http://stackoverflow.com/questions/42363850/template-for-class-returning-elements-of-rcppvectorrtype

vector-template.cpp

template <int RTYPE>
class MyNumVec {
public: 
    typedef Rcpp::Vector<RTYPE> vec_t;
    typedef typename Rcpp::traits::storage_type<RTYPE>::type storage_t;
    
private:
    const vec_t& x;
    
public:
    MyNumVec(const vec_t& y) 
        : x(y) 
    {}
    
    storage_t operator[](int i) const 
    { return x[i]; }
    
    operator vec_t() const 
    { return x; }
};