title: OpenMPI: error: ‘MPI’ has not been declared
body: <p>Always get the following errors while running the OpenMPI command "mpicxx hello_cxx.cc -o hello"</p>
  
  <pre><code>hello_cxx.cc: In function ‘int main(int, char**)’: 
  hello_cxx.cc:25:5: error: ‘MPI’ has not been declared
       MPI::Init();
       ^~~ hello_cxx.cc:26:12: error: ‘MPI’ has not been declared
       rank = MPI::COMM_WORLD.Get_rank();
              ^~~ hello_cxx.cc:27:12: error: ‘MPI’ has not been declared
       size = MPI::COMM_WORLD.Get_size();
              ^~~ hello_cxx.cc:31:5: error: ‘MPI’ has not been declared
       MPI::Finalize();
       ^~~
  </code></pre>