Re: [Rcpp-devel] Forcing a shallow versus deep copy

[romain]

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

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