[Rcpp-devel] can one modify array in R memory from C++ without copying it?

[Dirk Eddelbuettel]

On 4 November 2011 at 16:56, Darren Cook wrote:
| > Not a bug, this is expected behaviour.
| 
| Hello Romain,
| I did not mean the behaviour is a bug; I mean it is going to cause bugs.
| I was wondering if there is a way to stop the implicit data coercion,
| forcing the programmer to request it explicitly.

R does exactly that too

R> a <- 1
R> typeof(a)
[1] "double"
R> a <- 1L
R> typeof(a)
[1] "integer"
R> 
 
and Rcpp uses Proxy Classes so this is the Right Thing (TM) to do.  We think,
at least.

| E.g. Are the implicit type conversions happening with some extra copy
| constructors? If so, we could have conditional compilation to exclude
| those. Something like:
| 
|   #ifndef FORCE_EXPLICIT
|   IntegerMatrix(double*){...}
|   #endif
| 
| Then programmers who don't like surprises would define FORCE_EXPLICIT,
| and then their code would sometimes not compile and they would have to
| write an explicit conversion.
| 
| (I've not thought that through, I just wanted to demonstrate what I had
| in mind.)

If you feel really strongly about you could consider a patch that makes this
non-R behaviour you suggest an option.  To most of us who use Rcpp between R
and C++ it really is a feature.

Don't get me wrong though: I like your input here and maybe the implicit
nature of things needs to be stressed even more.

Dirk

 
| Darren
| 
| > 
| > If you pass a matrix of ints to the NumericVector ctor, Rcpp has no
| > choice but to coerce the data to a matrix of double, which means new
| > data, hence the original data does not get modified.
| > 
| > If you pass a matrix of double, no copy is required, therefore Rcpp
| > operates directly on the data.
| > 
| > Those are features.
| > 
| > 
| > 
| > Le 4 nov. 2011 à 08:01, Darren Cook <darren at dcook.org> a écrit :
| > 
| >>> I.e. the point of his code was to show that a matrix of doubles
| >>> gets modified, a matrix of ints does not.
| >> 
| >> Or change the first line from NumericMatrix to: Rcpp::IntegerMatrix
| >> r_m(mem);
| >> 
| >> Then the behaviour is reversed. The matrix of doubles does not get 
| >> modified, but the matrix of ints does!
| >> 
| >> Dirk, Romain, this is a bug-in-waiting. Is there any way to
| >> generate a warning when the implicit deep copy happens? Or
| >> alternatively when the pointer is being used implicitly... but my
| >> hunch is that I want to know when there is any implicit conversion
| >> between int and double: modern C++ style is to explicitly declare
| >> all type conversions with static_cast<> and friends.
| 
| 
| -- 
| Darren Cook, Software Researcher/Developer
| 
| http://dcook.org/work/ (About me and my work)
| http://dcook.org/blogs.html (My blogs and articles)
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