lock_guard_queue.cpp

#include <mutex>
#include <queue>
#include <iostream>

std::queue<int> q; // Queue which multiple threads might add/remove from
std::mutex m; // Mutex to protect this queue

template <class F> void lock(std::mutex& mut_ex, F f)
{
	std::lock_guard<std::mutex> guard{ mut_ex }; // Lock held from here to end of function
	f();
}

void AddToQueue(int i) { lock(m, [&] { q.push(i); }); }

int RemoveFromQueue()
{
	int i = -1;
	lock(m, [&] {
		if (!q.empty()) {
			i = q.front();
			q.pop();
		}});
	return i;
}

template <typename T>
class QueueIterator {
	using myQueue = std::queue < T > ;
public:
	QueueIterator(myQueue& queue) : queue_(queue) { }
	QueueIterator& operator++()	{ queue_.pop();	return *this; }
	T operator*() { return queue_.front(); }
	bool operator!=(QueueIterator const& other) { return !queue_.empty(); }
private:
	myQueue& queue_;
};

namespace std {
	template <typename T>
	QueueIterator<T> begin(std::queue<T>& queue) { return{ queue }; }
	template <typename T>
	QueueIterator<T> end(std::queue<T>& queue) { return{ queue }; }
}

int main()
{
	auto elements = { 1, 2, 3, 4 };
	for (auto&& e : elements)
		AddToQueue(e);
	RemoveFromQueue();
	RemoveFromQueue();
	for (auto&& e: q)
		std::cout << e << "\n";
}