Sample solution

template<typename T,
   template<typename> class MA,
   template<typename> class MB,
   Require<Ge<MA<T>>, Ge<MB<T>>> = true>
int scatter_by_row(const MA<T>& A, MB<T>& B, int root, MPI_Comm comm) {

   int nof_processes; MPI_Comm_size(comm, &nof_processes);
   int rank; MPI_Comm_rank(comm, &rank);

   MPI_Datatype rowtype_B = get_row_type(B);
   if (rank == root) {
      assert(A.numCols() == B.numCols());

      hpc::aux::UniformSlices<int> slices(nof_processes, A.numRows());
      std::vector<int> counts(nof_processes);
      std::vector<int> offsets(nof_processes);

      MPI_Datatype rowtype_A = get_row_type(A);
      for (int i = 0; i < nof_processes; ++i) {
         if (i < A.numRows()) {
            counts[i] = slices.size(i);
            offsets[i] = slices.offset(i);
         } else {
            counts[i] = 0; offsets[i] = 0;
         }
      }
      int recvcount = counts[rank];
      assert(B.numRows() == recvcount);

      return MPI_Scatterv(
         &A(0, 0), &counts[0], &offsets[0], rowtype_A,
         &B(0, 0), recvcount, rowtype_B, root, comm);
   } else {
      int recvcount = B.numRows();
      return MPI_Scatterv(nullptr, nullptr, nullptr, nullptr,
         &B(0, 0), recvcount, rowtype_B, root, comm);
   }
}

template<typename T,
   template<typename> class MA,
   template<typename> class MB,
   Require<Ge<MA<T>>, Ge<MB<T>>> = true>
int gather_by_row(const MA<T>& A, MB<T>& B, int root, MPI_Comm comm) {
   int nof_processes; MPI_Comm_size(comm, &nof_processes);
   int rank; MPI_Comm_rank(comm, &rank);

   MPI_Datatype rowtype_A = get_row_type(A);

   if (rank == root) {
      assert(A.numCols() == B.numCols());

      hpc::aux::UniformSlices<int> slices(nof_processes, B.numRows());
      std::vector<int> counts(nof_processes);
      std::vector<int> offsets(nof_processes);

      for (int i = 0; i < nof_processes; ++i) {
         if (i < B.numRows()) {
            counts[i] = slices.size(i);
            offsets[i] = slices.offset(i);
         } else {
            counts[i] = 0; offsets[i] = 0;
         }
      }
      int sendcount = counts[rank];
      assert(A.numRows() == sendcount);

      MPI_Datatype rowtype_B = get_row_type(B);

      return MPI_Gatherv(
         &A(0, 0), sendcount, rowtype_A,
         &B(0, 0), &counts[0], &offsets[0], rowtype_B, root, comm);
   } else {
      int sendcount = A.numRows();
      return MPI_Gatherv((void*) &A(0, 0), sendcount, rowtype_A,
         nullptr, nullptr, nullptr, nullptr, root, comm);
   }
}