RAII 想到AutoLock，Mutex类和C++11中的unique_lock

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#c++ #linux #c++11 #线程安全

RAII（Resource Acquisition Is Initialization）其定义直接引用The C++ Programming Language 4th Edition

“The technique of acquiring resources in a constructor and releasing them in a destructor, known as Resource Acquisition Is Initialization or RAII”

我浅显的理解就是资源的获取（锁，数据库链接，文件等）和释放分别在构造函数和析构函数中实现。由此想到了之前的一篇文章C++简洁实现线程安全单例类里面的两个类：AutoLock和Mutex，它们就是典型的RAII类，Mutex的构造函数中init了一个mutex，析构函数中则destroy了它，AutoLock在构造函数中获得了一个mutex，在析构函数中则释放了它，看代码就了然。

#define DISABLE_COPY_AND_ASSIGN(T) \  
        T(const T &); \  
        T& operator=(const T &)  
struct Recursive {};
class Mutex {  
public:  
    Mutex() {  
        pthread_mutex_init(&_mutex, NULL);  
    }  
    explicit Mutex(Recursive) {
        pthread_mutexattr_t attr;
        pthread_mutexattr_init(&attr);
        pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE_NP);
        pthread_mutex_init(&_mutex, &attr);
        pthread_mutexattr_destroy(&attr);
    }
    ~Mutex() {  
        pthread_mutex_destroy(&_mutex);  
    }  
      
    int lock() {  
        return pthread_mutex_lock(&_mutex);  
    }  
      
    int trylock() {  
        return pthread_mutex_trylock(&_mutex);  
    }  
      
    int unlock() {  
        return pthread_mutex_unlock(&_mutex);  
    }  
      
private:  
    pthread_mutex_t _mutex;  
    // RAII class should not be able to be copied
    DISABLE_COPY_AND_ASSIGN(Mutex); 
};  
  
template <class T>  
class AutoLock {  
public:  
    explicit AutoLock(T* t) : _t(t) {  
        _t.lock();  
    }  
    ~AutoLock() {  
        _t.unlock();  
    }  
private:  
    T* _t;  
    // RAII class should not be able to be copied
    DISABLE_COPY_AND_ASSIGN(AutoLock);  
};

其实在C++11中的std::mutex和std::unique_lock已经实现了上述两个类的功能，引用The C++ Programming Language 4th Edition中的代码：

mutex m; // used to protect access to shared data
// ...
void f()
{
	unique_lock<mutex> lck {m}; // acquire the mutex m
	// ... manipulate shared data ...
}

书中的解释是：

A thread will not proceed until lck’s constructor has acquired its mutex, m (§5.3.4).

The corresponding destructor releases the resource. So, in this example, unique_lock’s destructor

releases the mutex when the thread of control leaves f() (through a return, by “falling off the

end of the function,” or through an exception throw).

This is an application of the “Resource Acquisition Is Initialization” technique (RAII; §3.2.1.2, §13.3).

This technique is fundamental to the idiomatic handling of resources in C++.

Containers (such as vector and map), string, and iostream manage their

resources (such as file handles and buffers) similarly.