条件变量std::condition_variable是C++中用于线程间同步的重要工具，它提供了wait和notify接口。下图展示了Thread 2在wait接口阻塞，而Thread 1通过notify接口通知Thread 2继续执行。

以下是示例代码：

#include
#include
#include
#include
std::mutex mt;
std::queue 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(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 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需要先执行，以确保先等待再通知，否则若先通知再等待就会发生死锁。首先consume_data线程在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的使用示例

std::future与std::async配合使用，实现异步执行代码，并通过wait或get接口阻塞当前线程等待结果。下图展示了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。

以下是示例代码：

#include 
#include 
#include

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 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: " <