C++ is Often Used in R Packages

April 3, 2019
By

(This article was first published on R – Win-Vector Blog, and kindly contributed to R-bloggers)

The recent r-project article “Use of C++ in Packages” stated as its own summary of recommendation:

don’t use C++ to interface with R.

A careful reading of the article exposes at least two possible meanings of this:

  1. Don’t use C++ to directly call R or directly manipulate R structures. A technical point directly argued (for right or wrong) in the article.
  2. Don’t use C++/Rcpp to write R packages. A point implicit in the article. C++ and Rcpp (a package designed to allow the use of C++ from R) are not the same thing, but both are mentioned in the note.

One could claim the article is “all about point 1, which we can argue on its technical merits.” The technicalities involve discussion of C‘s setjmp/longjmp and how this differs from C++‘s treatment of RAII, destructors, and exceptions.

(edit: It has been pointed out to me that as there is no C++ interface to R that the point-1 interpretation is in some sense not technically possible. All C++ is in some sense forced to go through the C interface. Yes things can go wrong, but in strict technical sense you can’t directly “use C++ to interface with R“, C++ calls .C() or .Call() just as C does.)

However, in my opinion the overall tone of the article unfortunately reads as being about point 2. In fact after multiple readings of the article I remain uncomfortable saying if the article is in fact attempting to make point 2 or attempting to avoid point 2. Statements such as “Packages that are already using C++ would best be carefully reviewed and fixed by their authors” seem to accuse all existing C++ packages. But statements such as “one could use some of the tricks I’ve described here” seem to imply there are in fact correct ways to interface C++ with R (which for all we know, many C++ packages may already be using).

I think a point 2 interpretation of the article does the R community a disservice. So I hope the note is not in fact about point 2. And if it isn’t about point 2, I wish that had been stronger emphasized and made clearer.

For context Rcpp is the most popular package on CRAN. Based on CRAN data downloaded 2019/03/31: Rcpp is directly used in 1605 CRAN packages (or about 11% of CRAN packages), and indirectly used (brought in through Import/Depends/LinkingTo) by 6337 packages (or about 45% of CRAN packages). It has the highest reach of any CRAN package under each of those measures (calculation shared here), and even under a pagerank style measure.

Rcpp is something R users should be appreciative of and grateful for. Rcpp should not become the subject of fear, uncertainty, and doubt.

I apologize if I am merely criticizing my own mis-reading of the note. However, others have also written about discomfort with this note, and the original note comes from a position of authority (so does have a greater responsibility to be fairly careful in how it might be plausibly read).

To leave a comment for the author, please follow the link and comment on their blog: R – Win-Vector Blog.

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