Rcpp parallelize functions that return XPtr? [duplicate]

问题

Taking an example XPtr function:

test.cpp

#include <Rcpp.h>

// [[Rcpp::export]]
SEXP funx()
{
    /* creating a pointer to a vector<int> */
    std::vector<int>* v = new std::vector<int> ;
    v->push_back( 1 ) ;
    v->push_back( 2 ) ;

    /* wrap the pointer as an external pointer */
    /* this automatically protected the external pointer from R garbage 
     collection until p goes out of scope. */
    Rcpp::XPtr< std::vector<int> > p(v, true) ;

    /* return it back to R, since p goes out of scope after the return 
     the external pointer is no more protected by p, but it gets 
     protected by being on the R side */
    return( p ) ;
}

library(Rcpp)
sourceCpp("test.cpp")

xp <- funx()
xp
<pointer: 0x9618cc0>

But if I try to parallelize this I get null pointers

library(parallel)
out <- mclapply(1:2, function(x) funx())
out
[[1]]
<pointer: (nil)>

[[2]]
<pointer: (nil)>

Is it possible to achieve this kind of functionality?

Edit

It is worth noting that despite a duplicate question there appears to be no true solution to this problem. From what I understand now, an XPtr is not able to be multi-threaded. So essentially this cannot be done in R.

For example, when I put the function inside package test and try to use snow it still fails to return the pointers.

library(test)
library(snow)

fun <- function(){
  library(test)
  test:::funx()
}

cl <- makeCluster(2, type = "SOCK") 
clusterExport(cl, 'fun') 
clusterCall(cl, fun)
[[1]]
<pointer: (nil)>

[[2]]
<pointer: (nil)>

回答1:

Regarding

Is it possible to achieve this kind of functionality?

I would say the answer is a pretty firm 'nope' as the First Rule of Fight Club applies here: you simply cannot parallelise the underlying R instance merely by hoping it would work. Packages like RcppParallel are very careful about using non-R data structures for multithreaded work.

I may be too pessimistic but I would place the 'collection level' one level deeper, and only return its aggregated result to R.

来源：https://stackoverflow.com/questions/37167479/rcpp-parallelize-functions-that-return-xptr

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