c++ global variable result in segfault - r

Question

I'm working on turning some MCMC software written in c++ into an R-package using Rcpp and modules. In this regard I need to maintain a pointer that is a global variable, and that points to the latest object of some class constructed.
Here is a very simple example in the form of an R-script:
require(Rcpp)
require(inline)
inc <- '
using namespace Rcpp;
class test;
test* glob; //global pointer
class test{
private:
double foo;
public:
test(double foo_) : foo(foo_) {
glob=this; // the line causes segfault
};
double get_foo(){return foo;};
};
RCPP_MODULE(test){
class_<test>("test")
.constructor<double>()
.property("foo",&test::get_foo)
;
}
'
fx <- cxxfunction(signature(),plugin="Rcpp",include=inc);
test_module <- Module("test",getDynLib(fx))
test <- test_module$test
t1 <- new(test,1.0)
What I'm trying to get at is something like the following (in c++):
#include<iostream>
class test;
test* glob;
class test{
private:
double foo;
public:
test(double foo_) : foo(foo_) {glob=this;};
double get_foo(){return foo;};
};
int main(){
test t1(1.0);
test t2(2.0);
std::cout << (*glob).get_foo() << std::endl;
}
Which compiles and runs as it should.
Thanks in advance,
Regards, Tore Kleppe

Answer 1

That seems like two unrelated and simple errors.
First off, you need to make that pointer static. Things then work.
Second, using Rcpp Module with inline is no
longer the easiest way around, and we generally recommend using a package -- or Rcpp Attributes as I do below.
Correct code, including explicit messaging to stdout in the constructor:
#include <Rcpp.h>
using namespace Rcpp;
class test;
static test* glob = NULL; //global pointer
class test{
private:
double foo;
public:
test(double foo_) : foo(foo_) {
Rcpp::Rcout << "Seeing foo_ of " << foo_ << " in ctor\n";
glob=this; // the line causes segfault
};
double get_foo(){return foo;};
};
RCPP_MODULE(test){
class_<test>("test")
.constructor<double>()
.property("foo",&test::get_foo)
;
}
Then simple use from the command-line (using littler; R or Rscript are equivalent):
$ r -lRcpp -e 'sourceCpp("/tmp/tore.cpp"); tt <- new(test, 1.23); print(tt$foo)'
Seeing foo_ of 1.23 in ctor
[1] 1.23
$
Note how we can skip all the Module instantiation etc.