[Rcpp-devel] Read-only property in Rcpp module, can it be a Vector& or must it be a Vector

[Douglas Bates]

As the Grateful Dead said, "What a long, strange trip it's been."  I
have spent over a month trying to debug some code in the lme4a package
that uses Rcpp modules.  Partly I was getting the wrong numerical
answer, which was due to a dumb cut-and-paste error on my part - now
fixed - but more importantly I was getting memory protection errors.

Luke Tierney has added new facilities to the memory allocation and
garbage collection in R-devel to make it easier to detect when
unprotected memory is being accessed.  You compile R-devel with
-DTESTING_WRITE_BARRIER to enable this checking.

I found that I was consistently getting an indication of unprotected
memory being accessed in code that looks like

  obj1$method1(obj2$method2)

but only after many evaluations of this sequence, even with
gctorture(1) on.  obj2$method2 returns a numeric vector, which is the
argument to obj1$method1.  The declaration within the module is as a
read-only property

    .property("cu",        &reModule::cu,
	      "intermediate solution for u")

and the accessor function is defined as returning a Rcpp::NumericVector &

	const Rcpp::NumericVector    &cu() const {return d_cu;}

This is starting to seem like the situation that Dominick mentioned
where a reference to an object goes out of scope in the C++ code once
the value is passed to R but R is retaining the SEXP somehow.  This is
still pretty murky to me because the underlying SEXP is part of the
Rcpp::NumericVector structure and is protected but somehow the
reference gets stale?

Anyway I am going to change the accessor method to

        const Rcpp::NumericVector          cu() const {return d_cu;}

or, perhaps

        Rcpp::NumericVector                  cu() const {return clone(d_cu);}

which might cause unnecessary copying but at least will guarantee that
there are no references involved.  The reason I would clone d_cu is
because I want to ensure that d_cu cannot be modified at the R level.

So my questions are: Should I avoid returning a reference and always
return the vector itself?  Do I need to clone the object can I use
const Rcpp::NumericVector cu() const {return d_cu;}?