[Rcpp-devel] inplace modification more affect other varibles

[Dirk Eddelbuettel]

On 21 October 2014 at 20:22, Chenliang Xu wrote:
| Hello,
| 
| With the following inplace sorting example, I understand the value of `a` is
| sorted inplace, but it's strange to see the value of `b` is also modified. This
| can cause some hard to detect bug, since the cpp function may modify a variable
| defined in other scope.

Very well known issue -- maybe do a search for 'Rcpp::clone' ...

In a nutshell, SEXP objects are passed by a __pointer__ and changes do
therefore persist.  If you want distinct copies, use Rcpp::clone().
 
Dirk

| It seems that rcpp doesn't respect the named field, which is adopted by R to
| implement copy-on-modify. I don's see an easy fix on C++ side, since the called
| cpp function has no information about variable binding in R. A possible fix is
| adding a function `inplace` to R, which ensure the returned variable has named
| filed = 0 so is safe to modify inplace. Then, we have to call the function as
| `stl_sort_inplace(inplace(a))`, which seems odd but is also informative. It
| shows clearly that we are breaking the pass-by-value rule in R.
| 
| ```cpp
| #include <Rcpp.h>
| using namespace Rcpp;
| 
| // [[Rcpp::export]]
| void stl_sort_inplace(NumericVector x) {
|     std::sort(x.begin(), x.end());
| }
| 
| ```
| 
| ```r
| a <- seq(1, 0.1, -0.1)
| b <- a
| #  [1] 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
| 
| stl_sort_inplace(a)
| 
| a
| #  [1] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
| 
| b
| #  [1] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
| 
| a <- seq(1, 0.1, -0.1)
| pure_function <- function (x) {
|   y <- x
|   stl_sort_inplace(y)
|   print(y)
| }
| pure_function(a)
| a
| #  [1] 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
| 
| ```
| 
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-- 
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org