1 #include <Eigen/Core>
2 #include <iostream>
3 using namespace Eigen;
4 using namespace std;
5 
6 template<typename Derived>
7 Eigen::VectorBlock<Derived, 2>
firstTwo(MatrixBase<Derived> & v)8 firstTwo(MatrixBase<Derived>& v)
9 {
10   return Eigen::VectorBlock<Derived, 2>(v.derived(), 0);
11 }
12 
13 template<typename Derived>
14 const Eigen::VectorBlock<const Derived, 2>
firstTwo(const MatrixBase<Derived> & v)15 firstTwo(const MatrixBase<Derived>& v)
16 {
17   return Eigen::VectorBlock<const Derived, 2>(v.derived(), 0);
18 }
19 
main(int,char **)20 int 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