Conversion from std::vector<Eigen::Vector3d> to std::vector<Eigen::Vector3f>

Question

I came across the problem that I want to convert from a std::vector<Eigen::Vector3d> to a std::vector<Eigen::Vector3f> . I was wondering if there is a solution where I dont have to iterate over the points.

// mapping using iteration
std::vector< Eigen::Vector3d> tf{ {1,1,1},{1,1,1},{1,1,1} };
std::vector< Eigen::Vector3f> tf2;
tf2.reserve(tf.size());
std::transform(tf.begin(), tf.end(), std::back_inserter(tf2), [](const Eigen::Vector3d& p) {
    return p.cast<float>();
});

I tried some things like tf.data() and tried to cast that, but I didnt found a solution. I also looked into Eigen::Map<> class, but didnt really find a solution.

Answer 1

I don't think what you are asking is possible. Eigen::Map allows you to construct an Eigen data structure without needing to copy or move, it merely takes a view on existing contiguous data (typically from a std::array or std::vector ). The operation you are looking to do, casting from doubles to float, two distinct types with different memory layouts , is an explicit operation. You would be shrinking the size of the vector in half. It is not possible to achieve this by taking a different view on the same data.

Answer 2

Assuming Vector3d and Vector3f don't introduce any padding (which is true for all compilers which Eigen supports), you could use an Eigen::Map<const Matrix3Xd> and .cast<float>() that into an Eigen::Map<Matrix3Xf> over the destination vector:

std::vector< Eigen::Vector3d> tf{ {1,1,1},{1,1,1},{1,1,1} };
std::vector< Eigen::Vector3f> tf2(tf.size()); // target needs to be actually allocated

Eigen::Matrix3Xf::Map(tf2[0].data(), 3, tf2.size())
  = Eigen::Matrix3Xd::Map(tf[0].data(), 3, tf.size()).cast<float>();

With the upcoming 3.4 branch of Eigen you can also use iterators over the casted-map, like so:

Eigen::Map<Eigen::Matrix3Xd> input(tf[0].data(), 3, tf.size());
std::vector<Eigen::Vector3f> tf2(input.cast<float>().colwise().begin(),
                                 input.cast<float>().colwise().end());