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

From: Chris Weed (chrisweed_at_[hidden])
Date: 2005-12-21 19:51:30

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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