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