线程安全单例模式(C++)

线程安全单例模式(C++)

饿汉模式

#include<iostream>
using namespace std;

class Singleton
{
    public:
        static Singleton *GetInstance()
        {
            return m_pInstance;
        }

    private:
        static Singleton *m_pInstance;
        Singleton() { cout << "Singleton()" << endl; };
        ~Singleton() { cout << "~Singleton()" << endl; };

        class Garbo     /* Garbo意为垃圾工人，任务是在 Singleton 实例生命周期结束后(即进程即将结束）回收该实例 */
        {
            public:
                ~Garbo()
                {
                    if(Singleton::m_pInstance)
                        delete m_pInstance;
                }
        };
        static Garbo garbo;
};

Singleton::Garbo Singleton::garbo;      /* 一定要初始化，否则程序结束时不会调用~Garbo() */
Singleton *Singleton::m_pInstance = new Singleton;  /* main函数之前就生成一个实例 */

int main(void)
{
    Singleton *p1 = Singleton::GetInstance();
    Singleton *p2 = Singleton::GetInstance();
    if(p1 == p2)
    {
        cout << "p1 == p2" << endl;
    }
    return 0;
}

$ g++ Singleton.cpp -o Singleton
$ ./Singleton
Singleton()
p1 == p2
~Singleton()

懒汉模式

懒汉模式：即第一次调用该类实例的时候才产生一个新的该类实例，并在以后仅返回此实例。

需要用锁，来保证其线程安全性：原因：多个线程可能进入判断是否已经存在实例的if语句，从而非线程安全.

使用double-check来保证线程安全.但是如果处理大量数据时，该锁才成为严重的性能瓶颈。

#include<iostream>
#include<pthread.h>
using namespace std;

pthread_mutex_t mutex;

class Singleton
{
    public:
        static Singleton *GetInstance()
        {
            if(NULL == m_pInstance)
            {
                pthread_mutex_lock(&mutex);
                if(m_pInstance == NULL)
                    m_pInstance = new Singleton;
                pthread_mutex_unlock(&mutex);
            }
            return m_pInstance;
        }

    private:
        static Singleton *m_pInstance;
        Singleton() { cout << "Singleton()" << endl; }
        ~Singleton() { cout << "~Singleton()" << endl; }

        class Garbo
        {
            public:
                ~Garbo()
                {
                    if(m_pInstance)
                        delete m_pInstance;
                }
        };
        static Garbo garbo;
};
Singleton::Garbo Singleton::garbo;          /* 静态类成员初始化 */
Singleton *Singleton::m_pInstance = NULL;   /* 静态类成员初始化 */
int main(void)
{
    pthread_mutex_init(&mutex, NULL);

    Singleton *p1 = Singleton::GetInstance();
    Singleton *p2 = Singleton::GetInstance();
    if(p1 == p2)
        cout << "p1 == p2" << endl;

    pthread_mutex_destroy(&mutex);
    return 0;
}

$ g++ Singleton.cpp -o Singleton
$ ./Singleton
Singleton()
p1 == p2
~Singleton()