singleton模式 
#include "stdio.h"
class singleton
{
	private static singleton instance = new singleton();//不能在这里初始化
	singleton(){}
	~singleton(){}
public:
	static B * getInstance()
	{
		if(instance == NULL) 
			instance = new singleton;
		return b;
	}
};
singleton* singleton::b = NULL;  //静态成员初始化, 不初始化会出错
...
  
