[Rcpp-devel] Const correctness of NumericVector iterators

[Romain François]

Promoted this to an issue on github. https://github.com/RcppCore/Rcpp/issues/209

Should not be very hard to fix, but this is indeed a bug. The code should not compile if Rcpp respected const correctness. 

> Le 26 nov. 2014 à 14:20, Martyn Plummer <plummerM at iarc.fr> a écrit :
> 
> 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
> 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
> 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.
> 
> ### 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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