boost python: initialize C++ class that expects a pointer from python

I have a main C++ class Test of which member elements are continuously changed by other C++ classes. These other classes receive a pointer to the Test class in the constructor, which they should keep as a member. It is important that this is a pointer, and that all other classes all change the same Test object. A simplified version looks like this:

test.h

#ifndef TEST_H
#define TEST_H
class Test {
public:
    int a;
    Test();
    Test * get_ptr(void);
    int geta(void);
};
#endif // TEST_H

test.cpp

#include "test.h"
Test::Test() {this->a = 4;};    
Test * Test::get_ptr(void) {return new Test;};    
int Test::geta(void) {return a;};

changer.h

#ifndef CHANGER_H
#define CHANGER_H
#include "test.h"
class Changer {
public:
    Test * t_p;
    Changer(Test * t_ptr);
    void plus1(void);
};    
#endif // CHANGER_H

changer.cpp

#include "changer.h"
Changer::Changer(Test * t_ptr) {t_p = t_ptr;};
void Changer::plus1(void) {t_p->a += 1;};

boost.cpp

#include "test.h"
#include "changer.h"
#include "boost/python.hpp"    
using namespace boost::python;    
BOOST_PYTHON_MODULE(pnt) {
    class_<Test>("Test")
        .def("get_ptr", &Test::get_ptr, return_value_policy<manage_new_object>())
        .def("geta", &Test::geta)
        ;    
    class_<Changer>("Changer", init<Test*>())
        .def("plus1", &Changer::plus1)
        ;
}

This compiles fine, but when I use it in python I get a segmentation fault with this code:

In [1]: import pnt
   ...: t = pnt.Test()
   ...: t.geta()
   ...: c = pnt.Changer(t.get_ptr())
   ...: c.plus1()
   ...: t.geta()
   ...: 
Out[1]: free(): invalid pointer
Aborted (core dumped)

I got the part for the pointer from https://wiki.python.org/moin/boost.python/PointersAndSmartPointers but I can't seem to get it to work. I would greatly appreciate some hints on how to do this.