Hi Kevin, hi Dirk, K> All R data internally is a SEXPREC, ie, a C struct full of different K> bits of information. The data part of a SEXPREC is forced to a K> specific offset from the C structure, e.g. from Rinternals.h: K> K> /* Under the generational allocator the data for vector nodes comes K> immediately after the node structure, so the data address is a K> known offset from the node SEXP. */ K> #define DATAPTR(x) (((SEXPREC_ALIGN *) (x)) + 1) K> K> Since Rcpp objects are just proxy wrappers over R objects, for Rcpp to K> be able to reuse memory, we would have to construct the SEXPREC in the K> correct memory location so that the resulting DATAPTR call happens to K> fall where the data is. However, I'm not sure how we could reserve K> that space for the SEXPREC. K> K> Other data structures that just contain a pointer to their data, K> rather than requiring that they lie a specified offset away, don't K> have this problem (I imagine this is why e.g. Armadillo can reuse K> memory)
All right, that helps me a lot understanding why every conversion from an Armadillo object to an Rcpp object always seems to allocate fresh memory. If I understand correctly, that also implies that returning (potentially very large) RcppArmadillo objects to R requires a full copy of the data, while this is not the case for pure Rcpp objects. (And then I guess that's also the case in RcppEigen, so running there is not gonna save me... :)) D> To use an advanced constructor for Armadillo object requires Armadillo to D> provide such a constructor in the first place. See D> D> http://arma.sourceforge.net/docs.html D> D> and I can only spot them for outright matrix, rowvec, colvec objects, but D> not for ranges. I guess all I was trying to understand is that it's possible to create e.g. arma::mat objects with the range constructor while reusing memory. Because of the reasons Kevin pointed out, this is difficult/impossible for e.g. Rcpp::NumericMatrix objects. D> For Rcpp constructor, things are different too as Kevin kindly pointed out D> already. To sum up: R ----(reuse)----> Rcpp ----(reuse)----> RcppArmadillo but RcppArmadillo ----(fresh)----> Rcpp ----(reuse)----> R Correct? Cheers, Gregor _______________________________________________ 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
