a and b are the same object:
a <- seq(1, 0.1, -0.1)
b <- a
pryr::address( a )
[1] "0x7f9504534948"
So clone is what you need here.
Implementing copy on write for that kind of example is possible, but would
require a lot of additional code, i.e. the iterator would need to handle
the write operation.
An undesirable side effect of this is that such iterators would be quite
less performant, right now Rcpp is close to the metal and uses direct
pointers as iterators when it makes sense. A price that everyone would have
to pay. no go.
Instead, the responsibility is given to the user to clone explicitly when
changes will be made to the underlying object.
Romain
Le 22 oct. 2014 ? 04:13, Chenliang Xu <luckyrand at gmail.com> a ?crit :
Hi Dirk,
Thanks for your quick answer. I don't think Rcpp::clone is what I was
looking for. I know `stl_sort_inplace(a)` modify the value of `a`, but it
surprise me to see it modify `b`. And it may modify some other variables c,
d, e, f..., and it's hard to know which variables point to the same place.
On Tue, Oct 21, 2014 at 8:31 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
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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