Using the undeclared identifier 'MPI' when using the C ++ syntax for OpenMPI on MacOS

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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();

}


It seems your OpenMPI installation was built without C++ bindings. While OpenMPI still contains the C++ bindings, they actually have been removed from the standard years ago. You will probably have to rebuild OpenMPI to enable the C++ bindings, that depends on how you installed it in the first place.

Since they are not part of the standard, the recommendation is to not the C++ bindings in the first place. Instead the most common recommendation is to use Boost.MPI.

Unfortunately it appears that Boost.MPI is no longer actively developed (last bug closed 2 years ago), and there are some really nasty open issues, so I cannot recommend Boost.MPI. There are some cool MPI type-traits approaches, but I do not know of a widely accepted / actively developed wrapper.