eigen/doc/snippets/Tutorial_AdvancedInitialization_ThreeWays.cpp

const int size = 6;
MatrixXd mat1(size, size);
mat1.topLeftCorner(size / 2, size / 2) = MatrixXd::Zero(size / 2, size / 2);
mat1.topRightCorner(size / 2, size / 2) = MatrixXd::Identity(size / 2, size / 2);
mat1.bottomLeftCorner(size / 2, size / 2) = MatrixXd::Identity(size / 2, size / 2);
mat1.bottomRightCorner(size / 2, size / 2) = MatrixXd::Zero(size / 2, size / 2);
std::cout << mat1 << std::endl << std::endl;

MatrixXd mat2(size, size);
mat2.topLeftCorner(size / 2, size / 2).setZero();
mat2.topRightCorner(size / 2, size / 2).setIdentity();
mat2.bottomLeftCorner(size / 2, size / 2).setIdentity();
mat2.bottomRightCorner(size / 2, size / 2).setZero();
std::cout << mat2 << std::endl << std::endl;

MatrixXd mat3(size, size);
mat3 << MatrixXd::Zero(size / 2, size / 2),
    MatrixXd::Identity(size / 2, size / 2), MatrixXd::Identity(size / 2, size / 2), MatrixXd::Zero(size / 2, size / 2);
std::cout << mat3 << std::endl;