[Rcpp-devel] shallow/deep copy

[Zhongyi Yuan]

Hello Darren,

Thank you for your answer. It becomes much clearer to me now.
In particular, I really appreciate your "P.S.". Quite useful for beginners
like me.

Best,
Zhongyi

On Wed, Aug 31, 2011 at 1:40 AM, Darren Cook <darren at dcook.org> wrote:

> > NumericVector x = as<NumericVector>(x_); ( Btw, this is exactly the same
> as
> > NumericVector x(x_), both resulting in a shallow copy, isn't it? )
> produces
> > a shallow copy, while
> > vector<double> x = as< vector<double> >(x_); produces a deep copy.
> >
> > Is it because for NumvericVector the as() function returns the address of
> > x_? But why does it work differently for vector<double>?
>
> Hello Zhongyi,
> I think if you put the namespaces on, explicitly, it becomes clearer:
>
> Rcpp::NumericVector x = as<Rcpp::NumericVector>(x_);
> std::vector<double> x = as< std::vector<double> >(x_);
>
> (I'm assuming x_ is the parameter being passed in from R.)
>
> x_ is effectively a pointer to a block of memory owned by R.
> Rcpp classes are just wrappers around that same pointer. Hence a shallow
> copy is possible.
>
> std::vector is a block of memory allocated and owned by your C++ code,
> so the bytes need to be physically copied over.
>
> Darren
>
> P.S. You'll see a lot of C++ code that keeps the std:: namespace
> definition in, rather than a using namespace std; at the top of the file.
> It adds 5 characters each time, but on the other hand it *only* adds 5
> characters each time.
>
> I don't know who invented this convention, or all the reasons it is
> good, but I personally do it that way, without regret.
>
>
>
> --
> 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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