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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;
}