C99 _Complex and C++ complex<> interoperability.

[Gabriel Dos Reis]

"Smith-Rowland, Edward M" <ESmith-rowland@alionscience.com> writes:

| 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.

It is not in the ISO C++ standard.  It is purely a libstdc++
implementation detail.  However, I've taken deliberately chosen to
make  ISO C++ std::complex<> and ISO C _Complex interoperate where
possible.  It is therefore a GCC extension that I'm not willing to
formally document at this moment because there are other issues that
are resolved yet and that might change in the future.

| 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?

I don't see howw standardize something ISO C++ consciously chosed to
ignore.  Certainly, I'm not going to champion it.

| Finally, the circle would be complete if we could assign from
| std::complex to _Complex.
| Any ideas?

You can cast  a std::complex<T> to a  T _Complex*.  Yes, it is ugly.
But it is a guarantee.

-- Gaby