title: Swap two SEXP without copying
body: <p>Using SEXP as argument of a function don't allows user to exchange data between them by simple assignement. I used to copy each value with a tmp buffer to perform swapping.
  My question is :there is a possibilty to write a function that only swap data  like this:</p>
  
  <pre><code>// [[Rcpp::export]]
  void  swap(SEXP x, SEXP y){
    std::swap(x,y);
  
  } 
  </code></pre>
  
  <p>Then if I run this function with R, x and y will be swapped ?</p>