请问你用过哪些设计模式，介绍一下单例模式的多线程安全问题

//懒汉模式
class Singleton
{
private:
    Singleton(){}
private:
    static Singleton* instance;
public:
    static Singleton* getInstance();
};
Singleton* Singleton::instance = NULL;
Singleton* Singleton::getInstance()
{
    if( instance == NULL )
    {
         instance = new Singleton();
    }
    return instance;
}
/*这样的代码在多线程场景存在一个问题，当某个线程调用getInstance并执行到new Singleton时恰好被挂起
  此时另一个线程也调用getInstance发现instance仍然为空又会调用new Singleton于是就产生了多个Singleton实例，一个
  解决方法就是加锁,如下：
*/
class Singleton
{
private:
    Singleton(){}
private:
    static Singleton* instance;
    static pthread_mutex_t mutex;
public:
    static Singleton* getInstance();
};
Singleton* Singleton::instance = NULL;
pthread_mutex_t Singleton::mutex = PTHREAD_MUTEX_INITIALIZER;
Singleton* Singleton::getInstance()
{
    if( instance == NULL )
    {
        pthread_mutex_lock( &mutex );
        if( instance == NULL )
        {
            instance = new Singleton();
        }
        pthread_mutex_unlock( &mutex );
    }
    return instance;
}
//饿汉模式
class Singleton
{
private:
    Singleton(){}
private:
    static Singleton* instance;
public:
    static Singleton* getInstance();
};
Singleton* Singleton::instance = new Singleton(); //这儿其实是定义静态变量，类中只是声明，不要觉得私有变量为什么在类外还能访问
Singleton* Singleton::getInstance()
{
    return instance;
}