[Rcpp-devel] Creating pointers to objects and wrapping them

[Romain Francois]

Le 18/04/11 10:04, schattenpflanze at arcor.de a écrit :
> Dirk,
>
>> Could you mock up a small example?
> Here is a small, abstract example which demonstrates one of the
> scenarios where I would like to create and return object pointers. There
> are two classes, A and B. Instances of A contain pointers to instances
> of B. I would like to create the B objects pointed to by A on demand,
> and query A for these. (In the actual application, both A and B are
> rather complex mathematical objects, which require a lot of work to
> compute. They are not only created and stored, but also modified,
> swapped, removed, etc.)
>
> This is the basic idea:
>
> // B just stores an integer i here
> class B {
> public:
> B(int i) : i(i) {}
>
> int get_i() {
> return i;
> }
>
> private:
> int i;
> }
>
> // A contains a vector v of pointers to B
> class A {
> public:
> A(int n) : v(n) {}
>
> // THIS IS THE METHOD I WOULD LIKE TO IMPLEMENT:
> SEXP foo(int j) {
> if (!v[j]) {
> v[j] = new B(j);
> }
>
> // THIS DOES NOT WORK:
> return wrap(v[j]);
> }
>
> private:
> std::vector<B*> v;
> }
>
> // Rcpp module exposing both A and B
> RCPP_MODULE(MyModule) {
> class_<A>( "A" )
> .constructor<int>(
> "Create an instance of A with a given number of pointers")
> .method("foo",
> &A::foo,
> "(Create and) get one of the objects of type B.")
> ;
>
> class_<B>( "B" )
> .constructor<int>("Create an instance of B from an int.")
> .property("i",
> &B::get_i)
> ;
> }
>
> In R, I would then like to do, e.g., this:
>
> a <- new (A, 5)
> b <- a$foo(0)
> b$i
>
> I could rephrase the question as follows: How do I create objects of the
> exposed classes within C++ in a way which allows me to return them to R?

I see. There is no easy way to do this. But there should. This makes a 
nice item for someone's TODO list.

I don't have much spare time ATM, anyone with the required skills or a 
budget to fund such an extension to Rcpp modules ?


The way you try to implement it would require the class B to be 
wrappable. You can have a look at the Rcpp-extending vignette for how to 
enable wrap for custom classes.

Romain

>>> Meta question:
>>> Is there a search function for the Rcpp mailing list archive?
>> I just constrain Google to, say, site:gmane.org to restrict the search
>> to a
>> particular site. And I also use grep on my folder with the list messages.
> OK, thank you.
>
>
> Best regards,
> Peter
>
>
>
>
>
> Am 17.04.2011 10:45, schrieb schattenpflanze at arcor.de:
>> Hello,
>>
>> I have made progress exposing my C++ classes to R with Rcpp-Modules and
>> I really like the way it works. Thanks for this great package!
>>
>> There are, however, lots of aspects about Rcpp which I do not yet fully
>> understand. Here is one of them:
>> I have exposed a C++ class MyClass to R via an Rcpp module. Now I would
>> like to write a C++ function, which creates (pointers to) objects of
>> this class. The return type should ideally be a vector/list of pointers
>> to the created C++ objects. The pointers should have the same type as
>> those returned by the exposed constructors of MyClass in the module. In
>> particular, I want R to do the memory management and garbage collection
>> when I destroy the returned list of pointers.
>>
>> When I try to directly wrap an object of type MyClass or a pointer to it
>> or a List containing it, I get compiler errors. So I suppose this is not
>> the way to do it. I can create a vector of Rcpp::XPtrs and return it
>> successfully to R, but then I cannot use the objects in R in the same
>> way as those created by the constructor, namely object_pointer$method().
>>
>> Is there a way to achieve this or can you recommend a reasonable
>> alternative design?
>> How are XPtrs related to the objects a Rcpp-Module class constructor
>> returns, if at all?
>>
>>
>> Meta question:
>> Is there a search function for the Rcpp mailing list archive?
>>
>>
>> Thanks for your help,
>> Peter


-- 
Romain Francois
Professional R Enthusiast
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http://romainfrancois.blog.free.fr
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