C++条件变量condition_variable使用方法

一、条件变量概述

条件变量可以实现一个线程唤醒一个或多个其他等待中的线程，实现线程间的同步。生产者消费者模型就可以用条件变量来实现。

二、条件变量示例

源码

#pragma once
#include 
#include 
#include 
#include 
using namespace std;
class ConditionVariableTest
{
private:
	static mutex _queueMutex;
	static queue<int> _queue;
	static condition_variable _queueConditionVar;
public:
	void DoTest()
	{
		thread providerThread(Provider);
		int consumerNum = 2;
		thread** consumerThread = new thread*[consumerNum];
		for (int i = 0; i < 2; i++)
		{
			consumerThread[i] = new thread(Consumer,i+1);
		}		
		providerThread.join();

	}
	static void Provider() 
	{
		for (int i = 0; ; i+=2)
		{
			_queueMutex.lock();
			_queue.push(i);
			printf("provider v=%d\n", i);
			_queue.push(i+1);
			printf("provider v=%d\n", i+1);
			_queueMutex.unlock();
			_queueConditionVar.notify_all();
			this_thread::sleep_for(chrono::milliseconds(1000));//1000ms生产1个
			
		}
		
	}
	static void Consumer(int id)
	{
		while (true)
		{
			{//uniqueLock的作用域在大括号里，出了大括号，自动解锁
				unique_lock<mutex> uniqueLock(_queueMutex); //unique_lock是对mutex的封装，如果对mutex加了锁，在unique_lock析构时(这里也就是在出了大括号时），会自动解锁mutex
				_queueConditionVar.wait(uniqueLock, [] {return !_queue.empty(); });  //条件不满足会挂起
				//拿到uniqueLock的锁
				//等待收到_queueConditionVar的notify,并检查wait里的条件，直到这两个条件都满足

				int v = _queue.front();
				_queue.pop();
				printf("consumer%d v=%d\n", id, v);
				
			}

		}
	}
};

mutex ConditionVariableTest::_queueMutex;
queue<int> ConditionVariableTest::_queue;
condition_variable  ConditionVariableTest::_queueConditionVar;

输出

provider v=0
provider v=1
consumer1 v=0
consumer1 v=1
provider v=2
provider v=3
consumer2 v=2
consumer2 v=3
provider v=4
provider v=5
consumer2 v=4
consumer2 v=5
provider v=6
provider v=7
consumer2 v=6
consumer2 v=7
provider v=8
provider v=9
consumer2 v=8
consumer2 v=9
provider v=10
provider v=11
consumer1 v=10
consumer1 v=11