[Rcpp-devel] check validity of object created from Rcpp module

[Douglas Bates]

On Tue, Jun 26, 2012 at 7:04 PM, Dirk Eddelbuettel <edd at debian.org> wrote:
>
> On 26 June 2012 at 17:21, Jiqiang Guo wrote:
> | Dear List,
> |
> | I am wondering if there is a way to check whether an object created from Rcpp's
> | module is valid or not especially for objects loaded in another session.
> |
> | In detail, we know that we could not save and load objects created from Rcpp's
> | module [section 5 of http://cran.r-project.org/web/packages/Rcpp/vignettes/
> | Rcpp-modules.pdf].  But that does not prevent users doing that. So if an
> | object created from Rcpp's module is loaded in another session, we would end up
> | with errors or even segfault.  It would be nice we have a method to check if
> | it is valid or not so that when we are writing a package, we could do a check
> | before calling those methods of that object.
>
> It's a very good idea. I don't have a good idea as to how we'd do this though.

There has to be one or more external pointers in the R object
somewhere (although I spent some time looking at the Rcpp-modules code
and came away very confused so I can't tell you exactly where.)  That
pointer will become a NULL pointer after a serialize/unserialize
sequence so you need to check for its being NULL.  This is why the
reference classes in the development version of lme4 have a field
named ptr and a method named Ptr.  The Ptr method first checks if ptr
is a null pointer and, if so, regenerates the object then returns ptr.
 I can say from experience that this all gets rather complicated.
However I haven't looked at John Chambers' new code for Rcpp classes
in R and he might have addressed the issue.

>
> Dirk
>
> | An illustrative example:
> |
> | require("Rcpp")
> | require("inline")
> |
> | inc <- '
> | class Foo {
> | private:
> |   int x_;
> | public:
> |   Foo(int x) : x_(x) { }
> |   int getx() const { return x_;}
> | };
> |
> | RCPP_MODULE(foo) {
> |   class_<Foo>("Foo")
> |   .constructor<int>()
> |   .method("getx", &Foo::getx)
> |   ;
> | }
> |
> | fx <- cxxfunction(signature(), "", include = inc, plugin = "Rcpp", verbose =
> | FALSE)
> | m <- Module("foo", getDynLib(fx))
> | cppo  = new(m$Foo, 5)
> |
> | print(cppo$getx())
> | save("cppo", file = 'cppo.RData')
> |
> | rm(list = ls())
> | load("cppo.RData")
> | print(cppo$getx())
> |
> | For the above code, we know the last line would not work.  If we have a
> | function to test the validity of cppo,then if not valid, we would not call
> | cppo's function and are able to tell users that cppo is not valid any more.
> |
> | Best,
> | Jiqiang
> |
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> --
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com
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