12 - External Pointers

In cpp4r, you can use external_pointer. In Rcpp, you can use XPtr to create external pointers. These have significantly different syntax, and cpp4r does not provide an attr method for external pointers.

For example, the cpp11tesseract package defines:

typedef cpp4r::external_pointer<tesseract::TessBaseAPI, tess_finalizer> TessPtr;

You can then call TessPtr with:

TessPtr ptr(api);
return ptr;

As a result, the R equivalent that the OCR C++ function verifies that the engine is such that the following is true:

stopifnot(inherits(engine, "externalptr"))

The equivalent tesseract package, which uses Rcpp, defines:

typedef Rcpp::XPtr<tesseract::TessBaseAPI, Rcpp::PreserveStorage, tess_finalizer, true> TessPtr;

You can then call TessPtr with:

TessPtr ptr(api);
ptr.attr("class") = Rcpp::CharacterVector::create("tesseract");
return ptr;

Similarly, the Rcpp version checks the engine with:

stopifnot(inherits(engine, "tesseract"))