bad naming scheme and should probably use a more general template than Eigen::VexctorX?, but this does what i ened for now

eigen_hdk_conversion.hpp

#ifndef EIGEN_HDK_CONVERSION_HPP
#define EIGEN_HDK_CONVERSION_HPP
#include <UT/UT_SparseMatrix.h>
#include <Eigen/Sparse>

template <typename T, int Options = Eigen::ColMajor, typename StorageIndex>
UT_SparseMatrixELLT<T> eigen_to_hdk_ellt(const Eigen::SparseMatrix<T,Options,StorageIndex>& A) {

    UT_SparseMatrixELLT<T> B(A.rows(), A.nonZeros());
    int nz;
    for (int k=0; k<A.outerSize(); ++k) { 
        for (typename Eigen::SparseMatrix<T>::InnerIterator it(A,k); it; ++it) {
            std::cout << it.row() << ", " << it.col() << ": " <<  it.value() << std::endl;
            B.appendRowElement(it.row(), it.col(), it.value(), nz);
        }
    }
    return B;

}
template <typename T, int Options = Eigen::ColMajor, typename StorageIndex>
UT_SparseMatrixT<T,false> eigen_to_hdk(const Eigen::SparseMatrix<T,Options,StorageIndex>& A) {
    UT_SparseMatrixT<T,false> B(A.rows(),A.cols());

    for (int k=0; k<A.outerSize(); ++k) { 
        for (typename Eigen::SparseMatrix<T>::InnerIterator it(A,k); it; ++it) {
            B.addToElement(it.row(), it.col(), it.value());
        }
    }
    B.compile();
    return B;
}

template <typename T>
Eigen::SparseMatrix<T> hdk_to_eigen(const UT_SparseMatrixT<T,false>& A) {
    using Triplet = Eigen::Triplet<T>;
    std::vector<Triplet> ts;
    auto func =  [&](int r, int c, T v) {
        ts.emplace_back(r,c,v);
    };
    A.accept(func);
    Eigen::SparseMatrix<T> B(A.getNumRows(),A.getNumCols());
    B.setFromTriplets(ts.begin(),ts.end());
    return B;
}
template <typename T>
UT_VectorT<T> eigen_to_hdk(const Eigen::Matrix<T,Eigen::Dynamic,1>& A) {
    UT_VectorT<T> B(0,A.size()-1);
    for(int i = 0; i < A.rows(); ++i) {
        B(i) = A(i);
    }
    //std::copy(A.data(),A.data()+A.size(),B.getData());
    return B;
}
template <typename T>
Eigen::Matrix<T,Eigen::Dynamic,1> hdk_to_eigen(const UT_VectorT<T>& A) {
    Eigen::Matrix<T,Eigen::Dynamic,1> B(A.length());
    for(int i = 0; i < A.length(); ++i) {
        B(i) = A(i + A.getNL());
    }
    //std::copy(A.data(),A.data()+A.size(),B.getData());
    return B;
}
#endif//EIGEN_HDK_CONVERSION_HPP