C++的多线程编程(练习一下condition_variable)

        嗯，高考结束了，那就编写一个阅卷和查成绩的多线程吧。一个线程老师阅卷，其他三个线程查成绩。代码如下：

       

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <chrono>
#include <future>
#include <map>

// 定义一个简单的分数存储结构
typedef std::map<std::string, double> RecordType;

std::mutex ready_mutex;
std::condition_variable ready_condition;
RecordType record_table;

//模拟成绩查询
void query_score(std::string name)
{
    bool need_wait = true;
    std::cout << name << ": Now start query score..." << std::endl;
    while (need_wait)
    {
        // 等待条件变量，大括号限制互斥量的作用域，便于自动析构
        {
            std::unique_lock<std::mutex> unqique_lock_mutex(ready_mutex);
            // 设置等待超时30s
            std::cv_status status = ready_condition.wait_for(unqique_lock_mutex, std::chrono::seconds(30));
            if (std::cv_status::no_timeout == status)
            {// 如果还没有超时
                // 开始查询成绩
                auto record = record_table.find(name);
                if (record != record_table.end())
                {//查询到了对应的成绩，就输出
                    double score = record_table[name];
                    std::cout << name << "'s record is " << score << std::endl;
                    need_wait = false;
                }
                else
                {// 查不到成绩，就再次等待
                    continue;
                }
            }
            else
            {// 如果等到花儿也谢了
                std::cout << name << "'s record maybe lost!!!" << std::endl;
                need_wait = false;
            }
        }
    }
    std::cout << name << ": Now finish query record." << std::endl;
}

void rate_score()
{
    // 老师开始批卷
    std::cout << "Now the teacher start rating...." << std::endl;
    // 批卷一个同学要5秒，神速
    std::this_thread::sleep_for(std::chrono::seconds(5));
    // 批改完毕，上传成绩
    {
        std::lock_guard<std::mutex> lock_guard_the_mutex(ready_mutex);
        record_table.insert({ "Lucy", 550.5 });
    }
    // 上传完毕，需要知会一下正在查询的同学
    ready_condition.notify_all();
    std::this_thread::yield();

    // 继续批改下一个同学
    std::this_thread::sleep_for(std::chrono::seconds(6));
    // 再次提示，大括号是为了互斥锁的自动析构
    {
        std::lock_guard<std::mutex> lock_guard_the_mutex(ready_mutex);
        record_table.insert({ "Han Meimei", 655.0 });
    }
    ready_condition.notify_all();
    std::this_thread::yield();

    // 继续批改下一个同学
    std::this_thread::sleep_for(std::chrono::seconds(7));
    // 可以看出来老师批改越来越慢了
    {
        std::lock_guard<std::mutex> lock_guard_the_mutex(ready_mutex);
        record_table.insert({ "Li Lei", 635.0 });
    }
    ready_condition.notify_all();
    std::this_thread::yield();

    // 老师累了，批完闭眼休息一下，下班了
    std::this_thread::sleep_for(std::chrono::seconds(5));
    std::cout << "Now the teacher finish rating." << std::endl;
}


int main()
{
    // 各位同学开始查询成绩
    auto thread1 = std::thread(query_score, "Li Lei");
    // Han Meimei的手速慢了
    std::this_thread::sleep_for(std::chrono::seconds(1));
    auto thread2 = std::thread(query_score, "Han Meimei");
    // Lucy在国外，网络延迟了
    std::this_thread::sleep_for(std::chrono::seconds(3));
    auto thread3 = std::thread(query_score, "Lucy");

    // 老师开始批卷
    auto thread4 = std::thread(rate_score);

    // 等待成绩查询完毕和批卷完毕
    thread1.join();
    thread2.join();
    thread3.join();
    thread4.join();

    return 0;
}

 

以下是运行结果：

        知道这三位同学的同学，年纪应该都比较大了。