C99 _Complex and C++ complex<> interoperability.

["Smith-Rowland, Edward M" <ESmith-rowland at alionscience dot com>]

All,

This message is probably a combination of C++ standards (OT), gcc
implementation, and gcc-help so apologies in advance.

In discussions on the GSL mailing list about special functions and other
numerical issues, complaints about the lack of interoperability of C++
complex and C99 complex came up.  I'm trying to hunt down information on
these issues.

I have noticed that it is possible (since 4.0) to construct and assign
from a C99 complex number to a C++ complex number.

I think this is very cool.  This goes part way towards reconciling C99
and C++ as far as complex numbers are concerned.

I don't see this in the C++ standard or under g++ extensions in the
manual.

My questions are:
  Is this behavior standard or is this a gcc extension?
  If this is a gcc extension where is it documented?

Also, if this behavior is nonstandard is anyone thinking about
standardizing it?
Finally, the circle would be complete if we could assign from
std::complex to _Complex.
Any ideas?  There is a private functions that return _Complex from
std::complex.
Being able to have a nonmember assignment operator would really nail it.

Thank you.

This thing compiles:
----------------------------------------------------------
#include <complex>

int main(int, char**)
{

  std::complex<float> jf(0.0F, 1.0F);
  std::complex<double> jd(0.0, 1.0);
  std::complex<long double> jl(0.0L, 1.0L);

  float _Complex jcf;
  double __complex__ jcd;
  long double _Complex jcl;

  std::complex<float> zff(jcf);
  std::complex<float> zfd(jcd);
  std::complex<float> zfl(jcl);

  std::complex<double> zdf(jcf);
  std::complex<double> zdd(jcd);
  std::complex<double> zdl(jcl);

  std::complex<long double> zlf(jcf);
  std::complex<long double> zld(jcd);
  std::complex<long double> zll(jcl);

  std::complex<float> wff = jcf;
  std::complex<float> wfd = jcd;
  std::complex<float> wfl = jcl;

  std::complex<double> wdf = jcf;
  std::complex<double> wdd = jcd;
  std::complex<double> wdl = jcl;

  std::complex<long double> wlf = jcf;
  std::complex<long double> wld = jcd;
  std::complex<long double> wll = jcl;

  //jcd = jd;  //  Fails.

  //  Try some arithmetic.
  //std::complex<double> a = jcd + jd;  //  Fails.
  //std::complex<double> b = jd + jcd;  //  Fails.

  return 0;
}