怎么让一个类只能定义一个对象

//将构造函数定义为私有

//

#include<iostream>

using namespace std;

class Base
{
public:
static GetInstance()
{
if(m_base == NULL)
{
m_base =new Base;
}
return m_base;
}
private:
Base(){;};
static Base* m_base;
};
Base* Base::m_base=NULL;

int main()
{
Base* base = Base::GetInstance();
}