[Rcpp-devel] Passing XPtr between functions

[Charles Determan]

The fastLm examples are nice but they don't contain anything relating to
passing an XPtr between functions.  Regarding your comment about making an
XPtr around arma::mat 'innards' I'm not sure I fully understand.  I tried
creating an XPtr around a arma::mat but no success on passing the object
between functions.  It still returns that ~zeros.

// [[Rcpp::export]]
SEXP testXptr(SEXP A)
{
    arma::Mat<double> armaMat = as<arma::mat>(A);
    XPtr<arma::mat> pMat(&armaMat);
    return(pMat);
}

// [[Rcpp::export]]
void testReturn(SEXP ptrA, int nr, int nc)
{
    XPtr<arma::mat> ptrB(ptrA);
    arma::mat B = arma::mat( (double *) ptrB->memptr(),
          nr,
          nc,
          false);
    B.print("copied matrix");
}

Charles

On Wed, Jun 24, 2015 at 8:32 AM, Dirk Eddelbuettel <edd at debian.org> wrote:

>
> On 24 June 2015 at 08:22, Charles Determan wrote:
> | Thank you John,
> |
> | I am familiar with bigmemory (I am one of the current developers
> actually).
> | The project I am working on doesn't need the shared memory aspect so was
> | intending to avoid the dependency and just leverage the more familiar and
> | developed Armadillo library.  However your response informs me that I
> did not
> | fully understand how armadillo objects are handled.  I have some other
> ideas
> | with how I can address my problem but this was something I was hoping to
> apply
> | both for this project and for the sake of learning.
>
> Look at more current RcppArmadillo examples, and in particular _all_ the
> variants of the fastLm benchmark in examples.  By doing the explicit cast,
> you _forced_ a copy which may then have destroyed the Xptr link.  I *think*
> you could just do XPtr around arma::mat innards.  I would actually be
> interested in having something somewhere between bigmemory (where I was one
> of the first external users) and what we have in RcppArmadillo.
>
> Dirk, during a break of Rcpp class in Zuerich
>
> | Regards,
> | Charles
> |
> | On Tue, Jun 23, 2015 at 9:29 PM, John Buonagurio <
> jbuonagurio at exponent.com>
> | wrote:
> |
> |     Hi Charles,
> |
> |     > SEXP testXptr(SEXP A)
> |     > {
> |     >     arma::Mat<double> armaMat = Rcpp::as<arma::Mat<double> >(A);
> |     >     Rcpp::XPtr<double> pMat(armaMat.memptr());
> |     >     return(pMat);
> |     > }
> |
> |     armaMat is on the stack, so the Armadillo memptr is no longer valid
> when
> |     you return from the testXptr function.
> |
> |     One simple solution in your case would be to dynamically allocate
> with
> |     "new" [e.g. arma::mat *A = new arma::mat(...);], though I can't tell
> you
> |     off hand how object lifetime is managed with Armadillo objects.
> |
> |     If you're trying to preserve a matrix across function calls in R,
> have you
> |     looked into bigmemory? http://gallery.rcpp.org/articles/
> |     using-bigmemory-with-rcpp/
> |
> |     John
> |
> |
> |
> | _______________________________________________
> | Rcpp-devel mailing list
> | Rcpp-devel at lists.r-forge.r-project.org
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>
> --
> http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
>
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