[Rcpp-devel] Data Persistence when using inline

[Noah Silverman]

Darren,

That's an awesome answer! 

Thank you very much!!!!

--
Noah Silverman
UCLA Department of Statistics
8117 Math Sciences Building
Los Angeles, CA 90095

On Sep 7, 2011, at 6:04 PM, Darren Cook wrote:

>> If the code was in pure C++, it would be trivial to have an object
>> and then just pass a pointer to that object.  I can't figure out how
>> to do the same thing with R+Rcpp+inline.
> 
> I asked myself a similar question a few weeks back (actually it came
> from one of the questions at the end of the google talk [1] and, while
> both Dirk and Romain confidently said it was possible, at the time I
> couldn't see how to actually do it).
> 
> I think what you're looking for is Rcpp::XPtr. I.e. you have your C++
> code allocate and *own* the memory, and then it passes it back to R by
> wrapping it in XPtr. XPtr is a curious template as it does two distinct
> tasks: it returns a C++ pointer to R, and it also turns an R object back
> into a C++ pointer.
> 
> Below is the proof-of-concept example I came up with. I use a custom
> class (the real point of this example), but it could just as well be
> std::vector. This example also shows a couple of other things:
> 
>  1. Keeping the C++ class code in its own files.
> 
>  2. Compiling two functions at once; this isn't necessary for this
> example, it is just quicker. (test5b.R is also included below, and shows
> how to do this with two separate compiler invocations.)
> 
> (BTW, this example leaks memory. For long-running production code don't
> forget that your C++ code owns the memory and is responsible for
> deleting it.)
> 
> Darren
> 
> [1]: http://www.youtube.com/watch?v=UZkaZhsOfT4
> 
> 
> --------- test5.R ---------------
> 
> library('inline')
> 
> header=paste(readLines("test5.h"),collapse="\n")
> src=paste(readLines("test5.cpp"),collapse="\n")
> 
> use_src='
> Rcpp::XPtr< Example > xp(xp_); //I.e. Example xp=xp_;
> return wrap(xp->a + xp->b);
> '
> 
> fun=cxxfunction(
>    list(create=signature(),use=signature(xp_="externalptr")),
>    list(create=src,use=use_src),
>    includes=header,
>    plugin="Rcpp"
>    );
> 
> xp=fun$create()    #xp is an Example object
> print(xp)
> print(fun$use(xp))
> 
> 
> -------- test5.h ------------------------
> 
> class Example{
> public:
> int a,b;
> 
> public:
> Example(int a_,int b_):a(a_),b(b_){}
> 
> };
> 
> using namespace Rcpp;
> 
> 
> -------- test5.cpp ------------------------
> 
> Example *xp=new Example(10,20);
> return XPtr<Example>(xp,true);
> 
> 
> -------- test5b.R --------------------------
> 
> library('inline')
> 
> header=paste(readLines("test5.h"),collapse="\n")
> src=paste(readLines("test5.cpp"),collapse="\n")
> 
> use_src='
> Rcpp::XPtr< Example > xp(xp_); //I.e. Example xp=xp_;
> return wrap(xp->a + xp->b);
> '
> 
> fun_create=cxxfunction(
>    signature(),
>    src,
>    includes=header,
>    plugin="Rcpp"
>    );
> 
> fun_use=cxxfunction(
>    signature(xp_="externalptr"),
>    use_src,
>    includes=header,
>    plugin="Rcpp"
>    );
> 
> xp=fun_create()    #xp is an Example object
> print(xp)
> print(fun_use(xp))
> 
> 
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