写单例模式遇到的问题

//为什么这样写单例模式还是能构造两个对象
#include<stdlib.h>
 #include<pthread.h>
 #include<iostream>
 using namespace std;
 
 template<typename T>
 class Singleton
 {
     public:
         static T& get_instance()
         {
             pthread_once(&ponce_,&Singleton::init);
  //           assert(value!=NULL);
             return *value;
         }
     private:
         Singleton();
         ~Singleton();
 
         static void init()
         {
             cout<<"调用了初始化函数"<<endl;
             value = new T(10);
         }
 
     private:
         static pthread_once_t ponce_;
         static T* value;
 };
 
 template<typename T>
 pthread_once_t Singleton<T>::ponce_ = PTHREAD_ONCE_INIT;
 
 template<typename T>
 T* Singleton<T>::value = NULL;

int main()
 {
     int st = Singleton<int>::get_instance();
     cout<<st<<endl;
     int st1 = Singleton<int>::get_instance();
     cout<<st1<<endl;
     return 0;
 }