懒汉模式：
class singleton
{
protected:
singleton()
{
pthread_mutex_init(&mutex);
}
private:
static singleton* p;
public:
static pthread_mutex_t mutex;
static singleton* initance();
};
pthread_mutex_t singleton::mutex;
singleton* singleton::p = NULL;
singleton* singleton::initance()
{
if (p == NULL)
{
pthread_mutex_lock(&mutex);
if (p == NULL)
p = new singleton();
pthread_mutex_unlock(&mutex);
}
return p;
}
