[Rcpp-devel] Garbage collection PreserveObject.

[Dirk Eddelbuettel]

Houston, we have lift-off.  We have come full circle. Our use of preserve /
release followed Simon's answers to your questions on r-devel.

On 24 January 2010 at 00:44, Guillaume Yziquel wrote:
| I recently noticed that the garbage-collection interfacing toping has 
| been quite active at the beginning of the month on this list, so I'm 
| asking here. My question concerns essentially my OCaml binding, but I'll 
| try to make it as language agnostic as possible to somehow stay within 
| the scope of this list.

You are a nautical mile and a half off-topic for 'rcpp-devel', but we don't
mind.  We only charge after the third question.  Just kidding.
 
| Two questions:
| 
| -1- From what I gathered, it is fine to issue two PreserveObject 
| statements on the same SEXP, and then two ReleaseObject afterwards. It 
| seemed to me that reference counting was not needed, from my 
| understanding of the past discussion. Am I wrong?

I think you are right. We simply the object along, but AFAIK R just flips a
bit:

    void preserve(){ if( m_sexp != R_NilValue ) R_PreserveObject(m_sexp) ; } 
    void release() { if( m_sexp != R_NilValue ) R_ReleaseObject(m_sexp) ; } 

| -2- Once you have, in your C/C++ code, obtained a pointer to a SEXP from 
| R, do you need to PROTECT it, then do a PreserveObject, then UNPROTECT 
| it? Or is PreserveObject simply fine? (I guess so, but I'm damn unsure).

More the latter, but it is complicated. We had replaced all PROTECT /
UNPROTECT calls with preserve / release but we needed to go back for some
reason.  Romain has the gory details.

Dirk

-- 
Three out of two people have difficulties with fractions.