[Rcpp-devel] c++ class as argument in rcpp function (using modules)

[Yasir Suhail]

Hi

I looked up the previous thread "Creating pointers to objects and wrapping
them" from
http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2011-April/002170.html,
and it seems that providing a specialization of the wrap<>() function can
help us return classes. Perhaps similarly, one could provide a
specialization of the as<>() function for arguments? While wrap is
implemented with the Language("new" ... function, is there any example of
an as<>() specialization for custom classes? The vignette (
http://dirk.eddelbuettel.com/code/rcpp/Rcpp-extending.pdf) shows a
declaration for as, but no definitions.

Thanks!

Yasir



On Wed, Dec 21, 2011 at 5:45 AM, <romain at r-enthusiasts.com> wrote:

> Iirc, this is not implemented yet.
>
> There is a chance we might add this feature in the future.
>
> Cheers,
>
> Romain
>
>
>
> Le 21 déc. 2011 à 10:59, Yasir Suhail <yusuhail at gmail.com> a écrit :
>
> I need to pass a class as an argument in a function I am exposing to R.
> From the skeleton package module, let us say that I have
>
> class World {
> public:
>     World() : msg("hello"){}
>     void set(std::string msg) { this->msg = msg; }
>     std::string greet() { return msg; }
>     void addMsg(World& w2) {
> msg = msg + w2.msg;
>     }
> private:
>     std::string msg;
> };
>
> However, this gives errors of the sort
> /usr/local/lib/R/site-library/Rcpp/include/Rcpp/module/Module_generated_CppMethod.h:186:4:
> error: invalid initialization of non-const reference of type ‘World&’ from
> an rvalue of type ‘World’
> In file included from
> /usr/local/lib/R/site-library/Rcpp/include/RcppCommon.h:306:0,
>                  from /usr/local/lib/R/site-library/Rcpp/include/Rcpp.h:27,
>                  from rcpp_module.cpp:1:
> /usr/local/lib/R/site-library/Rcpp/include/Rcpp/traits/Exporter.h: In
> constructor ‘Rcpp::traits::Exporter<T>::Exporter(SEXPREC*) [with T = World,
> SEXPREC* = SEXPREC*]’:
> /usr/local/lib/R/site-library/Rcpp/include/Rcpp/as.h:51:51:   instantiated
> from ‘T Rcpp::internal::as(SEXPREC*, Rcpp::traits::r_type_generic_tag)
> [with T = World, SEXPREC* = SEXPREC*]’
> /usr/local/lib/R/site-library/Rcpp/include/Rcpp/as.h:75:89:   instantiated
> from ‘T Rcpp::as(SEXPREC*) [with T = World, SEXPREC* = SEXPREC*]’
> /usr/local/lib/R/site-library/Rcpp/include/Rcpp/module/Module_generated_CppMethod.h:186:4:
>   instantiated from ‘SEXPREC* Rcpp::CppMethod1<Class, void,
> U0>::operator()(Class*, SEXPREC**) [with Class = World, U0 = World&,
> SEXPREC* = SEXPREC*]’
> rcpp_module.cpp:86:1:   instantiated from here
> /usr/local/lib/R/site-library/Rcpp/include/Rcpp/traits/Exporter.h:31:37:
> error: no matching function for call to ‘World::World(SEXPREC*&)’
> rcpp_module.cpp:30:5: note: candidates are: World::World()
> rcpp_module.cpp:28:13: note:                 World::World(const World&)
> make: *** [rcpp_module.o] Error 1
>
> Looking at http://dirk.eddelbuettel.com/code/rcpp/Rcpp-extending.pdf, it
> seems that I can extend Rcpp::as to account for my classes, but I still
> have two questions:
> 1. How do I define the constructor World(SEXP)? Is there a working example
> somewhere for a user defined C++ class?
> 2. Can I also use pointers and pass-by-reference as these are more often
> used in real C++ code.
>
> In short, I'd appreciate a way to pass my C++ classes (as the class and
> also by ref. or pointer) that have been exposed to R as arguments to
> exposed functions. How do I achieve that?
>
> Thanks!
>
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