[Rcpp-devel] Forcing a shallow versus deep copy

[romain at r-enthusiasts.com]

Hello,

This comes up every now and then, I think we can find a syntax to 
initiate an arma::mat that would allow what you want.

It is not likely it will come via attributes. The idea is to keep them 
simple. The solutions I see below would eventually lead to clutter, and 
we are heading in the less clutter direction.

I'll think about it and propose something.

Romain

Le 2013-07-11 14:32, Changi Han a écrit :
> Hello,
>
> I think I (superficially) understand the difference between:
>
> // [[Rcpp::export]]
> double sum1(Rcpp::NumericMatrix M) {
>     arma::mat A(M.begin(), M.rows(), M.cols(), false);
>      return sum(sum(A));
> }
>
> // [[Rcpp::export]]
> double sum2(arma::mat A) {
>     return sum(sum(A));
> }
>
> Partly out of laziness, partly because sum2 is more elegant, and
> partly to avoid namespace pollution, I was wondering if there is a 
> way
> to "force" a "shallow" copy in sum2.
>
> If not, then may I submit a low priority feature request. An
> attribute? Some thing like:
>
> // [[Rcpp::export]]
> double sum2(arma::mat A) {
>     // [[ Rcpp::shallow ( A ) ]]
>      return sum(sum(A));
> }
>
> Or (akin to C++11 generalized attributes)
>
> // [[Rcpp::export]] { [[ Rcpp::shallow ( A ) ]] }
> double sum2(arma::mat A) {
>     return sum(sum(A));
>  }
>
> An alternative is to have an argument in sourceCpp that takes a
> list/vector of objects that are to be shallow or deep copied.
>
> For example in sum1, if M is changed within the function before
> casting to the arma::mat, then might be cleaner to add M to a
> list/vector of objects to be deep copied rather than cloning M within
> sum1: leads to one fewer variable name.
>
> Just a thought. I can certainly live with the additional step. As
> always, thanks for all the Rcpp goodness.
>
> Cheers,
> Changi Han