To be clear, C++ currently calls/uses the C99 interface for MPI (unless it uses the no-long-standard but possibly-still-present-in-libraries C++ bindings). That will still be true after these BigCount changes: C++ will see only the C99 symbols, just like any other non-C11-compliant compiler. Polymorphism will not be supported in C++ because it cannot use C11 _Generic (because that is part of the C11 standard and is *not* part of the C++ standard). It also cannot use C++ overloading because are choosing not to define any new C++ bindings in the MPI-4.0 standard. If libraries choose to supply C++ overloads, they may do so but they are not going to be part of the MPI-4.0 standard (they will have the same status as the Java bindings offered by Open MPI).

Cheers,

Dan.

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Dr Daniel Holmes PhD
Architect (HPC Research)
d.holmes@epcc.ed.ac.uk
Phone: +44 (0) 131 651 3465
Mobile: +44 (0) 7940 524 088
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On 31 Jul 2019, at 16:03, Jeff Squyres (jsquyres) via mpi-forum <mpi-forum@lists.mpi-forum.org> wrote:

On Jul 31, 2019, at 10:52 AM, Jeff Hammond <jeff.science@gmail.com> wrote:

You’re going to have to mention C++. You can’t just pretend that C++ supports C11 generic, because it explicitly doesn’t.

We are mentioning C++. Please re-read my prior email.

And you really should do this because it’s ridiculous not to use C++ polymorphism if we use C11’s.

There are three options:

1. Re-introduce C++ bindings, delay MPI-4.
2. Re-introduce C++ bindings, BigCount misses the MPI-4 train.
4. Do not re-introduce C++ bindings, BigCount catches the MPI-4 train.

The feedback from the Forum was that BigCount was a blocker/gating issue for MPI-4. Hence, this is why the BigCount WG is not planning at this time to re-introduce C++ bindings via BigCount.

There is a longer term plan (think: MPI-5) to introduce a full-featured set of C++ bindings to MPI -- one that does not necessarily have a 1:1 correspondence to the C bindings. That is a different, much longer effort, and will definitely not make it into MPI-4.

--
Jeff Squyres
jsquyres@cisco.com

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