#include <cstdio>

#include <iostream>

#include <list>

#include <mutex>

#include <thread>

// =========================================================

class list_wrapper {

std::list<int> my_list;

std::mutex m;

public:

void add_to_list(int const &x) {

std::lock_guard<std::mutex> lg(m);

my_list.push_front(x);

}

void size() {

std::lock_guard<std::mutex> lg(m);

int size = my_list.size();

std::cout << "size of the list is : " << size << std::endl;

}

// PROBLEM: Function breaks mutex locking of shared resource.

std::list<int> *get_data() { return &my_list; }

};

// =========================================================

struct Data {

void foo() { printf("Hello from foo()\n"); }

};

class DataWrapper {

Data data;

std::mutex m;

public:

// Allows running any function on the data that should be protected.

// In particular, the caller can retain a reference to the data.

template <typename Fn> void bar(Fn f) {

// Guard cannot guarantee safe usage of data.

std::lock_guard<std::mutex> lg(m);

f(data);

}

};

Data *unprotected_data;

void malicious_function(Data &data) { unprotected_data = &data; }

int main() {

DataWrapper wrapper;

wrapper.bar(malicious_function);

unprotected_data->foo();

}