单例设计模式的实现有关问题。代码如下

#include <iostream>   
using namespace std;   
//单例类的C++实现   
class Singleton   
{   
private:   
 Singleton();//注意:构造方法私有   
 virtual ~Singleton();   
 static Singleton* instance;//惟一实例   
 int var;//成员变量(用于测试)   
public:   
 static Singleton* GetInstance();//工厂方法(用来获得实例)   
 int getVar();//获得var的值   
 void setVar(int);//设置var的值   
};   
//构造方法实现   
Singleton::Singleton()   
{   
 this->var = 20;   
 cout<<"Singleton Constructor"<<endl;   
}   
Singleton::~Singleton()   
{   
    delete instance;   
}   
//初始化静态成员   
Singleton* Singleton::instance=new Singleton();   
Singleton* Singleton::GetInstance()   
{   
 return instance;   
}   
//seter && getter含数   
int Singleton::getVar()   
{   
 return this->var;   
}   
void Singleton::setVar(int var)   
{   
 this->var = var;   
}   
//main   
int main(int argc, char* argv[])   
{   
 Singleton *ton1 = Singleton::GetInstance();   
 Singleton *ton2 = Singleton::GetInstance();   
 cout<<"ton1 var = "<<ton1->getVar()<<endl;   
 ton1->setVar(150);   
 cout<<"ton2 var = "<<ton2->getVar()<<endl;   
 return 0;   
}