C++ 多线程笔记3

Note

std::thread 无拷贝构造函数，无法复制只能引用 &。

Tip

<condition_variable> 里包含了 <mutex> 头文件。

#include <iostream>
#include <thread>
#include <condition_variable>
#include <queue>
#include <vector>
#include <functional>
#include <atomic>

class ThreadPool {
private:
	std::mutex mtx;
	std::condition_variable condition;
	std::queue<std::function<void()>> tasks;
	std::atomic<bool> stop;
	std::vector<std::thread> threads;

public:
	ThreadPool(int num_threads) : stop{ false } {
		for (int i = 0; i < num_threads; i++) {
			threads.emplace_back([this]() {
				while (true)
				{
					std::unique_lock<std::mutex> lock(mtx);
					condition.wait(lock, [this]() { return !tasks.empty() || stop; });
					if (stop && tasks.empty()) return;
					std::function<void()> task(std::move(tasks.front()));
					tasks.pop();
					lock.unlock();
					task();
				}
				});
		}
	}
	~ThreadPool() {
		{	
			std::lock_guard<std::mutex> lock(mtx);
			stop = true;
		}
		condition.notify_all();
		for (auto& t : threads) {
			t.join();
		}
	}
	template<typename F, typename... Args> void enqueue(F&& f, Args&&... args) {
		std::function<void()> task = std::bind(std::forward<F>(f), std::forward<Args>(args)...);
		//auto task = [func = std::forward<F>(f), tup = std::make_tuple(std::forward<Args>(args)...)]() mutable { std::apply(std::move(func), std::move(tup)); }; C++17 引入 <tuple> 头文件 std::apply
		//auto task = [func = std::forward<F>(f), ...args = std::forward<Args>((args)...)]() mutable { func = std::move((args)...); }; C++20 才支持包扩展用于初始化捕获 [...args = ...]，用 lambda 替代 std::bind
		if (stop) throw std::runtime_error("enqueue on stopped ThreadPool");
		{
			std::lock_guard<std::mutex> lock(mtx);
			tasks.emplace(std::move(task));
			//std::cout<< "task loaded" << "\n";
		}
		condition.notify_one();
	}
};

int main() {
	ThreadPool threadpool(6);
	for (int i = 0; i < 500; i++) {
		threadpool.enqueue([i]() {
			std::cout << "task: " << i << " is running" << "\n";
			std::this_thread::sleep_for(std::chrono::milliseconds(10));
			std::cout << "task: " << i << " is done" << std::endl;
			});
	}

	return 0;
}