Illustrates that a NumericVector is a pointer

leftShift.cpp

/**
 * @title A NumericVector is Really a Pointer
 * @author G. Grothendieck completing Michael Meyer's post at https://stat.ethz.ch/pipermail/r-help/2015-August/431645.html
 * @license GPL (>= 2)
 * @tags NumericVector pointer
 * @summary Shows that a NumericVector is a pointer.
 */

// To run:  library(Rcpp); sourceCpp("leftShift.cpp")

#include <Rcpp.h> 
using namespace Rcpp; 

// [[Rcpp::export]]
NumericVector leftShift(NumericVector x){

  int n = x.size();
  Rprintf("C++: address of x prior to modification <%p>\n", (SEXP)x);
  for(int i=0;i<n-1;i++) x[i]=x[i+1];
  Rprintf("C++: address of x after modification <%p>\n", (SEXP)x);
  return x;
}

// next function is the same except x is a clone of the input

// [[Rcpp::export]]
NumericVector leftShiftClone(NumericVector x0){

  NumericVector x(clone(x0));
  int n = x.size();
  // the address of x0 will be the same as address of x0 in R
  // the x addresses will be the same as each other and different from x0
  Rprintf("C++: address of x0 <%p>\n", (SEXP)x0);
  Rprintf("C++: address of x prior to modification <%p>\n", (SEXP)x);
  for(int i=0;i<n-1;i++) x[i]=x[i+1];
  Rprintf("C++: address of x after modification <%p>\n", (SEXP)x);
  return x;
}


/*** R

library(pryr)

#########################################################
# x, the input, changes

x <- c(1, 2, 3)
address(x) # R: address of x prior to calling C++
y <- leftShift(x)
address(y) # R: address of output y
y

address(x) # R: address of x after calling C++
x # the input, x, has changed!

#########################################################
# x0, the input, is unchanged

x0 <- c(1, 2, 3)
address(x0) # R: address of x0 prior to calling C++
y0 <- leftShiftClone(x0)
address(y0) # R: address of y after calling C++ (same as x in C++)
y0

x0 # since leftShiftClone internally cloned x0 (the input), x0 did not change
address(x0) # R: address of x0 after calling C++ (unchanged)
*/

Also note this discussion of swapping without copying in R.
https://stackoverflow.com/questions/54970202/swap-two-sexp-without-copying/54970808#54970808