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

romain at r-enthusiasts.com romain at r-enthusiasts.com
Fri Nov 4 08:28:49 CET 2011

Not a bug, this is expected behaviour. 

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
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