Calling another cpp function in templated Rcpp function

test2.cpp

#include <Rcpp.h>
#include <string>
#include <vector>
// [[Rcpp::plugins("cpp11")]]

std::string nth_string(Rcpp::NumericVector& x, int i) {
  return std::to_string(x[i]);
}

double nth_numeric(Rcpp::NumericVector& x, int i) {
  return x[i];
}

template <typename T>
using RcppVec = Rcpp::Vector<Rcpp::traits::r_sexptype_traits<T>::rtype>;

template <int ITYPE, typename FTYPE>
auto apply_cpp_fun(Rcpp::Vector<ITYPE> const& x, FTYPE fun) ->
RcppVec<typename std::result_of<FTYPE>::type> {
  int n = x.size();
  RcppVec<typename std::result_of<FTYPE>::type>  res(n);

  for (int i = 0; i < n; i++) {
    res[i] = fun(x, i);
  }

  return res;
}

// [[Rcpp::export]]
Rcpp::NumericVector test1_cpp(Rcpp::NumericVector x) {
  return apply_cpp_fun(x, nth_numeric);
}