Use std::begin(), std::end() to convert ArrayXd to stl vector, - c++

Question

It seems to me that I should be able to use std::begin() and std::end() to convert ArrayXd to std::vector<double>; however, when I tried it in the following code, my attempt failed. My understanding is that .data() method on any Eigen object returns a pointer to a continuous block of memory similar to c style array. Therefore, since I can use std::begin(), std::end() on a c style array, I expected that to work with .data() as well. However, Eigen classes are templated, and I think this is what causes me problems, but don't see a way to fix this. How should this be done?
#include <iostream>
#include <vector>
#include <Eigen/Dense>
using namespace Eigen;
int main()
{
ArrayXd e_array(5);
e_array << 3,4,5,6,7;
double c_array[] = {1,2,3,4,5};
//Fails
// std::vector<double> my_vec(std::begin(e_array.data()), std::end(e_array.data()));
// Works
// std::vector<double> my_vec(e_array.data(), e_array.data() + 5);
// Works
// std::vector<double> my_vec(std::begin(c_array), std::end(c_array));
// Works
// std::vector<double> my_vec(c_array, c_array + 5);
return 0;
}
My error message(First lines, the whole message is long):
error: no matching function for call to
‘begin(Eigen::PlainObjectBase >::Scalar*)’
std::vector my_vec(std::begin(e_array.data()),
std::end(e_array.data()))

Answer 1

std::begin(vec.data()) cannot work because data() returns a raw pointer which cannot convey the number of elements in the vector. This version is the best one of yours:
std::vector<double> my_vec(e_array.data(), e_array.data() + 5);
And slightly better:
std::vector<double> my_vec(e_array.data(), e_array.data() + e_array.size());
And you may also be able to do this with many containers, but not with Eigen's ArrayXd in particular, because it lacks begin() and end() (related: http://eigen.tuxfamily.org/bz/show_bug.cgi?id=231).
std::vector<double> my_vec(foo.begin(), foo.end());