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

[Fab Tillier]

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> 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
>