singleton懒汉式与饿汉式以及线程安全（双检锁）

/*************************************************************************> File Name: Singleton.cpp> Created Time: Tue 31 Oct 2017 10:20:58 PM CST ************************************************************************/#include <iostream>using namespace std;#include <pthread.h>//懒汉式 （创建的时候采取new实例）以时间换取空间，线程不安全class singleton_L{    public:        static singleton_L* getinstance()        {            if(instance == NULL)            {                instance = new singleton_L();            }            return instance;        }        ~singleton_L()        {            if(instance != NULL)            {                delete instance;                instance = NULL;            }        }    private:        singleton_L(){}        static singleton_L*  instance;};singleton_L*  singleton_L::instance = NULL;//饿汉式（在定义实例的时候就去new对象）以空间换时间class singleton_E{    public:        static singleton_E* getinstance()        {            return instance;        }    private:        singleton_E(){}        static singleton_E* instance;};singleton_E* singleton_E::instance = new singleton_E();//懒汉式 线程安全式（双检锁）/*  所谓双重检查加锁机制，指的是：并不是每次进入getInstance方法都需要同步，而是先不同步，进入方法过后，先检查实例是否存在，如果不存在才进入下面的同步块，这是第一重检查。进入同步块过后，再次检查实例是否存在，如果不存在，就在同步的情况下创建一个实例，这是第二重检查。这样一来，就只需要同步一次了，从而减少了多次在同步情况下进行判断所浪费的时间。*/class singleton_L_Lock{    public:        static singleton_L_Lock* getinstance()        {            if(instance == NULL)            {                pthread_mutex_lock(&mutex);                if(instance == NULL)                    instance = new singleton_L_Lock();                pthread_mutex_unlock(&mutex);            }            return instance;        }        ~singleton_L_Lock()        {            if(instance != NULL)            {                delete instance;                instance = NULL;            }        }    public:        static pthread_mutex_t mutex;    private:        singleton_L_Lock()        {            pthread_mutex_init(&mutex, NULL);        }    private:        static singleton_L_Lock*  instance;};singleton_L_Lock* singleton_L_Lock::instance = NULL;pthread_mutex_t   singleton_L_Lock::mutex;  int main(){    singleton_L* p  = singleton_L::getinstance();    singleton_L* p1 = singleton_L::getinstance();    singleton_L* p2 = singleton_L::getinstance();    cout << p  << endl;    cout << p1 << endl;    cout << p2 << endl;        cout << "=================" << endl;    singleton_E* E  = singleton_E::getinstance();    singleton_E* E1 = singleton_E::getinstance();    singleton_E* E2 = singleton_E::getinstance();    cout << E  << endl;    cout << E1 << endl;    cout << E2 << endl;    cout << "=================" << endl;    singleton_L_Lock* L  = singleton_L_Lock::getinstance();    singleton_L_Lock* L1 = singleton_L_Lock::getinstance();    singleton_L_Lock* L2 = singleton_L_Lock::getinstance();    cout << L  << endl;    cout << L1 << endl;    cout << L2 << endl;        return 0;}