Le 06/05/10 13:12, Giuseppe Milicia a écrit :
Dirk,
Unfortunately the pieces of code that use static variables are lost in the
depth of my libraries.
I used to mix R and Java using rJava, when you do that you can keep object
references in R rather easily, that is not immediately possible with Rcpp.
Somehow I didn't think about static variables (after all we just have a shared
library loaded by the R process), so now I'm using static variables to make my
system stateful, it seems to work rather nicely :)
You can use external pointers to do similar things. For example, if your
"state" variable is an std::vector<int>, you can deal with it using
external pointers.
fx <- cppfunction( signature(), '
// create a pointer
std::vector<int> *v = new std::vector<int>(5) ;
// wrap it as an R external pointer
Rcpp::XPtr< std::vector<int> > pv(v,true);
return pv ;
' )
now you can return this to the R side :
> xp <- fx()
> str( xp )
<externalptr>
> xp
<pointer: 0x10056cd30>
> typeof( xp )
[1] "externalptr
Then you can pass it back to c++ :
fy <- cppfunction( signature( xp_ = "externalptr" ), '
// wrap it up again
Rcpp::XPtr< std::vector<int> > xp(xp_);
// now you can use xp as a pointer to std::vector<int>
xp->push_back( 10 ) ;
int size = xp->size() ;
return wrap( size ) ;
' )
> fy( xp )
[1] 6
> fy( xp )
[1] 7
> fy( xp )
[1] 8
This is what rJava uses. It is more work than with java because java has
runtime reflection, which is not available in C++.
See also:
http://romainfrancois.blog.free.fr/index.php?post/2010/01/08/External-pointers-with-Rcpp
In a Rcpp document I read (can't find it anymore, perhaps an old vignette?)
there was a paragraph about Rcpp being stateless
I'm not familiar with this, so it must be before I started to
contribute. Maybe Dirk knows better. Maybe this was from before Dirk
started to maintain.
In any case, please refer to the current documentation.
perhaps the vignette could mention that you can use static variables to keep
state around? It's obvious on hindsight, but somehow I didn't connect the dots
when I started to using Rcpp...
>
Cheers,
//Giuseppe
________________________________________
From: Dirk Eddelbuettel [[email protected]]
Sent: 30 April 2010 18:14
To: Giuseppe Milicia
Cc: [email protected]
Subject: Re: [Rcpp-devel] Rcpp& static variables
Hi Guiseppe,
Thanks for reposting here!
On 30 April 2010 at 16:07, Giuseppe Milicia wrote:
| Guys,
|
| I posted this on R-help and Dirk kindly pointed out that this list is a much
better place for this sort of questions...
|
| I was wondering whether anyone experimented with Rcpp and static
| variables. I remember reading that Rcpp is essentially stateless. That
| makes sense. However I just run a piece of code that contains a static STL
| structure, surprisingly it seems that those static variables are
| preserved...
I have not used it explicitly.
But I do use code in production that instantiates singletons etc to keep
e.g. connection handles to a backend. I initialize this on package load --
using a nice clean hook provided by R and then hold on to it until the
package unloads, which in practice is when the R session ends. That works
cleanly via constructor and deconstructor calls.
| The question is, can we use static variable to reliably preserve state
| between Rcpp calls? If this is the case, this should be a better way to
| store state than by passing state variables between R and C++.
I don't see why not. Can you mock up an example of what you are trying to do?
--
Regards, Dirk
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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