- use empty struct as fake_mpi_communicator type. this avoids accidential...

- use empty struct as fake_mpi_communicator type. this avoids accidential compatibility with a real mpi implementation.
- call MPI_Init only once.

[[Imported from SVN: r5924]]

dune/common/mpihelper.hh

@@ -77,7 +77,7 @@ namespace Dune
     /**
      * @brief The type of the mpi communicator.
      */
-    typedef int MPICommunicator;
+    struct MPICommunicator {};
 
     /**
      * @brief Get the default communicator.
@@ -86,7 +86,8 @@ namespace Dune
      */
     static MPICommunicator getCommunicator()
     {
-      return -1;
+      static MPICommunicator comm();
+      return comm;
     }
 
     static CollectiveCommunication<MPICommunicator>
@@ -208,7 +209,7 @@ namespace Dune
     {
       rank_ = -1;
       size_ = -1;
-      MPI_Init(&argc, &argv);
+      static int is_initialized = MPI_Init(&argc, &argv);
       MPI_Comm_rank(MPI_COMM_WORLD,&rank_);
       MPI_Comm_size(MPI_COMM_WORLD,&size_);