subset a vector and sort it

I'm looking into using some C++ for simple parts of my R package using the Rcpp package. I'm a C++ novice (but keen to learn!). I've implemented a few simple cpp programs using the excellent Rcpp - in fact that package has motivated me to learn C++...

Anyway, I've got stuck with a simple problem, which if I can fix would help lots. I have a NumericVector I want to subset and then sort. The code below sorts the whole vector (and would also deal with NAs, which is what I need).

My question is, say I want to extract a part of this vector, sort and have it available for other processing - how can I do that? For example, for a vector of length 10, how do I extract and sort the elements 5:10?

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
RcppExport SEXP rollP(SEXP x) {
  NumericVector A(x); // the data  
  A = sort_unique(A);  
  return A;
}

which I call from R:

sourceCpp( "rollP.cpp")
rollP(10:1)
# [1]  1  2  3  4  5  6  7  8  9 10