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

[Romain Francois]

Le 29/09/13 14:06, Simon Zehnder a écrit :
> 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));
> }

Try actually using the variable you pass in, as in:

void test2 (const Rcpp::CharacterVector &str)
{
	Rprintf("test2: %s\n", (char*) str(0));
}

Although it still exposes the bug.

You can use something like this in the meantime:

void test2 (const Rcpp::CharacterVector& charv)
{
     String x = charv[0] ;
     Rprintf("test2: %s\n", x.get_cstring());
}

It looks like the bug is about converting the result of charv(0) to a 
char*. Probably worth looking at the string_proxy class.

Romain

> 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?
>
>
> Best
>
> Simon

-- 
Romain Francois
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