线程同步：线程同步就是让多个线程按顺序访问临界区域，只有在当前线程访问临时区结束后，下一个线程才能继续访问。（临界区加锁即可）

#include <iostream>
#include <chrono>
#include <thread>
#include <mutex>
using namespace std;

int g_num = 0;  // 为 g_num_mutex 所保护
mutex g_num_mutex;

void slow_increment(int id)
{
    for (int i = 0; i < 3; ++i) 
    {
        g_num_mutex.lock();
        ++g_num;
        cout << id << " => " << g_num << endl;
        g_num_mutex.unlock();

        this_thread::sleep_for(chrono::seconds(1));
    }
}

int main()
{
    thread t1(slow_increment, 0);
    thread t2(slow_increment, 1);
    t1.join();
    t2.join();
}

lock_guard

lock_guard在使用上面提供的这个构造函数构造对象时，会自动锁定互斥量，而在退出作用域后进行析构时就会自动解锁，从而保证了互斥量的正确操作，避免忘记unlock()操作而导致线程死锁。

#include <iostream>
#include <chrono>
#include <thread>
#include <mutex>
using namespace std;

int g_num = 0;  // 为 g_num_mutex 所保护
mutex g_num_mutex;

void slow_increment(int id)
{
    for (int i = 0; i < 3; ++i)
    {
        { //这一对{}是lock_guard的作用域，
            lock_guard<mutex> lock(g_num_mutex);
            ++g_num;
            cout << id << " => " << g_num << endl;
        }

        this_thread::sleep_for(chrono::seconds(1));
    }
}

int main()
{
    thread t1(slow_increment, 0);
    thread t2(slow_increment, 1);
    t1.join();
    t2.join();
}