[Rcpp-devel] Forcing a shallow versus deep copy

wrote:

On Thu, Jul 11, 2013 at 8:32 AM, Changi Han <changi.han.wen at gmail.com>
wrote:

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.

I had just made a similar suggestion in my post about half an hour ago.