两个线程交替打印的各种解决方法

 

题目：https://leetcode-cn.com/problems/print-foobar-alternately/

 

方法1：模仿信号量机制，使用两个整数来控制先后执行顺序。

class FooBar {
private:
	int n;

public:
	FooBar(int n) {
		this->n = n;

	}

	void foo(function printFoo) {

		for (int i = 0; i < n; i++) {

			// printFoo() outputs "foo". Do not change or remove this line.
		//相当于信号量的wait操作：	
                while (s2 < 1) {
				std::this_thread::sleep_for(std::chrono::milliseconds(1));
			}
			s2--;  

			printFoo();
                //相当于信号量的inc操作：	
			s1++;
		}
	}

	void bar(function printBar) {

		for (int i = 0; i < n; i++) {

			// printBar() outputs "bar". Do not change or remove this line.
			while (s1 < 1) {
				std::this_thread::sleep_for(std::chrono::milliseconds(1));//注释掉会超时错误
			}
			s1--;

			printBar();
			s2++;
		}
	}

private:
	 std::atomic s1=0; //场景较为简单，不使用原子类型也可以
	 std::atomic s2=1;
	int s1 = 0;
	int s2 = 1;
};

方法2：借助c++ std::mutex锁，本质上与方法1一样，时间消耗略增多，空间消耗也略增多。

class FooBar {
private:
    int n;

public:
    FooBar(int n) {
        this->n = n;
        m1.lock();
        
    }

    void foo(function printFoo) {
        
        for (int i = 0; i < n; i++) {
            
        	// printFoo() outputs "foo". Do not change or remove this line.
           
            m2.lock();
        	printFoo();
            m1.unlock();
        }
    }

    void bar(function printBar) {
        
        for (int i = 0; i < n; i++) {
            
        	// printBar() outputs "bar". Do not change or remove this line.
            m1.lock();
        	printBar();
            m2.unlock();
        }
    }

private:
  std::mutex m1;
  std::mutex m2;
};

方法3：借助条件变量， 时间和空间效率，完胜前两种方法：

C++条件变量--std::condition_variable的使用方法：https://blog.csdn.net/qq_35865125/article/details/108859681

class FooBar {
private:
    int n;

public:
    FooBar(int n) {
        this->n = n;

        m_foo_printed = false;
        m_bar_printed = true;
    }

    void foo(function printFoo) {

        for (int i = 0; i < n; i++) {

            unique_lock lck(m_mtx);

            while(!m_bar_printed)
            {
                m_cv.wait(lck);  //等bar打印完
            }

            // printFoo() outputs "foo". Do not change or remove this line.
            printFoo();

            m_foo_printed = true;
            m_bar_printed = false;
            m_cv.notify_one();
        }
    }

    void bar(function printBar) {

        for (int i = 0; i < n; i++) {

            unique_lock lck(m_mtx);

            while(!m_foo_printed)
            {
                m_cv.wait(lck);  //等foo打印完
            }

            // printBar() outputs "bar". Do not change or remove this line.
            printBar();

            m_bar_printed = true;
            m_foo_printed = false;
            m_cv.notify_one();
        }
    }

private:
    mutex m_mtx;
    condition_variable m_cv;
    bool m_foo_printed; //foo打印完了
    bool m_bar_printed; //bar打印完了
};