C++ 多线程顺序打印

打印要求：

三个打印线程顺序进行。

线程要求如下：

线程A：打印A

线程B：打印B

线程C：打印C

打印结果：

A
B
C
A
B
C
A
B
C
A
B
C
A
B
C 

法一：需要锁和共享变量

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

using std::cout;
using std::endl;
using std::thread;
using std::mutex;
using std::condition_variable;
using std::unique_lock;

enum CVFLAG
{
    INIT,
    A_DONE,
    B_DONE,
    C_DONE
};

mutex mtx;
condition_variable cv;
CVFLAG flag = INIT;

void printA()
{
    unique_lock<mutex> lock(mtx);
    cout << "A" << endl;
    flag = A_DONE;
    cv.notify_all();
}

void printB()
{
    unique_lock<mutex> lock(mtx);
    cv.wait(lock, [] { return flag == 1; });
    cout << "B" << endl;
    flag = B_DONE;
    cv.notify_all();
}

void printC()
{
    unique_lock<mutex> lock(mtx);
    cv.wait(lock, [] { return flag == 2; });
    cout << "C" << endl;
    flag = C_DONE;
    cv.notify_all();
}

int main()
{
    int i = 0;
    while (i < 5)
    {
        thread tA(printA);
        thread tB(printB);
        thread tC(printC);
        tA.join();
        tB.join();
        tC.join();
        ++i;
    }
    return 0;
}

法二：简单粗暴，调整join的顺序

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

using std::cout;
using std::endl;
using std::thread;

void printA()
{
    cout << "A" << endl;
}

void printB()
{
    cout << "B" << endl;
}

void printC()
{
    cout << "C" << endl;
}

int main()
{
    int i = 0;
    while (i < 5)
    {
        thread tA(printA);
        tA.join();
        
        thread tB(printB);
        tB.join();
        
        thread tC(printC);
        tC.join();
        ++i;
    }
    return 0;
}