Jim, I think the main point of Tomas' post was to alert R users to the fact that there are very serious issues that you have to understand when interfacing R from C++. Using C++ code from R is fine, in many cases you only want to access R data, use some library or compute in C++ and return results. Such use-cases are completely fine in C++ as they don't need to trigger the issues mentioned and it should be made clear that it was not what Tomas' blog was about.
I agree with Tomas that it is safer to give an advice to not use C++ to call R API since C++ may give a false impression that you don't need to know what you're doing. Note that it is possible to avoid longjmps by using R_ExecWithCleanup() which can catch any longjmps from the called function. So if you know what you're doing you can make things work. I think the issue here is not necessarily lack of tools, it is lack of knowledge - which is why I think Tomas' post is so important. Cheers, Simon > On Mar 29, 2019, at 11:19 AM, Jim Hester <james.f.hes...@gmail.com> wrote: > > First, thank you to Tomas for writing his recent post[0] on the R > developer blog. It raised important issues in interfacing R's C API > and C++ code. > > However I do _not_ think the conclusion reached in the post is helpful >> don’t use C++ to interface with R > > There are now more than 1,600 packages on CRAN using C++, the time is > long past when that type of warning is going to be useful to the R > community. > > These same issues will also occur with any newer language (such as > Rust or Julia[1]) which uses RAII to manage resources and tries to > interface with R. It doesn't seem a productive way forward for R to > say it can't interface with these languages without first doing > expensive copies into an intermediate heap. > > The advice to avoid C++ is also antithetical to John Chambers vision > of first S and R as a interface language (from Extending R [2]) > >> The *interface* principle has always been central to R and to S > before. An interface to subroutines was _the_ way to extend the first > version of S. Subroutine interfaces have continued to be central to R. > > The book also has extensive sections on both C++ (via Rcpp) and Julia, > so clearly John thinks these are legitimate ways to extend R. > > So if 'don't use C++' is not realistic and the current R API does not > allow safe use of C++ exceptions what are the alternatives? > > One thing we could do is look how this is handled in other languages > written in C which also use longjmp for errors. > > Lua is one example, they provide an alternative interface; > lua_pcall[3] and lua_cpcall[4] which wrap a normal lua call and return > an error code rather long jumping. These interfaces can then be safely > wrapped by RAII - exception based languages. > > This alternative error code interface is not just useful for C++, but > also for resource cleanup in C, it is currently non-trivial to handle > cleanup in all the possible cases a longjmp can occur (interrupts, > warnings, custom conditions, timeouts any allocation etc.) even with R > finalizers. > > It is past time for R to consider a non-jumpy C interface, so it can > continue to be used as an effective interface to programming routines > in the years to come. > > [0]: > https://developer.r-project.org/Blog/public/2019/03/28/use-of-c---in-packages/ > [1]: https://github.com/JuliaLang/julia/issues/28606 > [2]: https://doi.org/10.1201/9781315381305 > [3]: http://www.lua.org/manual/5.1/manual.html#lua_pcall > [4]: http://www.lua.org/manual/5.1/manual.html#lua_cpcall > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
