org/ml/gcc/2005-01/msg01744.html
>> >> >> and the last two items here:
>> >> >> http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Deprecated-Features.html#Deprecated-Features
>> >> >
>> >> > Oh, well. Anyway, I have now uploaded a version of this to the vault:
>> >> > metamath.zip
>> >> > that is mpl-compliant and supports comparison and arithmetics with the
>> >> > same accuracy as normal double arithmetics. Does not support NaN.
>> >>
>> >> Does it get around the comparison problem somehow?
>> >>
>> >> I think the results when a floating point value is converted to an
>> >> integral that can't represent it are undefined (or something similar)
>> >> but at least most compilers won't stop you.  You might try to do some
>> >> awful thing like transforming
>> >>
>> >>       a < b
>> >>
>> >> into
>> >>
>> >>       long(b - a) > 0
>> >>
>> >
>> > I am not quite sure I understand the problem correctly.
>> 
>> The problem is that a < b is not a compile-time constant when a and b
>> are floating.  If you use such comparisons internally to your library,
>> it is nonportable.
>
> Ahh, but you have the same problem with a-b, or indeed with any
> operation.

Is that right?  I guess I thought there was a loophole if you
ultimately convert it to int.

> The only thing that compiles with comeau online in strict mode, is
> long(a), where a is a double literal, and this doesn't do much good.

Not much, no.

> The building up of a double is nonportable, and I haven't tried to
> tackle this problem.  So far, the only compilers I have tried that
> did not accept operators on doubles to form compile time constants,
> is Comeau in strict mode and Digital Mars.
>
> Borland, VC [6.5 ,8beta], GCC [?, 4.0.0> on the other hand,
> accepts the syntax.
>
> It is probably possible to create a less elegant way to construct a
> double_, not involving non-standard functionality.

I think you need to at least provide the option, for people who want to
write portable code.

It doesn't have to be all that ugly, you know.   Do you need more than
32 bits after the decimal point?

  double< 3,1415927 ,E<+6> >

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com