分享C++11代码片段－mutex

矫情吗；* 2022-07-16 01:55 210阅读 0赞

mutex即是互斥的意思。

直接看代码吧：

#include <iostream>
    #include <thread>
    #include <mutex>
    
    using namespace std;
    
    void RunUnsynchronized() {
      long sharedCounter = 0;
    
      /* simply increment in each thread the shared counter */
      auto increment = [&sharedCounter](){
        for(int i = 0; i < 1000; ++i)
          ++sharedCounter;
      };
    
      thread firstThread(increment);
      thread secondThread(increment);
    
      firstThread.join();
      secondThread.join();
    
      cout << "unsynchronized shared counter: " << sharedCounter << " (expected to be 2000)" << endl;
    }
    
    void RunSynchronized() {
      long sharedCounter = 0;
      mutex sharedMutex;
    
      /* locks the mutex and increments; guard is destroyed by leaving scope */
      auto increment = [&sharedCounter, &sharedMutex](){
        lock_guard<mutex> scopedMutex(sharedMutex);
    
        for(int i = 0; i < 1000; ++i)
          ++sharedCounter;
      };
    
      thread first_thread(increment);
      thread second_thread(increment);
    
      first_thread.join();
      second_thread.join();
    
      cout << "synchronized shared counter: " << sharedCounter << " (expected to be 2000)" << endl;
    }
    
    int main() {
      RunUnsynchronized();
      RunSynchronized();
    }

上面的代码挺简单的，这里就不做过多的解释了。只是有一个问题：

**为什么使用了std::lock\_guard 而不是 mutex.lock() 和 mutex.unlock()？**

先看一下cppreference上怎么说的：  
[http://en.cppreference.com/w/cpp/thread/mutex][http_en.cppreference.com_w_cpp_thread_mutex]

std::mutex is usually not accessed directly: std::unique\_lock and std::lock\_guard are used to manage locking in an exception-safe manner.

说白了，就是使用guard可以通过RAII确保异常安全，避免内存泄漏。

如果您不懂RAII，看这里：  
《c++中的RAII》  
[http://blog.csdn.net/wangshubo1989/article/details/52133213][http_blog.csdn.net_wangshubo1989_article_details_52133213]

**unique\_lock和lock\_guard有什么区别？**

lock\_guard使用起来比较简单，除了构造函数外没有其他member function，在整个区域都有效。

unique\_guard除了lock\_guard的功能外，提供了更多的member\_function，相对来说更灵活一些。

[http_en.cppreference.com_w_cpp_thread_mutex]: http://en.cppreference.com/w/cpp/thread/mutex
[http_blog.csdn.net_wangshubo1989_article_details_52133213]: http://blog.csdn.net/wangshubo1989/article/details/52133213