On 21 December 2011 at 16:29, Douglas Bates wrote: | On Wed, Dec 21, 2011 at 3:55 PM, Yasir Suhail <yusuh...@gmail.com> wrote: | > Hi | > | > I looked up the previous thread "Creating pointers to objects and wrapping | > them" | > from http://lists.r-forge.r-project.org/pipermail/rcpp-devel/2011-April/002170.html, | > and it seems that providing a specialization of the wrap<>() function can | > help us return classes. Perhaps similarly, one could provide a | > specialization of the as<>() function for arguments? While wrap is | > implemented with the Language("new" ... function, is there any example of an | > as<>() specialization for custom classes? The vignette | > (http://dirk.eddelbuettel.com/code/rcpp/Rcpp-extending.pdf) shows a | > declaration for as, but no definitions. | | It is not clear if you want to wrap the pointer to the object as an | "external pointer" in R or if you want to create an R object from your | C++ object. Wrapping an external pointer is straightforward. This | example from http://r-forge.r-project.org/scm/viewvc.php/pkg/lme4Eigen/src/external.cpp?view=markup&root=lme4 | | // generalized linear model (and generalized linear mixed model) response | | SEXP glm_Create(SEXP fam, SEXP y, SEXP weights, SEXP offset, SEXP mu, | SEXP sqrtXwt, SEXP sqrtrwt, SEXP wtres, SEXP eta, SEXP n) { | BEGIN_RCPP; | glmResp *ans = new glmResp(List(fam), y, weights, offset, mu, | sqrtXwt, sqrtrwt, wtres, eta, n); | return wrap(XPtr<glmResp>(ans, true)); | END_RCPP; | } | | SEXP glm_setN(SEXP ptr_, SEXP n) { | BEGIN_RCPP; | XPtr<glmResp>(ptr_)->setN(as<MVec>(n)); | END_RCPP; | } | | shows wrapping an external pointer to return it and then passing the | external pointer to be modify the C++ object. You should be aware, | however, that an external pointer cannot be save'd then loaded into | another R session (which makes sense if you think about it). You need | to do a bit of dancing if you want to do that. Essentially you must | save as R objects enough information to regenerate the C++ object then | detect if the external pointer points to NULL and regenerate the | object if it does.
Very nice. In the same spirit, the email by Richard Downe was also good, but maybe a little misleading (or maybe I am just tired and misread...). In essence it showed an "object factory" which given a string returned a new object. You can combine this with external pointers which makes it transparent in our context as external pointers can seamlessly travel back and forth between R and C++ as we have Rcpp::XPtr wrapping the corresponding SEXP. Dirk -- "Outside of a dog, a book is a man's best friend. Inside of a dog, it is too dark to read." -- Groucho Marx _______________________________________________ Rcpp-devel mailing list Rcpp-devel@lists.r-forge.r-project.org https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
