C++中实现多线程的三种方式

1 背景

力扣1116题 打印零和奇偶数。

2 方法

方法1：原子操作

class ZeroEvenOdd {
private:
    int n;
    atomic<int> flag = 0;
public:
    ZeroEvenOdd(int n) {
        this->n = n;
    }

    // printNumber(x) outputs "x", where x is an integer.
    void zero(function<void(int)> printNumber) {
        for (int i = 1; i <= n; ++i) {
            while (flag != 0) {
                this_thread::yield();
            }
            printNumber(0);
            if (i % 2 == 0) {
                flag = 2;
            } else {
                flag = 1;
            }
        }
    }

    void even(function<void(int)> printNumber) {
        for (int i = 2; i <= n; i += 2) {
            while (flag != 2) {
                this_thread::yield();
            }
            printNumber(i);
            flag = 0;
        } 
    }

    void odd(function<void(int)> printNumber) {
        for (int i = 1; i <= n; i += 2) {
            while (flag != 1) {
                this_thread::yield();
            }
            printNumber(i);
            flag = 0;
        }
    }
};

方法2：互斥锁和变量控制

class ZeroEvenOdd {
private:
    int n;
    mutex m;
    condition_variable cv;
    int flag = 0;
public:
    ZeroEvenOdd(int n) {
        this->n = n;
    }

    // printNumber(x) outputs "x", where x is an integer.
    void zero(function<void(int)> printNumber) {
        for (int i = 1; i <= n; ++i) {
            unique_lock<mutex> lock(m);
            cv.wait(lock, [&] {return flag == 0;});
            printNumber(0);
            if (i % 2 == 0) {
                flag = 2;
            } else {
                flag = 1;
            }
            cv.notify_one();
        }
    }

    void even(function<void(int)> printNumber) {
        for (int i = 2; i <= n; i += 2) {
            unique_lock<mutex> lock(m);
            cv.wait(lock, [&] {return flag == 2;});
            printNumber(i);
            flag = 0;
            cv.notify_one();
        }
    }

    void odd(function<void(int)> printNumber) {
        for (int i = 1; i <= n; i += 2) {
            unique_lock<mutex> lock(m);
            cv.wait(lock, [&] {return flag == 1;});
            printNumber(i);
            flag = 0;
            cv.notify_one();
        }
    }
};

方法3：信号量

#include 

class ZeroEvenOdd {
private:
    int n;
    sem_t printOdd, printEven, numDone;
public:
    ZeroEvenOdd(int n) {
        this->n = n;
        sem_init(&printOdd, 0, 0);
        sem_init(&printEven, 0, 0);
        sem_init(&numDone, 0, 1);
    }

    // printNumber(x) outputs "x", where x is an integer.
    void zero(function<void(int)> printNumber) {
        for (int i = 1; i <= n; ++i) {
            sem_wait(&numDone);
            printNumber(0);
            if (i % 2 == 0) {
                sem_post(&printEven);
            } else {
                sem_post(&printOdd);
            }
        }
    }

    void even(function<void(int)> printNumber) {
        for (int i = 2; i <= n; i += 2) {
            sem_wait(&printEven);
            printNumber(i);
            sem_post(&numDone);
        }       
    }

    void odd(function<void(int)> printNumber) {
        for (int i = 1; i <=n; i += 2) {
            sem_wait(&printOdd);
            printNumber(i);
            sem_post(&numDone);
        }
    }
};