[Rcpp-devel] Error when handling default values

[Alessandro Mammana]

Hi all,
once again I have a bug and no idea what I am doing wrong.
I started to handle optional arguments directly at the C++ level, so
that I don't have to write a wrapper function in R. The pattern below
has always worked so far, but today it crashed. It was very hard to
reproduce, It took ages to nail it down to the minimal example below.


in a file:

// [[Rcpp::export]]
void reproduce_error(int n,
        SEXP ucs = R_NilValue,
        SEXP mConst = R_NilValue){

    if (Rf_isNull(ucs)){
        IntegerVector v1(n);
        IntegerVector v2(n);
        ucs = (SEXP)  List::create(Named("map")=v1, Named("values")=v2);
    }
    if (Rf_isNull(mConst)){
        NumericVector v1(n);
        mConst = (SEXP) v1;
    }

    List ucs_list(ucs);
    IntegerVector map = ucs_list["map"];
    Rcout << "before" << std::endl;
    IntegerVector uniqueCS = ucs_list["values"];
    Rcout << "after" << std::endl;
    NumericVector multinomConst(mConst);
}

from R's terminal:

>library(microbenchmark)
>sourceCpp("scratchpad.cpp")
>microbenchmark(reproduce_error(1000000), times=100)
before
after
before
after
before
after
before
after
before
after
before
after
before
after
before
after
before
after
before
after
before
Error: not compatible with INTSXP

What am I doing wrong?
Do you have other ideas about how I could handle NULL values?
Maybe I should avoid to rewrap the list in a SEXP, but this method
allows me to write very little code and no need for an "else" after
"if (Rf_isNull(ucs))"

I think the problem has to do with the way R manages its memory.
Correct me if I am wrong. If ucs was a normal stl container (with a
particular NULL state), then when I do:

ucs = (SEXP)  List::create(Named("map")=v1, Named("values")=v2);

ucs becomes the owner of the list, and the list is the owner of
vectors v1 and v2 (no idea if they are getting copied there by the
way..., with stl vectors I guess they would get copied), so when ucs
goes out of scope it tries to free the memory allocated for the list,
and the list tries to free the memory allocated for v1 and v2.

But because ucs is not a normal stl container, I guess I need some
other rules of thumb to understand how I should manage memory.

Thanks a lot in advance, and, don't get me wrong, I still love Rcpp :D


-- 
Alessandro Mammana, PhD Student
Max Planck Institute for Molecular Genetics
Ihnestraße 63-73
D-14195 Berlin, Germany