I am putting various responses together, for clarity. On Jun 27 2012, Marc Snir wrote:

The standard does not say much about datatypes that do not correspond to primitive datatypes of the host language. I would suggest that is we define an MPI_Complex type for a language that that does not have a primitive complex, then the implementation will decide how it defines a complex (e.g., two successive doubles) and perform reductions accordingly. A good quality implementation will make C++/C interaction easier.

That assumes that there is only one, and there may not be. Inter alia, complex IS a primitive datatype in Fortran, but not in C+, and only sort-of in C. On Jun 28 2012, Rolf Rabenseifner wrote:

I would propose to fill the gap by - defining in C and Fortran MPI_CPP_BOOL for C++ bool - defining in C and Fortran MPI_CPP_COMPLEX, MPI_CPP_DOUBLE_COMPLEX, MPI_CPP_LONG_DOUBLE_COMPLEX,

Yes. That would fix the problem.

if the C/C++ standards do not require that C "... _Complex" and C++ "Complex<...>" is internally implemented in an identical way,

They don't.

To be decided by the C/C++ specialists: - Is MPI_CPP_..._COMPLEX needed or is MPI_C_..._COMPLEX enough for C++? - If MPI_CPP_..._COMPLEX is needed, what are the external 32 sizes? (I expect, it's external32 and therefore same as for C.) - Which is the external32 size for MPI_CPP_BOOL?

The former, certainly - as others have said, the representation will almost always be the same. C++ bool is not required to be one byte, and its representation is unspecified, but will almost always be one byte and 1.

To be decided by the whole Forum: - Do we make this MPI-2.2 errata to help the C++ programmer using the MPI C language binding?

Speaking for my users, yes, please!

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On Jun 27, 2012, at 3:10 PM, Jeff Squyres wrote:

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Don't forget a few points:

- As Jim pointed out, all MPI datatypes are supposed to be available in all language bindings. Hence, if we have (for example) MPI_CXX_COMPLEX, it needs to be available in both C and the various Fortran bindings.

- This is not as wonky as it sounds;

With Fortran's C interoperability, it's not at all wonky, and an increasing number of Fortran+C programs are doing it.

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- That being said, handling reductions on MPI_CXX_COMPLEX from C-based MPI implementations may be a little problematic if the types are not equivalent between C and C++, for example...

Yes, that's the only issue that needs more than the representation. In all cases, it may be necessary to call into the owning language to do the reduction. Regards, Nick Maclaren.