[Rcpp-devel] Yet Another Design Question

[Romain Francois]

Le 13/04/10 00:14, Douglas Bates a écrit :
>
> One of the advantages of using S4 classes and methods in R is that
> there are formal definitions of classes.  I plan eventually to write R
> code that will take an S4 class definition and create a C++ class
> definition and constructor code to mirror the R class using Rcpp
> templates.
>
> Right now my question is related to constructors.  If I have a slot in
> an S4 class that is itself an S4 object, is there a way to chain the
> constructor so that I generate the internal object without first
> creating an empty object.
>
> To be specific, I have an S4 class "reModule" that contains sparse
> matrices of S4 class "dgCMatrix" and some information that is atomic.
> The S4 definition is (more-or-less)
>
> setClass("reModule", representation(Lambda = "dgCMatrix", Zt =
> "dgCMatrix", theta = "numeric", Lind = "integer"))
>
> and the definition of dgCMatrix is
>
> setClass("dgCMatrix", representation(Dim = "integer", i = "integer", p
> = "integer", x = "numeric")
>
> Using Rcpp I define C++ classes
>
> class dgCMatrix {
> public:
>      dgCMatrix(Rcpp::S4);
>
>      Rcpp::IntegerVector Dim, i, p;
>      Rcpp::NumericVector x;
> };
>
> class reModule {
> public:
>      reModule(Rcpp::S4);
>
>      dgCMatrix Lambda, Zt;
>      Rcpp::NumericVector theta;
>      Rcpp::IntegerVector Lind;
> };
>
> If I define a constructor for the reModule without chaining to
> constructors for the dgCMatrix objects then I need to define a
> dgCMatrix::dgCMatrix() constructor and then redefine the Lambda and Zt
> members inside the reModule constructor.  I think a more idiomatic C++
> constructor definition is to chain to the other constructor as in
>
> using namespace Rcpp;
>
> reModule::reModule(S4 xp) :
>      Lambda(S4(xp.slot("Lambda"))),
>      Zt(S4(xp.slot("Zt"))),
>      ...
> {
> }
>
> but the compiler doesn't like constructing an S4 object from an
> Rcpp::RObject::SlotProxy.

This we can fix (I guess). I have not been using the "S4" class too much 
beyond hello world classes.

The code generation idea sounds great. I'm curious to see this going. 
Would you generate a c++ class skeleton by marshalling the S4 class 
definition, would this include methods ? There might be scope for having 
this R to C++ compiler in a separate package

> The way I have gotten around this is to use
>     Lambda(S4(SEXP(xp.slot("Lambda"))))
> etc.
>
> Is that a reasonable construction to use?

Yes, in that it gets the job done. but using the first construct should 
work.

-- 
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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