[Rcpp-devel] How to Get Pointer of Underlying Object of Rcpp Module Class

[Tianqi Chen]

Sorry about the in-complete message. I sent it earlier but did not get it
posted. The question is about how to Get XPtr out from underlying Rcpp S4
object, and how to construct such object from C++ side.

Tianqi

On Tue, Oct 6, 2015 at 2:40 PM, Tianqi Chen <tianqi.tchen at gmail.com> wrote:

> Dear Rcpp Developers:
>
> ```c++
> class MyClass {
> public:
>    static void SaveList(SEXP list, const std::string& filename) {
>           Rcpp::List data_lst(list);
>           std::vector<MyClass *> internals(data_lst[i]);
>           for (size_t i = 0; i < data_lst.size(); ++i) {
>                SEXP obj = data_lst[i];
>                // Need to Get internal pointers from S4 MyClass object
>                internals[i] = GetPtr<MyClass>(obj);
>           }
>           SomeRealSave(internals, filename);
>    }
>    static void LoadList(const std::string& filename) {
>           std::vector<MyClass > internals;
>           SomeReadLoad(&internals);
>           Rcpp::List out(internals.size());
>           for (size_t i = 0; i < internals.size(); ++i) {
>               // need to construct a MyClass object from C++ side
>               out[i] = MyClassConstructor(internals[i]);
>           }
>          return out;
>    }
> };
>
> RCPP_MODULE(MyClass) {
>      class_<MyClass>("MyClass");
>
>      function("save.myclass.list", &MyClass::SaveList);
>      function("load.myclass.list", &MyClass::LoadList);
> }
> ```
>
> To be specific, We have ways to Save/Load the pointers to the MyClass
> object (in the SomeRealSave and SomeRealLoad). In our specific use-case, It
> was an Array on GPU.
>
> So the question is how is there any solution to  GetPtr<MyClass> and MyClassConstructor,
> so we can get the internal pointer of MyClass from SEXP(which is the S4
> object), and construct the corresponding S4 object given the information of
> MyClass.
>
>
> What was wanted is indeed possible if we choose to only expose
> XPtr<MyClass>, so we can simply do
> ```
>     MyClass * GetPtr<MyClass>(SEXP obj) {
>          XPtr<MyClass>(obj).get();
>     }
>     MyClassConstructor(MyClass *ptr) {
>         return XPtr<MyClass>(ptr);
>    }
>
> ```
>
> But it would be really nice we can directly return a wrapped S4 object
> instead of raw ptr
>
> Tianqi
>
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