Re: [boost] [MPL.Math] compile-time double macro and printing

From: Chris Weed (chrisweed_at_[hidden])
Date: 2005-12-22 10:56:07

I suspect this is probably just a big/little endian problem. I don't
know if boost has a typedef for this.
Chris

On 12/21/05, Chris Weed <chrisweed_at_[hidden]> wrote:
> Hi,
> The arguments in reverse order (shown below) worked for my Sun system.
>
> struct encoding
>    {
>        unsigned long sign:1;
>        unsigned long exp:11;
>        unsigned long mantissa1:20;
>        unsigned long mantissa2;
>    };
>
> Thanks,
> Chris
>
> On 12/21/05, Peder Holt <peder.holt_at_[hidden]> wrote:
> > On 12/19/05, Chris Weed <chrisweed_at_[hidden]> wrote:
> > > Hi,
> > > I am trying to simply print a compile-time double using gcc 4.0.0 on Sun.
> > > The following prints out : 1.9036e+185
> > >
> > > #include <boost/metamath/double.hpp>
> > > #include <boost/metamath/double_macros.hpp>
> > > #include <iostream>
> > >
> > > int main()
> > > {
> > >   typedef BOOST_DOUBLE(3.3) D;
> > >   D d;
> > >   std::cout << (double)d << std::endl;
> > >   return 0;
> > > }
> > >
> > > Am I doing something wrong?
> > > Chris
> >
> > This has probably got something to do with the way I translate a
> > mpl::double_ to a double. I use a union to represent the mapping
> > between a double and the mantissa,exponent and sign (represented as
> > integers) This mapping is most likely different for different systems,
> > so we need to either device a better scheme for converting from
> > compile-time to runtime double, or use different mappings for
> > different systems.
> >
> > The relevant code is found in:
> > boost\mpl\math\double_\aux_\get_value.hpp
> >
> >     struct encoding
> >     {
> >         unsigned long mantissa2;
> >         unsigned long mantissa1:20;
> >         unsigned long exp:11;
> >         unsigned long sign:1;
> >     };
> >
> >     union converter_
> >     {
> >         double value;
> >         encoding encode;
> >     };
> >
> >     template <typename T>
> >     double get_value(const T& arg)
> >     {
> >         converter_ converter;
> >         converter.encode.sign = T::sign;
> >         converter.encode.exp = T::exponent + 1023;
> >         converter.encode.mantissa1 = T::mantissa::part1 >> 10;
> >         converter.encode.mantissa2 =
> >             (T::mantissa::part1 << 22)
> >           | (T::mantissa::part2 >> 9);
> >         return converter.value;
> >     }
> >
> > Try fiddling with the order of the elements in the encoding struct, e.g.
> >     struct encoding
> >     {
> >         unsigned long mantissa1:20;
> >         unsigned long exp:11;
> >         unsigned long sign:1;
> >         //Move this to the end
> >         unsigned long mantissa2;
> >     };
> >
> > On the other hand, this encoding is probably documented somewhere...
> >
> > Regards,
> > Peder
> >
> > >
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