romainfrancois commented on pull request #8256:
URL: https://github.com/apache/arrow/pull/8256#issuecomment-724712441
I've added back an `as_sexp()` for `std::shared_ptr<...>` so that we can
return the actual type instead of the catch-all R6, so for example:
```cpp
// [[arrow::export]]
std::shared_ptr<arrow::Field> Field__initialize(
const std::string& name, const std::shared_ptr<arrow::DataType>& field,
bool nullable = true) {
return arrow::field(name, field, nullable);
}
```
`as_sexp()` uses `cpp11::to_r6<>()` which in turns uses `r6_class_name<>()`
to decide the R6 class name.
```cpp
template <typename T>
SEXP as_sexp(const std::shared_ptr<T>& ptr) {
return cpp11::to_r6<T>(ptr);
}
```
I have not been able to replace this template function with a template
struct so far:
```cpp
template <typename T>
const char* r6_class_name(const std::shared_ptr<T>&);
```
this was giving errors about the type not being fully defined or something.
I'm using a macro to help generating the many functions:
```cpp
#define DEFAULT_R6_CLASS_NAME(CLASS, NAME) \
namespace cpp11 { \
template <> \
const char* r6_class_name<CLASS>(const std::shared_ptr<CLASS>&) { \
return NAME; \
} \
}
```
so that for example:
```cpp
DEFAULT_R6_CLASS_NAME(parquet::ArrowReaderProperties,
"ParquetArrowReaderProperties")
DEFAULT_R6_CLASS_NAME(parquet::ArrowWriterProperties,
"ParquetArrowWriterProperties")
DEFAULT_R6_CLASS_NAME(parquet::WriterProperties, "ParquetWriterProperties")
DEFAULT_R6_CLASS_NAME(parquet::arrow::FileReader, "ParquetFileReader")
DEFAULT_R6_CLASS_NAME(parquet::WriterPropertiesBuilder,
"ParquetWriterPropertiesBuilder")
DEFAULT_R6_CLASS_NAME(parquet::arrow::FileWriter, "ParquetFileWriter")
```
So that's still a little bit of housekeeping, and it's a bit brute force,
but neither the R code (no more `shared_ptr()` or `..dispatch()`) nor the C++
side have to deal with conversion from a `std::shared_ptr<arrow::*>` to the
right R6 class.
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