[Rcpp-devel] Some questions regarding internals of Rcpp objects

Hi Ulrich,

These are really several questions.  I'll try to address them all, with some
luck maybe Romain, Doug, Christian, ... will correct or complement as needed.

| Dear list members,
| 
| I have started developing a package using the latest version of Rcpp just
| recently, so I am more or less an Rcpp newbie. Since I am not sufficiently
| proficient in advanced C++ (and admittedly too lazy too) to analyze the Rcpp
| source code, I have some questions regarding what actually happens inside Rcpp.
| Let me bother you with an example:
| 
| RcppExport SEXP test(SEXP mat)
| {
| BEGIN_RCPP
|     Rcpp::NumericMatrix matC(mat);
|     Rcpp::NumericMatrix matD(mat);
| 
|     matC(1, 1) = 1;
|     matD(1, 1) = -matD(1, 1);
| 
|     return(matC);
| END_RCPP
| }
| 
| If I call this function on some matrix, then the function returns the input
| matrix with its element [2, 2] set to 1, but that's all. The change in matD
| only seems to have an effect on the local copy matD. If I replace "return(matC)
| " by "return(matD)", then the function returns the input matrix with the sign
| of its element [2, 2] inverted, so the change in matC has no effect. That Rcpp
| creates local copies, i.e. that matC and matD are independent objects each
| holding local copies of the argument mat, would perfectly explain these

This has been discussed here before.  The matrices both receive a _pointer_
(as a SEXP is a pointer) to the original memory of the R-level matrix you
pass in.  

You need to explicitly request deep copies via clone() if you want that.  See
this variant:

R> fun <- cxxfunction(signature(mat="numeric"), plugin="Rcpp", body='
+      Rcpp::NumericMatrix matC(mat);
+      Rcpp::NumericMatrix matD(mat);
+ 
+      matC(1, 1) = 1;
+      matD(1, 1) = -matD(1, 1);
+ 
+      return(Rcpp::List::create(Rcpp::Named("C")=matC,
+                                Rcpp::Named("D")=matD));
+ ');
R> fun(matrix(42,2,2))
$C
     [,1] [,2]
[1,]   42   42
[2,]   42   -1

$D
     [,1] [,2]
[1,]   42   42
[2,]   42   -1

R> 

So C and D are really the same thing.


| results. Previously, however, I thought Rcpp would only wrap an R/SEXP object
| into an Rcpp object without making a copy. If it was so, no matter whether the
| function returns matC or matD, the function would return the input matrix with
| -1 on position [2, 2]. Did I get this point wrong? I suppose it would save time
| and memory not to create a copy, in particular, since arguments to functions
| are local copies already. Is there a way to avoid copying?
| 
| A related question is what the "=" operator does. Does
| 
|     Rcpp::NumericVector x = ...;
| 
| create a copy or is it only a pointer that is assigned?

IIRC the pointer is copied, yet still points to same memory address.
 
| My final question concerns the "longevity" of Rcpp objects. Suppose I do the
| following:
| 
|     {
|         Rcpp::NumericVector vec(10);
|         double *p = vec.begin();
|     }
| 
| Can I use *p outside this block or is the pointer killed along with the
| variable vec after the block is closed? According to my limited knowledge, I

p as a variable is gone outside of this { } scope. So is vec but as you say ...

| would assume that it is only the reference vec that is not available outside
| the block, but that the corresponding R object still exists and will be deleted
| by the R garbage collector only after the whole function has terminated.

... we do not explicitly destroy in the destructor but let R take care of
it. So it will live "some random amount" of time you have no control over. Eeek.

In a nutshell, if you want longer-living objects, assign in R before you call
a function -- you are pretty much guaranteed it will be there.

If you need persistence between different calls from R you need an object
that persists, and there are many possible C++ tricks to do that.
 
| Any help is gratefully appreciated! I am sorry if these are stupid questions or
| if any of them have been discussed previously.

Great questions, but some of the material has been discussed before.  We
probably need a better FAQ or better search engine...

Cheers, Dirk
| 
| Thanks and best regards,
| Ulrich
| 
| 
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