#include <Eigen/Core> #include <iostream> template<typename Derived> Eigen::VectorBlock<Derived, 2> firstTwo(Eigen::MatrixBase<Derived>& v) { return Eigen::VectorBlock<Derived, 2>(v.derived(), 0); } template<typename Derived> const Eigen::VectorBlock<const Derived, 2> firstTwo(const Eigen::MatrixBase<Derived>& v) { return Eigen::VectorBlock<const Derived, 2>(v.derived(), 0); } int main(int, char**) { Eigen::Matrix<int,1,6> v; v << 1,2,3,4,5,6; std::cout << firstTwo(4*v) << std::endl; // calls the const version firstTwo(v) *= 2; // calls the non-const version std::cout << "Now the vector v is:" << std::endl << v << std::endl; return 0; }