How do I use Eigen/Armadillo for linear algebra in C++?

#include <Eigen/Dense>
#include <armadillo>

int main() {
    // Example using Eigen
    Eigen::MatrixXd a(2, 2);
    a << 1, 2,
         3, 4;
    std::cout << "Eigen Matrix:\n" << a << std::endl;

    // Example using Armadillo
    arma::mat b(2, 2);
    b << 1 << 2
      << 3 << 4;
    std::cout << "Armadillo Matrix:\n" << b << std::endl;

    return 0;
}