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

Dear Dirk,

Thanks for your detailed comments!

Regarding the first question: as I wrote, I was actually assuming that
Rcpp would not make deep copies.

[...]
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.

Fine. If so, I actually wonder what's wrong with my function

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
}

I tried it on matrix(42, 2, 2) too and, surprise, it returns -1 as
element [2, 2] - just as in your example. So far so good. However, if I
apply it to matrix(1:16, 4, 4) (and this is an example similar to the
one I tried before, which actually made me write this question to the
list), I get the following (note the 1 at [2, 2]):

test(matrix(1:16, 4, 4))

[,1] [,2] [,3] [,4]
[1,] 1 5 9 13
[2,] 2 1 10 14
[3,] 3 7 11 15
[4,] 4 8 12 16

If it's not the copying, what else is wrong here? I actually don't get
it. Very mysterious!

Regarding my third question, I have to admit that my example was flawed.
Stupid me! I wanted to write something like the following:

double *p;

{
Rcpp::NumericVector vec(10);
p = vec.begin();
}

As I understand your reply, I could use p outside the block, as it still
points to the data inside the R object created inside the block, right?

Thanks again for your very helpful answers!

Best regards,
Ulrich