[GitHub] [arrow] romainfrancois commented on pull request #8256: ARROW-9001: [R] Box outputs as correct type in call_function

[GitBox <gi...@apache.org>]

romainfrancois commented on pull request #8256:
URL: https://github.com/apache/arrow/pull/8256#issuecomment-720361840


   Last commit goes in that direction with: 
   
   ```cpp
   namespace cpp11 {
   
   template <typename T>
   std::string r6_class_name(const std::shared_ptr<T>& x) ;
   
   template <typename T>
   SEXP to_r6(const std::shared_ptr<T>& x) {
     if (x == nullptr) return R_NilValue;
   
     auto r_class_name = cpp11::r6_class_name<T>(x);
     cpp11::external_pointer<std::shared_ptr<T>> xp(new std::shared_ptr<T>(x));
     SEXP r6_class = Rf_install(r_class_name.c_str());
   
     // make call:  <symbol>$new(<x>)
     SEXP call = PROTECT(Rf_lang3(R_DollarSymbol, r6_class, arrow::r::symbols::new_));
     SEXP call2 = PROTECT(Rf_lang2(call, xp));
   
     // and then eval in arrow::
     SEXP r6 = PROTECT(Rf_eval(call2, arrow::r::ns::arrow));
   
     UNPROTECT(3);
     return r6;
   }
   }
   
   class R6 {
   public:
   
     template <typename T>
     R6(const std::shared_ptr<T>& x) : data_(cpp11::to_r6<T>(x)){}
   
     template <typename T>
     R6(std::unique_ptr<T> x) : data_(cpp11::to_r6<T>(std::shared_ptr<T>(x.release()))){}
   
     R6(SEXP data) : data_(data){}
   
     operator SEXP() const {
       return data_;
     }
   
   private:
     SEXP data_;
   };
   ```
   
   so that the functions "just" return `R6` and the the `R6` constructor finds the right R6 class name by calling one of the `r6_class_name` methods. 
   
   This however means defining many of these, most of them being trivial: 
   
   ```cpp
   template <> inline std::string r6_class_name<arrow::Field>(const std::shared_ptr<arrow::Field>& array_data) {
     return "Field";
   }
   ```
   
   but some of them with some notion of dispatch: 
   
   ```cpptemplate <> std::string r6_class_name<ds::Dataset>(const std::shared_ptr<ds::Dataset>& dataset) {
     auto type_name = dataset->type_name();
   
     if (type_name == "union") {
       return "UnionDataset";
     } else if (type_name == "filesystem") {
       return "FileSystemDataset";
     } else if(type_name == "in-memory"){
       return "InMemoryDataset";
     } else {
       return "Dataset";
     }
   }
   ```
   


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