C++多线程编程（二）

转自：https://www.youtube.com/watch?v=IBu5ka1MQ7w

unique_lock<mutex>、once_flag、call_once

// https://www.youtube.com/watch?v=IBu5ka1MQ7w
//Lazy Initialization
#include<iostream>
#include<fstream>
#include<vector>
#include<algorithm>
#include<thread>
#include<mutex>
#include<string>


using namespace std;

class LogFile
{
public:
	LogFile() {
		//_f.open("log.txt");
	};
	~LogFile() {};

	void share_print(string id,int val) {

		//unique_lock<mutex> locker2(_mu_open);//_f只打开一次，但每个进程都要lock一下再判断，浪费时间！
		//if (!_f.is_open()) {
		//	_f.open("log.txt");//Lazy Initialization 
		//}

		call_once(_flag, [&]() {_f.open("log.txt"); });//只打开一次



		//lock_guard<mutex> guard(_mu);   
		//unique_lock<mutex>比lock_guard<mutex>更加灵活，可以随时 lock()或者 unlock()
		//unique_lock<mutex>可以移动（move）!!
		unique_lock<mutex> locker(_mu,defer_lock);//defer_lock 延迟上锁！
		//do somthing else

		locker.lock();
		_f << "From " <<id << ":" << val << endl;
		locker.unlock();

		/*locker.lock();
		_f << "......:)..." << endl;
		locker.unlock();*/

		//unique_lock<mutex> locker2 = move(locker);


	}
private:
	mutex _mu;
	mutex _mu_open;
	ofstream _f;
	once_flag _flag;

};

void thread_function(LogFile &log,string id,int val) {

	log.share_print(id, val);
}
int main() {

	LogFile log;
	ofstream f;
	f.open("log.txt");
	f << "dsdads" << endl;
	//log.share_print("main thread", 1);

	thread t1(&thread_function ,ref(log), " thread 1", 1);
	thread t2(&thread_function, ref(log), " thread 2", 2);
	thread t3(&thread_function, ref(log), " thread 3", 3);
	t1.join();
	t2.join();
	t3.join();
	//getchar();
	return 0;

}