C++11 线程同步接口std::condition_variable和std::future的简单使用

std::condition_variable

条件变量std::condition_variable有wait和notify接口用于线程间的同步。如下图所示，Thread 2阻塞在wait接口，Thread 1通过notify接口通知Thread 2继续执行。

具体参见示例代码：

#include<iostream> #include<mutex> #include<thread> #include<queue> std::mutex mt; std::queue<int> data; std::condition_variable cv; auto start=std::chrono::high_resolution_clock::now();  void logCurrentTime() { 	auto end = std::chrono::high_resolution_clock::now(); 	auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count(); 	std::cout << elapsed << ":"; } void prepare_data() {	 	logCurrentTime(); 	std::cout << "this is " << __FUNCTION__ << " thread:" << std::this_thread::get_id() << std::endl; 	for (int i = 0; i < 10; i++) 	{ 		data.push(i); 		logCurrentTime(); 		std::cout << "data OK:" << i << std::endl; 	} 	//start to notify consume_data thread data is OK! 	cv.notify_one(); }   void consume_data() { 	logCurrentTime(); 	std::cout << "this is: " << __FUNCTION__ << " thread:" << std::this_thread::get_id() << std::endl; 	std::unique_lock<std::mutex> lk(mt); 	//wait first for notification 	cv.wait(lk);  //it must accept a unique_lock parameter to wait  	while (!data.empty()) 	{ 		logCurrentTime(); 		std::cout << "data consumed: " << data.front() << std::endl; 		data.pop(); 	} }   int main() { 	std::thread t2(consume_data); 	//wait for a while to wait first then prepare data,otherwise stuck on wait 	std::this_thread::sleep_for(std::chrono::milliseconds(10)); 	std::thread t1(prepare_data); 	t1.join(); 	t2.join(); 	return 0; } 

输出结果

分析

主线程中另启两个线程，分别执行consume_data和prepare_data，其中consume_data要先执行，以保证先等待再通知，否则若先通知再等待就死锁了。首先consume_data线程在从wait 处阻塞等待。后prepare_data线程中依次向队列写入0-10，写完之后通过notify_one 通知consume_data线程解除阻塞，依次读取0-10。

std::future

std::future与std::async配合异步执行代码，再通过wait或get接口阻塞当前线程等待结果。如下图所示，Thread 2中future接口的get或wait接口会阻塞当前线程，std::async异步开启的新线程Thread1执行结束后，将结果存于std::future后通知Thread 1获取结果后继续执行.

具体参见如下代码：

#include <iostream> #include <future> #include<thread>  int test() { 	std::cout << "this is " << __FUNCTION__ << " thread:" << std::this_thread::get_id() << std::endl;; 	std::this_thread::sleep_for(std::chrono::microseconds(1000)); 	return 10; } int main() { 	std::cout << "this is " <<__FUNCTION__<<" thread:" << std::this_thread::get_id() << std::endl;; 	//this will lanuch on another thread 	std::future<int> result = std::async(test);  	std::cout << "After lanuch a thread: "<< std::this_thread::get_id() << std::endl;  	//block the thread and wait for the result 	std::cout << "result is: " <<result.get()<< std::endl;  	std::cout << "After get result "<< std::endl;  	return 0; } 

输出结果

分析

主程序中调用std::async异步调用test函数，可以看到main函数的线程ID 27428与test函数执行的线程ID 9704并不一样，说明std::async另起了一个新的线程。在test线程中，先sleep 1000ms，所以可以看到"After lanuch a thread:"先输出，说明主线程异步执行，不受子线程影响。而"After get result "最后输出，说明get()方法会阻塞主线程，直到获取结果。