[Rcpp-devel] Const correctness of NumericVector iterators

[Romain François]

And now sent a pull request. Not sure this will be merged in with the new commandment that « Thou shalt not break abi ». 
See: https://github.com/RcppCore/Rcpp/pull/211

Romain

> Le 29 nov. 2014 à 13:52, Romain François <romain at r-enthusiasts.com> a écrit :
> 
> 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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