win_get_handle Function

   function win_get_handle(this) result(mpi_win_out)
      class(win_t), intent(in) :: this
      integer :: mpi_win_out

      if (.not. this%is_valid) then
         error stop "Cannot get MPI_Win from null window"
      end if
      mpi_win_out = this%m_win
   end function win_get_handle