datatypes.cc
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	#include <iostream>
	#include <mpi.h>
	#include <vector>

	int main() {
	MPI_Init(NULL, NULL);

	int prank;
	MPI_Comm_rank(MPI_COMM_WORLD, &prank);

	MPI_Count lb, extent, size;

	struct Test_t {
	double d[2];
	int i;
	};

	std::vector<Test_t> foo(100);

	std::array<int, 2> block_lengths = {2, 1};
	std::array<MPI_Aint, 2> displacements;
	std::array<MPI_Datatype, 2> old_types = {MPI_DOUBLE, MPI_INT};

	MPI_Aint addr0, addr1;
	MPI_Get_address(&foo[0], &addr0);
	MPI_Get_address(&foo[0].d[0], &displacements[0]);
	MPI_Get_address(&foo[0].i, &displacements[1]);

	displacements[0] = MPI_Aint_diff(displacements[0], addr0);
	displacements[1] = MPI_Aint_diff(displacements[1], addr0);

	MPI_Datatype mpi_test_t, mpi_test_vector_t;

	MPI_Type_create_struct(2, block_lengths.data(), displacements.data(),
	old_types.data(), &mpi_test_t);

	MPI_Get_address(&foo[1], &addr1);
	addr1 = MPI_Aint_diff(addr1, addr0);

	MPI_Type_create_resized(mpi_test_t, 0, addr1, &mpi_test_vector_t);
	MPI_Type_commit(&mpi_test_vector_t);

	MPI_Type_get_extent_x(mpi_test_t, &lb, &extent);
	MPI_Type_size_x(mpi_test_t, &size);
	std::cout << "MPI Datatype: mpi_test_t, size: " << size
	<< " extent: " << extent << std::endl;
	std::cout << "C++ Datatype: Test_t, size: " << sizeof(Test_t) << std::endl;

	MPI_Type_free(&mpi_test_vector_t);
	MPI_Finalize();

	return 0;
	}