[Rcpp-devel] Passing XPtr of Eigen Map object between functions

[Charles Determan]

I have a question similar to a previous question I asked here regarding the
passing of XPtr objects between functions (
https://www.mail-archive.com/rcpp-devel@lists.r-forge.r-project.org/msg08389.html).
Exploring the same topic with the Eigen library I have run in to another
curious situation.  As per the previous suggestion I was under the
impression that I need to create a `new` object which I then wrap in an
XPtr to be passed between functions.  However, this becomes a problem when
trying to work with RcppEigen.  I try to pass in a matrix as MatrixXf to be
mapped but the resulting output shows some numbers matching and others
completely wrong.  This is likely me not understanding how Eigen objects
are handled in memory but I am confused in that if I pass the matrix in as
an arma::fmat object which I then Map to an Eigen MatrixXf it works without
issue.

Thoughts???


R code

library(Rcpp)
sourceCpp("test.cpp")

A <- matrix(rnorm(10), 5)

# This works
testFloatReturn(testArmaFloatXptr(A), 5, 2)

# This does not
testFloatReturn(testEigenFloatXptr(A), 5, 2)

C++ Code

#include <RcppArmadillo.h>
#include <RcppEigen.h>

using Eigen::Matrix;
using Eigen::Dynamic;
using Eigen::Map;
using Eigen::MatrixXf;

template<class T>
using MapMat = Eigen::Map<Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic>
>;

using namespace Rcpp;


// [[Rcpp::export]]
SEXP testArmaFloatXptr(arma::fmat A)
{
    MapMat<float> *B = new MapMat<float>(A.memptr(), (int) A.n_rows, (int)
A.n_cols);
    XPtr<MapMat<float> > pMat(B);
    return(pMat);
}

// [[Rcpp::export]]
SEXP testEigenFloatXptr(Eigen::MatrixXf A)
{
    MapMat<float> *B = new MapMat<float>(&A(0), A.rows(), A.cols());
    XPtr<MapMat<float> > pMat(B);
    return(pMat);
}

// [[Rcpp::export]]
void testFloatReturn(XPtr<MapMat<float> > ptrA, int nr, int nc)
{
    MapMat<float> A = *ptrA;
    cout << A << std::endl;
}


Regards,
Charles
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