条件变量和互斥锁在多线程同步过程中经常被使用，以下测试程序测试其使用。

目录

1.测试程序1

2.测试程序2 

 3.运行结果思考

---

1.测试程序1

#include <mutex>
#include <deque>
#include <iostream>
#include <thread>
#include <condition_variable>

class MCTest {
public:
	MCTest() : m_work(true), m_max_num(30), m_next_index(0) {
	}
	
	void producer_thread() {
		while (m_work) {
			std::this_thread::sleep_for(std::chrono::milliseconds(500));
			std::unique_lock<std::mutex> lk(m_cvMutex);
			m_cv.wait(lk);
			m_data.push_back(m_next_index++);
			std::cout << "producer " << m_next_index << ", queue size: " << m_data.size() << std::endl;
			m_cv.notify_all();
		}
	}
	void consumer_thread() {
		while (m_work) {
            std::unique_lock<std::mutex> lk(m_cvMutex);
            m_cv.wait(lk);
		    int data = m_data.front();
		    m_data.pop_front();
		    std::cout << "consumer " << data << ", deque size: " << m_data.size() << std::endl;
		    m_cv.notify_all();
		}
	}
private:
	bool m_work;
	std::mutex m_cvMutex;
	std::condition_variable m_cv;
	std::deque<int> m_data;
	size_t m_max_num;
	int m_next_index;
};

int main() {
	MCTest obj;
	std::thread tp = std::thread(&MCTest::producer_thread, &obj);
	std::thread tc = std::thread(&MCTest::consumer_thread, &obj);
    tp.join();
    tc.join();
	return 0;
}

运行结果：

2.测试程序2 

#include <mutex>
#include <deque>
#include <iostream>
#include <thread>
#include <condition_variable>

class MCTest {
public:
	MCTest() : m_work(true), m_max_num(30), m_next_index(0) {
	}
	
	void producer_thread() {
		while (m_work) {
			std::this_thread::sleep_for(std::chrono::milliseconds(500));
			std::unique_lock<std::mutex> lk(m_cvMutex);
			m_cv.wait(lk, [this]() -> bool { return this->m_data.size() < this->m_max_num; });
			m_data.push_back(m_next_index++);
			std::cout << "producer " << m_next_index << ", queue size: " << m_data.size() << std::endl;
			m_cv.notify_all();
		}
	}
	void consumer_thread() {
		while (m_work) {
            std::unique_lock<std::mutex> lk(m_cvMutex);
            m_cv.wait(lk, [this] { return !this->m_data.empty(); });
		    int data = m_data.front();
		    m_data.pop_front();
		    std::cout << "consumer " << data << ", deque size: " << m_data.size() << std::endl;
		    m_cv.notify_all();
		}
	}
private:
	bool m_work;
	std::mutex m_cvMutex;
	std::condition_variable m_cv;
	std::deque<int> m_data;
	size_t m_max_num;
	int m_next_index;
};

int main() {
	MCTest obj;
	std::thread tp = std::thread(&MCTest::producer_thread, &obj);
	std::thread tc = std::thread(&MCTest::consumer_thread, &obj);
    tp.join();
    tc.join();
	return 0;
}

运行结果：

 3.运行结果思考

为什么测试1程序没有任何输出，出现死锁，而程序2正常交替执行？

程序1条件变量在得到通知之前会一直wait，如果线程1获取了锁后，阻塞于wait调用，释放了互斥锁，等待通知。此时线程2执行，线程2获取锁后，阻塞于阻塞于wait调用，并释放互斥锁，等待唤醒。本质上是处于死锁状态。

程序2条件变量处于wait阻塞时，除了得到通知会解除阻塞外，第二个参数为true时，wait函数也会返回，所以避免了死锁的存在。

总结一下，wait函数参数2含义如下：

（1）如果参数为true，即使没有收到通知，wait也会返回，此时本线程互斥量已经加锁
（2）如果参数为false，在没有收到通知时，解锁互斥量wait一直阻塞
（3）如果没有参数，与false一样