Eigen facade or view for structured data in STL vector/array (Eigen3 and C++14 required)

EigenSTLFacade.cpp

#include <vector>
#include <iostream>
#include <type_traits>
#include <Eigen/Eigen>

/**
 * @brief Eigen::Map facade to access data members in STL vectors/arrays
 *
 * No data is copied and modifications of the Eigen object
 * modifies the original container and vice versa.
 *
 * Example:
 *
 *     struct Particle {
 *         double mass=2.0;
 *         Eigen::Vector3d velocity;
 *         int type;
 *     };
 *
 *    std::vector<Particle> v(10);
 *    auto m1 = asEigenVector(v.begin, v.end(), &Particle::velocity); --> 3x10 maxtix view
 *    auto m2 = asEigenMatrix(v.begin, v.end(), &Particle::mass);     --> 1x10 vector view
 *
 * @warning Be careful that objects are properly aligned and divisible with `sizeof<double>`
 */
template<typename matrix=Eigen::MatrixXd, typename dbl=typename matrix::Scalar, class iter, class memberptr>
    auto asEigenMatrix(iter begin, iter end, memberptr m) {
        typedef typename std::iterator_traits<iter>::value_type T;
        static_assert( sizeof(T) % sizeof(dbl) == 0, "value_type size must multiples of double");
        const size_t s = sizeof(T) / sizeof(dbl);
        const size_t cols = sizeof(((T *) 0)->*m) / sizeof(dbl);
        return Eigen::Map<matrix, 0, Eigen::Stride<1,s>>((dbl*)&(*begin.*m), end-begin, cols).array();
    }

template<typename matrix=Eigen::MatrixXd, typename dbl=typename matrix::Scalar, class iter, class memberptr>
    auto asEigenVector(iter begin, iter end, memberptr m) {
        typedef typename std::iterator_traits<iter>::value_type T;
        static_assert( std::is_same<dbl&, decltype(((T *) 0)->*m)>::value, "member must be a scalar");
        return asEigenMatrix<matrix>(begin, end, m).col(0);
    }

struct Particle {
    double mass=2.0;
    Eigen::Vector3d velocity;
    int type;
};

int main() {
    std::vector<Particle> d(5);
    auto m = asEigenVector( d.begin(), d.end(), &Particle::mass);    // 1x5 vector
    auto v = asEigenMatrix( d.begin(), d.end(), &Particle::velocity);// 3x5 matrix

    d[1].mass = 0;            // Map points to original data
    v = v.setRandom();        // random positions
    auto p = v.colwise() * m; // momenta, p=mv

    std::cout << "masses:\n\n" << m << "\n\n";
    std::cout << "velocities:\n\n" << v << "\n\n";
    std::cout << "momenta:\n\n" << p << "\n";
}