Andre,
For Rcpp, you can use the XPtr class template. Say you want to deal
with pointers to Foo. You can create an R external pointer like this:
SEXP create_xp_foo(){
Foo* foo = new Foo ;
XPtr<Foo> xp( foo ) ;
return xp ;
}
Now, "xp" is an R external pointer, i.e. a SEXP of type EXTPTRSXP, that
you can send back to the R side. This constructor assumes ownership of
the pointer, so when the SEXP is no longer protected, it becomes
candidate for garbage collection, and then the finalizer deletes the
object using C++ delete operator. The XPtr template lets you control
these finalizers. We can discuss further if you want to know more.
If you then send it back to the C++ side, you can get back the Foo
pointer like this:
SEXP use_foo( SEXP xp ){
XPtr<Foo> foo(xp);
// conversion operator
Foo* p_foo = foo ;
do_whatever( p_foo ) ;
return R_NilValue ;
}
The XPtr template defines C++ operators so that the XPtr<Foo> object
can be converted to a Foo*.
You can find details about the XPtr template in Rcpp documentation or
perhaps just reading the source at r-forge:
https://r-forge.r-project.org/scm/viewvc.php/pkg/Rcpp/inst/include/Rcpp/XPtr.h?view=markup&root=rcpp
This simply builds on the macros that the R api offers, so perhaps
"writing R extensions" or "R internals" are good sources of information.
Romain
Le 2012-12-25 18:15, Simon Urbanek a écrit :
On Dec 25, 2012, at 6:39 AM, andre__ wrote:
Hi,
I am using swig to build a wrapper for an c-function to use it in R.
I would
like to define a generic function, which gives back a void pointer.
On the R
side, I know what this pointer is pointing to, whether it is an
integer or
an string, or something else... but I need to somehow reinterpret
the
resulting "externalptr" into another form...
a function that looks like the following:
*void* test(){
std::string str="test";
return str.c_str();
}*
when I call this from R:
*str <- test()
typeof(str) % result is: "externalptr"*
how could I reinterpret this to a charcterarray, to a numeric, to
list, or
...
The simple answer is you can't. External pointers are entirely opaque
to R, so *you* have to write that code that interprets them.
Obviously
it's then up to you to create the corresponding R object from the
external pointer.
You may want to have a look at interface packages like Rcpp, rJava,
Rserve, ... to understand how objects are converted to/from other
languages.
Cheers,
Simon
Thanks for any suggestions ;)
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