[Mpi-forum] C++ types inaccessible after #281

[Jeff Squyres]

Yes, I think that Jed has a good point here.

There are a few C++-specific datatypes for which it would be worth introducing new C datatypes (e.g., MPI_CXX_COMPLEX).  It's too bad that no one realized this until now, because this won't make the MPI-3 deadline.  But it could be part of MPI-3.1 or errata, or some such.

Is there a C++ person out there who could draft a proposal about this?


On Jun 26, 2012, at 1:02 AM, Jed Brown wrote:

> I'm all for getting rid of the C++ bindings and see no compelling reason for anyone to use them.
> 
> Predefined types are inconsequential to MPI-1 functionality, but they are special for one-sided, so availability of types is highly relevant to applications. This is a non-cosmetic change imposed by #281 that doesn't seem to have been discussed.
> 
> On Jun 25, 2012 8:51 PM, "Fab Tillier" <ftillier at microsoft.com> wrote:
> Ah, thanks, sorry for being dense (I'm not familiar with the C++ bindings as we don't ship them for MSMPI).
> 
> Ticket 281 removes the entire C++ bindings from the standard, MPI::COMPLEX included.  You'd need to convince your MPI library vendor to keep supplying you with an MPI-2.2 compliant version of the C++ bindings (which is still allowed by ticket #281).
> 
> Hopefully someone with more experience with the C++ bindings can chime in, but I'd expect the answer to your original question is yes, ticket #281 intentionally removes all C++ support from MPI-3.
> 
> -Fab
> 
> Jed Brown wrote on Mon, 25 Jun 2012 at 21:31:55
> 
> > MPI::COMPLEX is not the same as MPI_C_COMPLEX (and the latter is not
> > available on systems without C99).
> 
> > On Jun 25, 2012 8:21 PM, "Fab Tillier" <ftillier at microsoft.com> wrote:
> > Hi Jed,
> >
> > Maybe I'm not following you, but what predefined MPI datatype supports
> > std::complex today?  My understanding is that there is no support for
> > std::complex in MPI today, and that ticket 281 does not change this at
> > all.
> >
> > Thanks,
> > -Fab
> >
> > Jed Brown wrote on Mon, 25 Jun 2012 at 20:42:53
> >
> >> I see that std::complex will not be accessible in a predefined type if
> >> #281 passes in its current form. Do you intend for there to be a
> >> supported/recommended way for a C++ caller to get a complex predefined
> >> (so that one-sided can be used) type? Note that C99 is not a subset of
> >> any C++ and C99 complex cannot be used portably from C++ (even with
> >> compiler suites that happen to support both).
> >>
> >> On Jun 25, 2012 7:07 PM, "Fab Tillier" <ftillier at microsoft.com
> >> <mailto:ftillier at microsoft.com> > wrote: Hi Jed,
> >>
> >> Jed Brown wrote on Mon, 25 Jun 2012 at 16:43:54
> >>
> >>> Is it intended that #281 would make std::complex inaccessible? Should
> >>> there be a MPI_CXX_COMPLEX, MPI_CXX_DOUBLE_COMPLEX,
> >>> MPI_CXX_LONG_DOUBLE_COMPLEX, and (possibly) MPI_CXX_BOOL?
> >>
> >> I don't think ticket #281 does anything with respect to std::complex
> >> accessibility.  The MPI standard defines the MPI_C_COMPLEX and family
> >> as mapping to the C99 _Complex type (see Table 3.2, Predefined MPI
> >> datatypes corresponding to C datatypes).  The standard is clear that
> >> there are no corresponding C++ bindings.
> >>
> >>> Note that C99 complex is not a substitute because Microsoft does not
> >>> implement it and it has different semantics regarding numerical
> >>> stability. (Well, C99 semantics are specified and useful, std::complex
> >>> stability is not specified by any C++ standard and in practice, is not
> >>> implemented in a stable way.) This is not such a big deal for use with
> >>> collectives because user-defined MPI_Ops can be used, but only
> >>> predefined types can be used with one-sided operations, so presence of
> >>> complex datatypes (or lack thereof) is relevant to applications.
> >>
> >> The fact that Microsoft does not support C99 is lame.  You can probably
> >> get away using std::complex as a stand-in for C99 _Complex (but then
> >> you're on your own), or move to a compiler that does (I believe the
> >> Intel compiler supports C99 on Windows).
> >>
> >> -Fab
> >>
> >
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-- 
Jeff Squyres
jsquyres at cisco.com
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