eigen_unpacking.h

#include <iostream>
#include <Eigen/Core>

//Mimics http://stackoverflow.com/a/40958644

template <typename Scalar, int D, int E, int... Is>
auto eigen_unpack_impl(const Eigen::Matrix<Scalar, D, E>& a, std::integer_sequence<int,Is...>) {
    return std::tie( a(Is)... );
}

template <typename Scalar, int D, int E>
auto eigen_unpack(const Eigen::Matrix<Scalar, D, E>& a) {
    static_assert(D == 1 || E == 1, "ONLY VECTORS");
    return eigen_unpack_impl(a,std::make_integer_sequence<int,D*E>{});
}


int main(int argc, char * argv[]) {
    int a,b,c;
    std::tie(a,b,c) = eigen_unpack(Eigen::Vector3d(2.0,3.0,4.0));
    std::cout << a << " " << b << " " << c << std::endl;
    std::tie(a,b,c) = eigen_unpack(Eigen::RowVector3d(2.0,3.0,4.0));
    std::cout << a << " " << b << " " << c << std::endl;

    Eigen::Vector3d w(6,7,8);
    //auto& [x,y,z] = eigen_unpack(w);
}