二级指针的应用场景之一：在函数外定义指针，在函数中对指针进行赋值，函数调用结束后，实现指针赋值；指针引用代替二级指针实现函数内指针赋值。

 函数内二级指针完成函数外定义指针赋值：

struct teacher{
	int id;
	char name[64];
};

int get_mem(struct teacher **tpp){
	struct teacher *tp = NULL;
	tp = (struct teacher*)malloc(sizeof(struct teacher));
	if(tp == NULL){
		return -1;
	}
	tp->id = 100;
	strcpy(tp->name,"lisi");
	*tpp = tp;
	return 0;
}

int get_mem2(struct teacher* &tpp){
	tpp = (struct teacher*)malloc(sizeof(struct teacher));
	if(tpp == NULL){
		return -1;
	}
	tpp->id = 1000;
	strcpy(tpp->name,"wang5");
	return 0;
}

void free_teacher(struct teacher **tpp){
	if(tpp == NULL)
		return;
	struct teacher *tp = *tpp;
	if(tp != NULL){
		free(tp);
		*tpp = NULL;
	}
}
int main(void){
	
	struct teacher *tp = NULL;
	get_mem(&tp);
	cout<<tp->id<<" "<<tp->name<<endl;
	free_teacher(&tp);
	return 0;
}

用指针引用实现函数外定义指针，在函数内对指针赋值：

struct teacher{
	int id;
	char name[64];
};


int get_mem2(struct teacher* &tpp){
	tpp = (struct teacher*)malloc(sizeof(struct teacher));
	if(tpp == NULL){
		return -1;
	}
	tpp->id = 1000;
	strcpy(tpp->name,"wang5");
	return 0;
}

void free_teacher2(struct teacher* &tpp){
	if(tpp != NULL){
		free(tpp);
		tpp = NULL;
	}
}
int main(void){
	
	struct teacher *tp = NULL;
	get_mem2(tp);
	cout<<tp->id<<" "<<tp->name<<endl;
	free_teacher2(tp);
	return 0;
}