[Rcpp-devel] Const correctness of NumericVector iterators

[Dirk Eddelbuettel]

Hi Martyn,

On 26 November 2014 at 13:20, Martyn Plummer wrote:
| This is related to David Shih's thread about modifying input arguments
| but I think it needs its own thread.
| 
| I found a problem when I tried porting some Rcpp code to run outside of
| Rcpp by providing my own NumericVector implementation based on STL

Interesting. I have seen that use case.  Typically I have 'real' C++ code
using STL which I'd then bring to R(cpp).

| vectors. In Rcpp it is possible to obtain a non-constant iterator for a
| constant vector, whereas STL does not allow this.
| 
| In particular, it is possible to modify a const NumericVector. The

"Not on purpose" but mostly because we get a SEXP we cannot protect better?

| compiler allows this because the actual contents of the NumericVector
| are in auxiliary memory. But this breaks the abstraction of the
| NumericVector class.
| 
| An example is given below.  The "sumsquares" function calculates the sum
| of squares of a vector. Internally, it calls the "workhorse" function,
| which is declared to take a constant reference as an argument. So it
| looks safe to pass the input vector to "workhorse" without copying.
| However the implementation of "workhorse" does in fact alter the vector.

Isn't that an argument for just using STL vectors ?

Otherwise, patches/pull requests always welcome...

Dirk

| 
| ### BEGIN SS.cc
| #include <Rcpp.h>
| 
| using Rcpp::NumericVector;
| 
| static double workhorse(NumericVector const &v);
| 
| // [[Rcpp::export]]
| double sumsquares(NumericVector v)
| {
|     //Since workhorse takes a constant reference we think we do not need
|     //to copy v...
|     return workhorse(v);
| }
| 
| double workhorse(NumericVector const &v)
| {
|     double y = 0;
|     for (NumericVector::iterator i = v.begin(); i != v.end(); ++i) {
|         double &x = *i;
|         //... but this function does alter its argument
|         x *= x;
|         y += x;
|     }
|     return y;
| }
| ### END SS.cc
| 
| In R we have 
| 
| > library(Rcpp)
| > sourceCpp("SS.cc")
| > x <- c(1,2,3)
| > sumsquares(x)
| [1] 14
| > x #is modified
| [1] 1 4 9
| 
| Martyn
| 
| 
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