doc/snippets/Tutorial_ReshapeMat2Mat.cpp

MatrixXf M1(2,6);    // Column-major storage
M1 << 1, 2, 3,  4,  5,  6,
      7, 8, 9, 10, 11, 12;

Map<MatrixXf> M2(M1.data(), 6,2);
cout << "M2:" << endl << M2 << endl;