[Rcpp-devel] R_PreserveObject and R_ReleaseObject

[Dirk Eddelbuettel]

On 16 August 2018 at 15:20, Simon Dirmeier wrote:
| Thanks, Dirk. I am just trying to understand how R works under the 
| surface :/.
| Correct me where I am wrong please: if I have another method called 
| /get/, then I would need to preserve the objects like I did until it is 
| removed from the map?
| 
| template<typename T>
| class map
| {
| public:
| 
|     // as before
| 
|     SEXP get(T& t)
|      {
|          return Rcpp::wrap(map_[t]);
|      }
| 
| private:
|      std::unordered_map<T, SEXP> map_;
| };

You don't need to do anything else because of Rcpp. Using `wrap()` to convert
"whatever" (as long it is supported / has a converter) is the way to go, as
our documentation states in several places and as many other packages do.

Dirk
 
| Best,
| Simon
| 
| Am 16.08.18 um 14:00 schrieb Dirk Eddelbuettel:
| > On 16 August 2018 at 10:10, Simon Dirmeier wrote:
| > | Dear all,
| > |
| > | I have a question about /R_PreserveObject /and/R_ReleaseObject
| > | /and//thought this would be good place to post it.
| > | I apologize if the question is not suitable or naive.
| >
| > This is the place.
| >   
| > | Suppose I have something like the class below for which I use
| > | Rcpp-modules to extend it to R (code not shown).
| > | I use "insert" to add key-value pair to a map and "remove" to remove an
| > | entry.
| > |
| > | I am not having memory issues or so, but the solution seems wrong to me
| >
| > I would agree. You are using a Standard Template Type; these manage their memory.
| >
| > The two functions you found are R internals for setting reference counters on
| > SEXP objects. Think of them as 'free floating' objects -- the ones we receive
| > from R or return to R. Yours is not one of those, and that it may contain
| > SEXP does not matter as you do not seem to return these SEXPs to R.  If you
| > did then you would have do that.
| >
| > And even then you should not need to as Rcpp maps every R with one of our
| > classes.  These have now about 10 years of testing behind and _work_.  So why
| > not take advantage of that?
| >
| > Of course, this is C so if you know what you are doing, you can whatever you
| > want.  But then you would not need to ask this question :)
| >
| > Dirk
| >
| > | and I am concerned that it causes problems.
| > | Could you please give me some advice if this implementation is wrong or
| > | unefficient?
| > |
| > | Thank you in advance.
| > | Best,
| > | Simon
| > |
| > | template<typename T>
| > | class map
| > | {
| > | public:
| > |      map() = default;
| > |
| > |      size_t size()
| > |      {
| > |          return map_.size();
| > |      }
| > |
| > |      void insert(T& t, SEXP u)
| > |      {
| > |          SEXP s = Rf_duplicate(VECTOR_ELT(u, i));
| > |          R_PreserveObject(s);
| > |
| > |          map_.insert(std::pair<T, SEXP>(t[i], s));
| > |      }
| > |
| > |      void remove(T& t)
| > |      {
| > |              auto iter = map_.equal_range(t);
| > |              for (auto it = iter.first; it != iter.second; ++it)
| > |              {
| > |                  R_ReleaseObject(it->second);
| > |              }
| > |              map_.erase(t);
| > |      }
| > |
| > | private:
| > |      std::unordered_map<T, SEXP> map_;
| > | };
| > |
| > |
| > |
| > | _______________________________________________
| > | Rcpp-devel mailing list
| > | Rcpp-devel at lists.r-forge.r-project.org
| > | https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
| >
| 
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