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 <dar...@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) > _______________________________________________ > Rcpp-devel mailing list > Rcpp-devel@lists.r-forge.r-project.org > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel >
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