一、call_once 单例模式 Singleton 

大家可以先看这篇文章：https://zh.cppreference.com/w/cpp/thread/call_once

/*
    std::call_once
    void call_once( std::once_flag& flag, Callable&& f, Args&&... args );
*/
#include <iostream>
#include <mutex>
#include <thread>

std::once_flag flag1, flag2;

void simple_do_once() {
    std::call_once(flag1, []() {
        std::cout << "简单样例:调用一次\n";
    });
}

void test1() {
    std::thread st1(simple_do_once);
    std::thread st2(simple_do_once);
    std::thread st3(simple_do_once);
    std::thread st4(simple_do_once);
    st1.join();
    st2.join();
    st3.join();
    st4.join();
}

void may_throw_function(bool do_throw) {
    if (do_throw) {
        std::cout << "抛出：call_once 会重试\n"; // 这会出现不止一次
        throw std::exception();
    }
    std::cout << "没有抛出，call_once 不会再重试\n"; // 保证一次
}

void do_once(bool do_throw) {
    try {
        std::call_once(flag2, may_throw_function, do_throw);
    }
    catch (...) {}
}

void test2() {
    std::thread t1(do_once, true);
    std::thread t2(do_once, true);
    std::thread t3(do_once, false);
    std::thread t4(do_once, true);
    t1.join();
    t2.join();
    t3.join();
    t4.join();
}
int main() {
    test1();
    test2();
    return 0;
}

call_once 应用在单例模式，以及关于单例模式我的往期文章推荐：C++ 设计模式----“对象性能“模式_爱编程的大丙 设计模式-CSDN博客https://heheda.blog.csdn.net/article/details/131466271

懒汉是一开始不会实例化，什么时候用就什么时候new，才会实例化
饿汉在一开始类加载的时候就已经实例化，并且创建单例对象，以后只管用即可
--来自百度文库

#include <iostream>
#include <thread>
#include <mutex>
#include <string>

// 日志类：在整个项目中，有提示信息或者有报错信息，都通过这个类来打印
// 这些信息到日志文件，或者打印到屏幕上。显然，全局只需要一个日志类的
// 对象就可以完成所有的打印操作了。不需要第二个类来操作，这个时候就可以
// 使用单例模式来设计它

std::once_flag onceFlag;
class Log {
public:
    Log(const Log& log) = delete;
    Log& operator=(const Log& log) = delete;
    // static Log& getInstance() { 
    //     static Log log; // 饿汉模式
    //     return log;
    // }
    static Log& getInstance() { // 懒汉模式
        std::call_once(onceFlag, []() {
            std::cout << "简单样例：调用一次\n";
            log = new Log;
        });
        return *log;
    }
    void PrintLog(std::string msg) {
        std::cout << __TIME__ << msg << std::endl;
    }
private:
    Log() {};
    static Log* log; 
};
Log* Log::log = nullptr;

void func() {
    Log::getInstance().PrintLog("这是一个提示");
}

void print_error() {
    Log::getInstance().PrintLog("发现一个错误");
}

void test() {
    std::thread t1(print_error);
    std::thread t2(print_error);
    std::thread t3(func);
    t1.join();
    t2.join();
    t3.join();
}

int main() {
    test();
    return 0;
}

二、condition_variable 与其使用场景

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

std::queue<int> queue;
std::condition_variable cond;
std::mutex mtx;

void Producer() {
    for (int i = 0; i < 10; i++) {
        {
            std::unique_lock<std::mutex> locker(mtx);
            queue.push(i);
            cond.notify_one();
            std::cout << "Producer : " << i << std::endl;
        }
        std::this_thread::sleep_for(std::chrono::microseconds(100));
    }
}

void Consumer() {
    while (1) {
        std::unique_lock<std::mutex> locker(mtx);
        cond.wait(locker, []() {
            return !queue.empty();
            });
        int value = queue.front();
        queue.pop();
        std::cout << "Consumer :" << value << std::endl;
    }
}

int main() {
    std::thread t1(Producer);
    std::thread t2(Consumer);
    t1.join();
    t2.join();
    return 0;
}

