1#include <Eigen/Core> 2#include <iostream> 3using namespace Eigen; 4using namespace std; 5 6template<typename Derived> 7Eigen::Block<Derived, 2, 2> 8topLeft2x2Corner(MatrixBase<Derived>& m) 9{ 10 return Eigen::Block<Derived, 2, 2>(m.derived(), 0, 0); 11} 12 13template<typename Derived> 14const Eigen::Block<const Derived, 2, 2> 15topLeft2x2Corner(const MatrixBase<Derived>& m) 16{ 17 return Eigen::Block<const Derived, 2, 2>(m.derived(), 0, 0); 18} 19 20int main(int, char**) 21{ 22 Matrix3d m = Matrix3d::Identity(); 23 cout << topLeft2x2Corner(4*m) << endl; // calls the const version 24 topLeft2x2Corner(m) *= 2; // calls the non-const version 25 cout << "Now the matrix m is:" << endl << m << endl; 26 return 0; 27} 28