Re: [boost] Review request: extended complex number library

Subject: Re: [boost] Review request: extended complex number library
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2012-03-07 17:36:13

On 03/07/2012 11:20 PM, Christopher Jefferson wrote:
>
> On 7 Mar 2012, at 22:12, Mathias Gaunard wrote:
>
>> On 03/06/2012 07:49 PM, Vicente J. Botet Escriba wrote:
>>
>>> you are right. The goal been to provide a faster library implies that
>>> the Boost library should use the standard library when the standard is
>>> more efficient, and we could expect that the standard is faster for the
>>> scope it covers.
>>
>> What about correctness?
>>
>> Most implementations of standard library functions on complex numbers are not correct from a numerical point of view.
>
> Really? Can you give some examples? Have you reported these issues to the various compiler designers?

Verbatim from the libstdc++ headers

   // 26.2.5/13
   // XXX: This is a grammar school implementation.
   template<typename _Tp>
     template<typename _Up>
     complex<_Tp>&
     complex<_Tp>::operator*=(const complex<_Up>& __z)
     {
       const _Tp __r = _M_real * __z.real() - _M_imag * __z.imag();
       _M_imag = _M_real * __z.imag() + _M_imag * __z.real();
       _M_real = __r;
       return *this;
     }

This is incorrect for infinite types and causes undue overflow or underflow.

See the C99 or C11 standards, annex G.
It also comes with a possible implementation.

C++ has no equivalent to this annex AFAIK.