use of undeclared identifier 'MPI' when using C++ syntax for OpenMPI on MacOS

Question

I have my OpenMPI and gcc installed on my MacOS. C syntax for OpenMPI could run perfectly. However, the c++ syntax for OpenMPI are always not recognized. For example, line with void MPI::Init(int &argc, char**&argv); shows error of use of undeclared identifier 'MPI'.

Could anybody give some detailed procedure to fix this problem?

#include "mpi.h"
#include <stdio.h>
#include <iostream>

using namespace std;

int main(int argc, char** argv) {


void MPI::Init(int &argc, char**&argv);

// Get the number of processes
int world_size;
MPI_Comm_size(MPI_COMM_WORLD, &world_size);
//int Comm::Get_size() const
// Get the rank of the process
int world_rank;
MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);

// Get the name of the processor
char processor_name[MPI_MAX_PROCESSOR_NAME];
int name_len;
MPI_Get_processor_name(processor_name, &name_len);

// Print off a hello world message
printf("Hello world from processor %s, rank %d out of %d processors\n",
       processor_name, world_rank, world_size);


// Finalize the MPI environment. No more MPI calls can be made after this
MPI_Finalize();

}

Answer 1

I had the same issue when installing open-mpi using homebrew in Mac OS X (I am a newbie in Mac OS X coming from Debian/Ubuntu Linux). Fortunately, I solved it. As Zulan said, your open-mpi installation was built without C++ bindings. However, I think the best solution is to rebuild it directly from the source code. I downloaded the .tar files directly from its homepage, and if you look closer you will see that the C++ bindings are by default disabled (do the following inside the untar folder):

$ ./configure -help

--enable-mpi-cxx enable C++ MPI bindings (default: disabled)

So, basically I rebuilt open-mpi directly adding that flag, as follow:

$ ./configure --enable-mpi-cxx

Then, just make sure you know where you have installed it (if you used --prefix= or the default) to link it to your makefile properly.

for more information about this issue look here: https://github.com/open-mpi/ompi/issues/3173.