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> 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> 15 firstTwo(const MatrixBase<Derived>& v) 16 { 17 return Eigen::VectorBlock<const Derived, 2>(v.derived(), 0); 18 } 19 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
