#include <Eigen/Core> #include <iostream> template <typename Derived> Eigen::Block<Derived> topLeftCorner(Eigen::MatrixBase<Derived>& m, int rows, int cols) { return Eigen::Block<Derived>(m.derived(), 0, 0, rows, cols); } template <typename Derived> const Eigen::Block<const Derived> topLeftCorner(const Eigen::MatrixBase<Derived>& m, int rows, int cols) { return Eigen::Block<const Derived>(m.derived(), 0, 0, rows, cols); } int main(int, char**) { Eigen::Matrix4d m = Eigen::Matrix4d::Identity(); std::cout << topLeftCorner(4 * m, 2, 3) << std::endl; // calls the const version topLeftCorner(m, 2, 3) *= 5; // calls the non-const version std::cout << "Now the matrix m is:" << std::endl << m << std::endl; return 0; }