[Rcpp-devel] RFC: Rcpp modules vs. RefClass

Hao Ye hye at ucsd.edu
Tue Nov 29 20:12:00 CET 2016

> (I guess one downside of Rcpp modules is the use of external pointers,
> which implies they do not serialize / deserialize well?)

I think not at all, unless you can force the same memory location for a new
R session and everything it loads. This is documented in the Rcpp-modules
vignette -- I tried it anyway, and it crashed my R session. :)

Hao Ye
hye at ucsd.edu

On Tue, Nov 29, 2016 at 11:02 AM, Kevin Ushey <kevinushey at gmail.com> wrote:

> To be brief -- Rcpp modules are effectively in maintenance mode at this
> point; we don't plan to extend / improve modules beyond resolving issues if
> and when they come up. We did get to the bottom of the posted issue -- R
> packages that use modules need to import the whole Rcpp namespace.
> I'm not quite sure what the parallel between reference classes and modules
> are here -- Rcpp modules allow you to basically expose C++ classes to R
> with a relatively minimal amount of boilerplate; of course you can write
> your own R functions (maybe as part of a reference class) to do the same,
> but the techniques are in the end different.
> In sum, I think Rcpp modules are more geared towards developers who are
> primarily C++ programmers who just want a super-simple way to expose a C++
> class to R; other methods are going to lean towards being more R-centric. I
> can't really call one method 'better' or 'worse'; they're just different.
> (I guess one downside of Rcpp modules is the use of external pointers,
> which implies they do not serialize / deserialize well?)
> Kevin
> On Mon, Nov 28, 2016 at 8:40 PM, Christian Gunning <xian at unm.edu> wrote:
>> Dear List,
>> The following is a general request for advice / comment on modern Rcpp
>> development best-practices for package & class development. I looked over
>> the Rcpp gallery, and didn't see anything obvious that answers my
>> questions  - perhaps this discussion could serve as a prototype for a new
>> post?
>> ## Background
>> I've used Rcpp modules for several projects where in-place modification
>> was required for performance reasons. I like the interface - it encourages
>> clean code, and yields a nice mix of performance and encapsulation.
>> In the past, the lack of serialization has been a minor annoyance.
>> Honestly, it's not something I need, but I dislike having invalid objects
>> in the work-space after a quit/restart. I've spent a little time thinking
>> about work-arounds, which essentially boil down to moving back and forth
>> from an R list object.
>> Looking towards the future, I also looked at the recent Rcpp dev history.
>> It looks like modules has had some maintenance issues - for example, the
>> last edits there (i..e, PR 454) were reverted due to Windows toolchain
>> issues (i.e., https://github.com/RcppCore/Rcpp/issues/463).  From my
>> outside perspective, it appears that the modules code is A) hard, and B)
>> not a current dev priority.
>> ## A possible alternative: RefClass
>> I'm able to achieve similar behavior (in-place modification using named
>> methods, relatively tidy code) using a combination of R RefClasses and Rcpp
>> attributes. This solves the issue of serialization, and yields reasonably
>> clean code. This has the added benefit of allowing easy mixing of R and C++
>> code in class methods.
>> From a user perspective, RefClass methods are a nice place for Rcpp
>> functions that modify args in-place, since RefClass implies side-effects.
>> And, in terms of style, if all C++ method functions return void, and have
>> const-correct arglists, then the C++ function signatures provide something
>> of a interface spec.
>> Minimal example:
>> https://gist.github.com/helmingstay/17d5d9f241c4170a29d1681db0111065
>> ## Summary of observations:
>> * RefClass + attributes achieves similar outcomes to Rcpp modules, with
>> somewhat better support (serialization, documentation, future?).
>> * Unique to Rcpp modules is the ability to auto-magically generate
>> RefClass-like R bindings for existing C++ class-based code.
>> * For "mere mortals", Rcpp modules now look less attractive for routine
>> use, given available alternatives (i.e. for anything but binding
>> auto-generation)
>> ## Questions:
>> A) Any obvious problems with the above observations?
>> B) Are there any *gotchas* with using Rcpp "modify-in-place" functions
>> inside RefClass methods?
>> C) Is anyone else doing this? Any suggested improvements on the above?
>> Thanks much,
>> Christian Gunning
>> --
>> A man, a plan, a cat, a ham, a yak, a yam, a hat, a canal – Panama!
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