repos_assist/GameAssist/IMGProc/ThreadPool.h

#ifndef THREAD_POOL_H
#define THREAD_POOL_H

#include <vector>
#include <queue>
#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <future>
#include <functional>
#include <stdexcept>

class ThreadPool
{
public:
	ThreadPool(size_t);
	template<class F, class... Args>
	auto enqueue(F&& f, Args&&... args)
		->std::future<typename std::result_of<F(Args...)>::type>;
	~ThreadPool();
	size_t getThreadNum()const {
		return workers.size();
	}
private:
	// need to keep track of threads so we can join them
	std::vector< std::thread > workers;
	// the task queue
	std::queue< std::function<void()> > tasks;

	// synchronization
	std::mutex queue_mutex;
	std::condition_variable condition;
	bool stop;
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads) :stop(false)
{
	for (size_t i = 0;i < threads;++i)
	{
		workers.emplace_back([this] {
			for (;;)
			{
				std::function<void()> task;

				{
					std::unique_lock<std::mutex> lock(this->queue_mutex);
					this->condition.wait(lock,
						[this] { return this->stop || !this->tasks.empty(); });
					if (this->stop && this->tasks.empty())
						return;
					task = std::move(this->tasks.front());
					this->tasks.pop();
				}

				task();
			}
			});
	}
}

// add new work item to the pool
template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args)
-> std::future<typename std::result_of<F(Args...)>::type>
{
	using return_type = typename std::result_of<F(Args...)>::type;

	auto task = std::make_shared< std::packaged_task<return_type()> >(
		std::bind(std::forward<F>(f), std::forward<Args>(args)...)
		);

	std::future<return_type> res = task->get_future();
	{
		std::unique_lock<std::mutex> lock(queue_mutex);

		// don't allow enqueueing after stopping the pool
		if (stop)
			throw std::runtime_error("enqueue on stopped ThreadPool");

		tasks.emplace([task]() { (*task)(); });
	}
	condition.notify_one();
	return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
	{
		std::unique_lock<std::mutex> lock(queue_mutex);
		stop = true;
	}
	condition.notify_all();
	for (std::thread& worker : workers)
		worker.join();
}
#endif