C++ template function which can take Eigen::vector or std::vector

Question

I was using std::vector in my program and now I made a realization that Eigen::VectorXd will reduce my task in great deal. So I shifted to using Eigen. But I don't want to change the program entirely so I thought of using Templates for some previously defined functions.

I'm new to C++ and template programming is little bit confusing I'm not able to think beyond the below program which is wrong.

using namespace Eigen;
template<class T>
T getvec(T& var)
{
    T res;
    res[0] = var[0]*3;
    res[1]=var[1]*3;
     res[2]=var[2]*3;
    return res;
}

int main(){


    std::vector<double> a(3,1);
    Eigen::VectorXi b(3);
    b.setOnes();

     auto x= getvec(a);
    auto y=getvec(b);
    }

Is it possible to write a template function which can take std::vector or Eigen::VectorXd as paramters?

I am in need of a of function as below which can take both type.

std::vector<double> getTransform(std::vector<double>& vec)
{
     std::vector<double> res;
     res[0] = vec[0]*3.14;
     ...........
     ...........
     return  res;
}

Answer 1

Try this:

template <typename T>
T  getTransform(const T& vec)
{
    T res;
    res.resize(vec.size());
    for (size_t i = 0; i < size_t(vec.size()); ++i)
    {
        res[i] = vec[i]*3.14;
    }
    return  res;
}