The issue I am encountered with is… the same vector we read as double is to be used in another function, but with different data type as mentioned bellow. So I need a conversion function which performs this fact. I have a class as follows template <class T> class NRVec { private: int nn; // size of array. upper index is nn-1 T *v; public: NRVec(); explicit NRVec(int n); // Zero-based array NRVec(const T &a, int n); //initialize to constant value NRVec(const T *a, int n); // Initialize to array inline const T & operator[](const int i) const; inline int size() const; ~NRVec(); }; I have a vector defined as follows typedef const NRVec<DP> Vec_I_DP; I have a another class having a function svdfit() and this function takes a argument (vector) with datatype from the class NRVec. void AutoCompensation::svdfit(Vec_I_DP &x, Vec_I_DP &y) The problem arises when I have my own vectors with type typedef double DP; vector <DP> v,Vect_XX_AT, Vect_YY_AT; Now instead of using Vec_I_DP, I want to use my vector Vec_XX_AT. The problem is difference in the datatype. So I wanted to have a conversion function which converts vector from datatype Vec_I_DP(Vec_I_DP &x) to DP(vector <DP> Vect_XX_AT) as shown above. How to do it?? Merci

What you'll need to do is to create an array of doubles using double *arr=new double[Vect_XX_AT.size()] (now put a delete[] somewhere in such a way as it will cause a compiler error - forcing yourself to deal with it before proceeding), populate it from Vect_XX_AT, then you can use the NRVec(const T *a,int n) constructor.