c++设计模式之单例模式

#include <iostream>using namespace std;class singleton{private:    singleton(){}    static singleton* p;public:    static singleton* instance()    {        if(NULL==p)        {            p=new singleton();        }        return p;    }};singleton* singleton::p=NULL;int main(){    singleton* a1=singleton::instance();    singleton* a2=singleton::instance();        cin.get();}