static singletons in shared libraries

inc.h

class Registry {
public:
  //change to:
  //static Registry* get() __attribute__ ((visibility ("hidden") )) {
  static Registry* get() {
    static Registry inst;
    std::cout << "Registry in: " << &inst << std::endl;
    return &inst;
  }
  int size() {
    return val;
  }
  void inc() {
    val += 12;
  }
private:
  Registry() {val=42;}
  int val;
};

lib.cpp

#include <iostream>
#include "inc.h"
extern "C" {
  int foo() {
    std::cout << "in foo found myVar: " << Registry::get()->size() << std::endl;
    Registry::get()->inc();
    return 42;
  }
}

Makefile

all:
	g++ -o main test.cpp -ldl -std=c++11
	g++ -o libmylib.so -shared -fPIC -std=c++11 lib.cpp
	cp libmylib.so libmylib2.so

clean:
	rm -rf *.so main *~

out

########## with __attribute__ ###################
hello
foo is at: 0x7efd6600aba0
Registry in: 0x7efd6620c07c   <-----------
in foo found myVar: 42
Registry in: 0x7efd6620c07c   <-----------
foo is at: 0x7efd65e07ba0
Registry in: 0x7efd6600907c
in foo found myVar: 42
Registry in: 0x7efd6600907c

############ without __attribute__ ##################
hello
foo is at: 0x7f4391bccca0
Registry in: 0x7f4391dce084
in foo found myVar: 42
Registry in: 0x7f4391dce084
foo is at: 0x7f43919c9ca0
Registry in: 0x7f4391dce084
in foo found myVar: 54
Registry in: 0x7f4391dce084

test.cpp

#include <iostream>
#include <dlfcn.h>

void lookup(const char* path) {
  void* handle = dlopen(path, RTLD_LAZY);
  if (!handle) {
    std::cout << "Error loading library: '" << path << "'\n" << dlerror() << std::endl;
  }

  const char* name = "foo";
  void* func = dlsym(handle, name);
  if (!func) {
    std::cout << "Error getting function '" << name << "' from library\n" << dlerror() << std::endl;
  }

  void (*foo)() = (void(*)())func;
  std::cout << "foo is at: " << func << std::endl;
  foo();
}

int main() {
  std::cout << "hello" << std::endl;

  const char* path = "libmylib.so";
  lookup(path);
  const char* path2 = "libmylib2.so";
  lookup(path2);

  
  
  return 0;
}