[Rcpp-devel] Understanding the behaviour of const CharacterVector as a function parameter

[Simon Zehnder]

Hi Dirk,

thanks for the quick response! 

I do not yet understand your comment towards the const correctness in regard to SEXP, but I will take a closer look at the doxygen of Rcpp. 

New Version of Rcpp: Cool! Thanks for the tip, as I am curious now, I take the Vienna CRAN. The changes from Romain are the ones discussed lately on the rcpp-devel list regarding the shallow copy I assume. 

Enjoy your day with your family!


Best

Simon

 
On Sep 29, 2013, at 2:16 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

> 
> On 29 September 2013 at 14:06, Simon Zehnder wrote:
> | Dear Rcpp::Users and Rcpp::Devels,
> | 
> | I would like to understand a certain behaviour of my code I encountered lately. 
> | 
> | I am working with CharacterVector and the following behaviour occurred:
> | 
> | void test1 (Rcpp::CharacterVector &charv)
> | {
> | 	Rprintf("test1: %s\n", (char*) charv(0));
> | }
> | 
> | void test2 (const Rcpp::CharacterVector &str)
> | {
> | 	Rprintf("test2: %s\n", (char*) charv(0));
> | }
> | 
> | Using a string like "2013-05-04 20:23:21" for the Rcpp::CharacterVector gives the following outputs:
> | 
> | test1: 2013-05-04 20:23:21
> | 
> | test2:  `
> | 
> | This does also not change if I use a cast to const char* in test2. I tried something similar with strings and printing the c_str() of them, there the 'const' keyword does not make a difference - it always prints the correct string.
> | 
> | Is this something specific to the Rcpp::CharacterVector, that uses a string_proxy for its elements returned by the operator ()? Is there a way to use const Rcpp::CharacterVector and get the behaviour of test1? 
> 
> Looks like a bug. (But note that const correctness of types build around SEXP
> is at best a promise -- we cannot undo the fact the the SEXP _is_ a pointer.)
> 
> But hold on for a day til Rcpp 0.10.5 reaches your mirror, or grab it from
> CRAN in Vienna. It brings a lot of excellent changes, among them some fine
> work by Romain dealing with exactly that. Full announcement coming later once
> I am back from running and kid's soccer game.
> 
> Dirk
> 
> -- 
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com