1#include <Eigen/Core>
2#include <iostream>
3using namespace Eigen;
4using namespace std;
5
6template<typename Derived>
7Eigen::Block<Derived>
8topLeftCorner(MatrixBase<Derived>& m, int rows, int cols)
9{
10  return Eigen::Block<Derived>(m.derived(), 0, 0, rows, cols);
11}
12
13template<typename Derived>
14const Eigen::Block<const Derived>
15topLeftCorner(const MatrixBase<Derived>& m, int rows, int cols)
16{
17  return Eigen::Block<const Derived>(m.derived(), 0, 0, rows, cols);
18}
19
20int main(int, char**)
21{
22  Matrix4d m = Matrix4d::Identity();
23  cout << topLeftCorner(4*m, 2, 3) << endl; // calls the const version
24  topLeftCorner(m, 2, 3) *= 5;              // calls the non-const version
25  cout << "Now the matrix m is:" << endl << m << endl;
26  return 0;
27}
28