[Rcpp-devel] Const correctness of NumericVector iterators

[Romain Francois]

This is probably related to this: 


	template <int RTYPE>
	struct r_vector_const_iterator {
		typedef typename storage_type<RTYPE>::type* type ;
	};

There should be a const there somewhere. 

FWIW, more modern implementations of R/C++ are more safe on that issue: 

$ Rcpp11Script /tmp/martyn.cpp
file33e7a74d9ba.cpp:18:33: error: cannot initialize a variable of type 'NumericVector::iterator' (aka 'double *') with an rvalue of
      type 'const_iterator' (aka 'const double *')
   for (NumericVector::iterator i = v.begin(); i != v.end(); ++i) {
                                ^   ~~~~~~~~~
1 error generated.
make: *** [file33e7a74d9ba.o] Error 1
Error in dyn.load(basename(dynlib)) :
  unable to load shared object '/private/var/folders/k8/gmp11jks4hl_nwcnk82fbr8m0000gn/T/RtmpKvVUpq/file33e7a74d9ba.so':
  dlopen(/private/var/folders/k8/gmp11jks4hl_nwcnk82fbr8m0000gn/T/RtmpKvVUpq/file33e7a74d9ba.so, 6): image not found
Calls: sourceCpp -> <Anonymous> -> dyn.load
In addition: Warning message:
running command '/Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB 'file33e7a74d9ba.cpp'' had status 1
Execution halted


If I change NumericVector::iterator to auto, I then get: 

$ Rcpp11Script /tmp/martyn.cpp
file34f617c08b2.cpp:19:16: error: binding of reference to type 'double' to a value of type 'const double' drops qualifiers
       double &x = *i;
               ^   ~~
1 error generated.
make: *** [file34f617c08b2.o] Error 1
Error in dyn.load(basename(dynlib)) :
  unable to load shared object '/private/var/folders/k8/gmp11jks4hl_nwcnk82fbr8m0000gn/T/Rtmp74ZJLp/file34f617c08b2.so':
  dlopen(/private/var/folders/k8/gmp11jks4hl_nwcnk82fbr8m0000gn/T/Rtmp74ZJLp/file34f617c08b2.so, 6): image not found
Calls: sourceCpp -> <Anonymous> -> dyn.load
In addition: Warning message:
running command '/Library/Frameworks/R.framework/Resources/bin/R CMD SHLIB 'file34f617c08b2.cpp'' had status 1
Execution halted

Hard compiler error/GOOD THING. 


> 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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