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

[Simon Zehnder]

Hi Romain,

thanks for the quick explanation of the idea behind the generic_proxy! - In addition, I am very glad, I do not have to understand everything what is going on under the hood. So I can concentrate on what I can do with it! 

Best

Simon

On Sep 29, 2013, at 8:46 PM, Romain Francois <romain at r-enthusiasts.com> wrote:

> Le 29/09/13 20:36, Simon Zehnder a écrit :
>> Hi Romain,
>> 
>> thanks for this fix!
>> On Sep 29, 2013, at 5:26 PM, Romain Francois <romain at r-enthusiasts.com> wrote:
>> 
>>> Hello,
>>> 
>>> What acts as a proxy for const CharacterVector& does not do its proxy job. Instead it gives direct access to the underlying array of SEXP that character vector use.
>>> 
>> I need some time to get this from the Rcpp source code to see what exactly is going on there - I don't have this deep understanding of the class structure, yet.
> 
> You don't really need to understand how it is implemented.
> 
>>> This has been fixed in Rcpp11. The relevant addition is the const_string_proxy class. See this commit which can easily be applied to Rcpp. https://github.com/romainfrancois/Rcpp11/commit/f5e1600f7acbf3bef39325c06ef3ac5ddf8dc66a
>>> 
>> Your new rep Rcpp11 looks very interesting! I have to distribute my package not on CRAN, but at least to colleagues working on Windows machines using the Rtools package. Rtools relies on gcc 4.6 and from http://gcc.gnu.org/projects/cxx0x.html I conclude, that it does not support all features - I guess for Rcpp11 it needs at least gcc 4.7?
>> On my own machine I can use it without a problem, but for the distribution among the Windows machines I fear I have to rely on the Rcpp CRAN version and use the nonconst reference as parameter.
>> 
>> I am curious, what feature of C++11 does enable the const_string_proxy?
>> 
>>> The commit in Rcpp11 also has removed a few things from the proxy class that I don't judge needed anymore because I'm cleaning things. This might not apply to Rcpp with its more strict compatibility requirements.
>>> 
>> I see, that the class 'generic_proxy' has gone. What was its intention in Rcpp?
> 
> I don't think so. These classes implement a proxy pattern. That is classic c++ pattern.
> 

> In essence, when you have a List and you call its operator[], what you get is a generic_proxy. This class's job is to define getters and setters in terms of operator= and implicit conversion operators so that you can do things like this:
> 
> List z ;
> RObject x = z[0] ;
> z[0] = 2 ;
> 
> The proxy classes take care of all the plumbing here.
> 
> But again, you should not need to know about this.
> 
>> Best
>> 
>> Simon
>> 
>> 
>>> Romain
>>> 
>>> Le 29/09/13 15:24, Romain Francois a écrit :
>>>> 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
>>> Professional R Enthusiast
>>> +33(0) 6 28 91 30 30
>>> 
>>> _______________________________________________
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>>> Rcpp-devel at lists.r-forge.r-project.org
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>> 
>> 
> 
> 
> -- 
> Romain Francois
> Professional R Enthusiast
> +33(0) 6 28 91 30 30
>