[Rcpp-devel] Best way to return raw array

Dirk Eddelbuettel edd at debian.org
Fri Sep 2 14:19:15 CEST 2011

On 2 September 2011 at 11:10, Darren Cook wrote:
| > | I've extended Christian Gunning's speed test with an STL and C/C++ 
| > | version; I was about to post but then I got a bit stuck with using 
| > | Rcpp::wrap() for a raw block of memory. I'm using this: | |
| > src1cpp<-' | int nn=as<int>(n); | double *p=new double[nn]; | ... |
| > NumericVector ret(p,p+nn); | delete p; | return ret; | '
| > 
| > That strikes me as plain wrong code.
| Hello Dirk,
| Perhaps I can squeeze an answer out of you by changing it to this:
| double *p=third_party_function(nn);
| NumericVector ret(p,p+nn);
| delete p;
| return ret;

Still strikes me as wrong; look eg RcppGSL to see how we deal with a C API.

Maybe this would do

  double *p=third_party_function(nn);
  NumericVector ret(nn);           // new memory
  copy(ret.begin(), ret.end(), p); // untested
  delete p;
  return ret;

| where third_party_function() is C legacy code that is documented as
| returning a block of memory of size nn that the client should take
| ownership of.

Yes -- "client should take ownership of" is paramount, and for that we need
memory managed by R.  Rcpp data structures do that, just doing a random C
level allocation does not.
| How do I return it?
| (I took a look at the convolve examples but they all build up the result
| in a Rcpp object. I cannot see an example where you have the result
| ready-made in a block of memory and just need to return it.)

Maybe there is reason for that? Consider my last email... ;-)
| > c) The whole point of what we do with Rcpp is to NOT have to deal
| > with new / delete and or malloc / free.  Even if you think it's cool
| > and know how to it in plain, it is simply against the whole spirit
| > ...STL idioms are really much better.
| You'll enjoy my timing post then (as the STL does not just equal the raw
| array version, it beats it).
| But I think we see the raison d'etre of Rcpp differently; for me it is:
|   * Optimizing key R code;
|   * Interfacing with 3rd party C/C++ libraries;
|   * Doing the above two while bypassing the ugly verbose code of the
| usual way to write R extensions.
| Or, in a sound bite: "Rcpp is not just for C++ newbies" ;-)

Obviously agreed on all point, but there is no need to mislead the newbies,
and to poison them with bad C habits just because that's what may have
happened in your and my rough youth.


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