三、C++11 手撕线程池 + 单例模式（call_once）

• ThreadPool.h

#include <iostream>
#include <thread>
#include <mutex>
#include <string>
#include <condition_variable>
#include <queue>
#include <vector>
#include <functional>
std::once_flag onceFlag;
class ThreadPool {
private:
    ThreadPool();
public:
    ThreadPool(const ThreadPool& obj) = delete;
    ThreadPool& operator=(const ThreadPool& obj) = delete;
    static ThreadPool& getInstance();
    ~ThreadPool();
    template<class F, class... Args>
    void enqueue(F&& f, Args&&... args) {
        std::function<void()> task =
            std::bind(std::forward<F>(f), std::forward<Args>(args)...);
        {
            std::unique_lock<std::mutex> locker(mtx);
            tasks.emplace(std::move(task));
        }
        cond.notify_one();
    }
    inline void setNum(int num) {
        threadNum = num;
    }
    inline void printNum() {
        std::cout << "线程数量为:" << threadNum << std::endl;
    }
private:
    static ThreadPool* pool;
    std::vector<std::thread> threads;// 线程数组
    std::queue<std::function<void()>> tasks;//任务队列
    std::mutex mtx;// 互斥锁
    std::condition_variable cond;//条件变量
    bool stop;
    int threadNum;// 线程数量
};

• ThreadPool.cpp

#include "ThreadPool.h"
#include <iostream>
ThreadPool* ThreadPool::pool = nullptr;
ThreadPool::ThreadPool() {
    stop = false;
    threadNum = 4;
    for (int i = 0; i < threadNum; ++i) {
        threads.emplace_back([this]() {
            while (1) {
                std::unique_lock<std::mutex> locker(mtx);
                cond.wait(locker, [this]() {
                    return !tasks.empty() || stop;
                    });
                if (stop && tasks.empty()) {
                    return;
                }
                std::function<void()> task(std::move(tasks.front()));
                tasks.pop();
                task();// 执行这个任务
            }
            });
    }
}

ThreadPool::~ThreadPool() {
    {
        std::unique_lock<std::mutex> locker(mtx);
        stop = true;
    }
    cond.notify_all();
    for (auto& t : threads) {
        t.join();
    }
}

ThreadPool& ThreadPool::getInstance() { // 懒汉模式
    std::call_once(onceFlag, []() {
        std::cout << "懒汉模式：调用一次" << std::endl;
        pool = new ThreadPool();
        });
    return *pool;
}

•  main.cpp

#include <iostream>
#include "ThreadPool.h"
#include <thread>
void addTask() {
    ThreadPool& pool = ThreadPool::getInstance();
    pool.setNum(8);
    for (int i = 0; i < 10; ++i) {
        pool.enqueue([i]() {
            std::cout << "task : " << i << " is runing!" << std::endl;
            std::this_thread::sleep_for(std::chrono::microseconds(10));
            std::cout << "task : " << i << " is done!" << std::endl;
            });
    }
}

void test() {
    std::thread t1(addTask);
    std::thread t2(addTask);
    std::thread t3(addTask);
    t1.join();
    t2.join();
    t3.join();
}

int main() {
    test();
    return 0;
}

运行结果：

懒汉模式：调用一次
task : 0 is runing!
task : 0 is done!
task : 0 is runing!
task : 0 is done!
task : 1 is runing!
task : 1 is done!
task : 0 is runing!
task : 0 is done!
task : 1 is runing!
task : 1 is done!
task : 1 is runing!
task : 1 is done!
task : 2 is runing!
task : 2 is done!
task : 3 is runing!
task : 3 is done!
task : 2 is runing!
task : 2 is done!
task : 4 is runing!
task : 4 is done!
task : 3 is runing!
task : 3 is done!
task : 2 is runing!
task : 2 is done!
task : 3 is runing!
task : 3 is done!
task : 4 is runing!
task : 4 is done!
task : 5 is runing!
task : 5 is done!
task : 4 is runing!
task : 4 is done!
task : 5 is runing!
task : 5 is done!
task : 6 is runing!
task : 6 is done!
task : 7 is runing!
task : 7 is done!
task : 8 is runing!
task : 8 is done!
task : 9 is runing!
task : 9 is done!
task : 6 is runing!
task : 6 is done!
task : 7 is runing!
task : 7 is done!
task : 5 is runing!
task : 5 is done!
task : 6 is runing!
task : 6 is done!
task : 8 is runing!
task : 8 is done!
task : 9 is runing!
task : 9 is done!
task : 7 is runing!
task : 7 is done!
task : 8 is runing!
task : 8 is done!
task : 9 is runing!

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未完待续~