[Rcpp-devel] "Error during wrapup"

Romain Francois romain at r-enthusiasts.com
Wed May 28 21:44:28 CEST 2014

(now with some links):

Le 28 mai 2014 à 16:31, John Mous <john.mous0 at gmail.com> a écrit :

> The object really is just built as part of the return statement. i.e. the lines from my prior e-mail exist as-is in the full code,

Sure. What happens is that Rcpp::export generates something that calls wrap( std::map<std::string,int> ). 

> there's just more that actually builds the variables X1-X4 beforehand.

It should not be relevant. 

> So I'm not sure where to debug from the client side. I'm a C/C++ developer, but have no experience with Rcpp internals or the general interface between R and C. I can insert some debug statements on the Rcpp side if you can guide me to where wrap is defined. 

well wrap is defined here: 

It then performs a series of dispatch, that eventually lead to something that handles std::map<std::string,int>: 

template <typename InputIterator, typename T>
inline SEXP range_wrap_dispatch___impl__cast( InputIterator first, InputIterator last, ::Rcpp::traits::false_type ){
	size_t size = std::distance( first, last ) ;
	const int RTYPE = ::Rcpp::traits::r_sexptype_traits<typename T::second_type>::rtype ;
	Shield<SEXP> x( Rf_allocVector( RTYPE, size ) );
	Shield<SEXP> names( Rf_allocVector( STRSXP, size ) ) ;
	typedef typename ::Rcpp::traits::storage_type<RTYPE>::type CTYPE ;
	CTYPE* start = r_vector_start<RTYPE>(x) ;
	size_t i =0;
	std::string buf ;
	for( ; i<size; i++, ++first){
		start[i] = (*first).second ;
		buf = (*first).first ;
		SET_STRING_ELT( names, i, Rf_mkChar(buf.c_str()) ) ;
	::Rf_setAttrib( x, R_NamesSymbol, names ) ;
	return wrap_extra_steps<T>( x ) ;

what I suspect to be the problem is this line:

 ::Rf_setAttrib( x, R_NamesSymbol, names ) ;

> What Romain says makes sense to me, despite my lack of expertise in this area.. the really intermittent nature of the problem and the fact that I can't recreate it in a small / fast running example suggests that perhaps this manifests when R happens to garbage collect at an unfortunate time (if I understood correctly). Thanks again.

Something like that. as hadley hinted, please try this under gc torture, or preferably through a debugger. 

> On Wed, May 28, 2014 at 10:12 AM, Dirk Eddelbuettel <edd at debian.org> wrote:
> On 28 May 2014 at 10:02, John Mous wrote:
> | Hmm, unfortunately the GitHub version failed also.
> Darn.
> | The attributes on the failed
> | object are a little different though, here's what they look like:
> |
> | Browse[1]> str(results)
> |  atomic [1:4] 1 1 2270 0
> |  - attr(*, "")= symbol sim
> |  - attr(*, "value")= promise to  NULL
> I am not sure what we can do without a reproducible example. :-/
> The code just got a review / refreshment over the last few months.
> You best bet may the slow and tedious insertion of debug statements to see
> when / if the object changes.
> Dirk
> --
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
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