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

[Romain Francois]

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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>
>


-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30