RWLock.h

#pragma once

#include <mutex>

using namespace std;

class RWLock {
	mutex m_mutex;				// re-entrance not allowed
	condition_variable m_readingAllowed, m_writingAllowed;
	bool m_writeLocked = false;	// locked for writing
	size_t m_readLocked = 0;	// number of concurrent readers

public:
	size_t getReaders() const {
		return m_readLocked;
	}

	void lockR() {
		unique_lock<mutex> monitor(m_mutex);
		while (m_writeLocked)
		{
			m_readingAllowed.wait(monitor);
		}
		m_readLocked++;
	}

	void unlockR() {
		unique_lock<mutex> monitor(m_mutex);
		m_readLocked--;
		if (m_readLocked == 0)
		{
			m_writingAllowed.notify_all();
		}
	}

	void lockW() {
		unique_lock<mutex> monitor(m_mutex);
		while (m_writeLocked && m_readLocked != 0)
		{
			m_writingAllowed.wait(monitor);
		}
		m_writeLocked = true;
	}

	void unlockW() {
		unique_lock<mutex> monitor(m_mutex);
		m_writeLocked = false;
		m_readingAllowed.notify_all();
	}
};