The STL transform function can be used to pass a single function over a vector. Here we use a simple function square().

#include <Rcpp.h>

using namespace Rcpp;

inline double square(double x) { return x*x ; }

// [[Rcpp::export]]
std::vector<double> transformEx(const std::vector<double>& x) {
    std::vector<double> y(x.size());
    std::transform(x.begin(), x.end(), y.begin(), square);
    return y;
}

  x <- c(1,2,3,4)
  transformEx(x)


[1]  1  4  9 16

A second variant combines two input vectors.

inline double squaredNorm(double x, double y) { return sqrt(x*x + y*y); }

// [[Rcpp::export]]
NumericVector transformEx2(NumericVector x, NumericVector y) {
    NumericVector z(x.size());
    std::transform(x.begin(), x.end(), y.begin(), z.begin(), squaredNorm);
    return z;
}

  x <- c(1,2,3,4)
  y <- c(2,2,3,3)
  transformEx2(x,y)


[1] 2.236 2.828 4.243 5.000