友元函數 無法訪問 類私有成員

#include<iostream> 

02	using namespace std;

03	class cylinder

04

{

05	friend istream operator>>(istream& is,cylinder &cy);

06

public:

07	inline double square()

08	{       return length*(width+height)*2+width*height*2;    }

09	inline double volume()

10	{      return length*width*height;      }

11

private:

12	double length;

13	double width;

14	double height;

15

};

16	istream operator>>(istream is,cylinder &cy)

17

{

18	cout<<"input length:"<<endl;

19	is>>cy.length;

20	cout<<"input width:"<<endl;

21	is>>cy.width;

22	cout<<"input height:"<<endl;

23	is>>cy.height;

24	return is;

25

}

26	int main()

27

{

28	cylinder first;

29	cin>>first;

30	cout<<first.square()<<endl;

31	cout<<first.volume()<<endl;

32

return 0;

33

}

這些代碼在VC6.0中不能被編譯通過:提示不能訪問私有成員,沒有這個訪問權限

改成這樣就可以了,代碼如下:

view sourceprint?

01	#include<iostream>

02	using std::cin;

03	using std::endl; using std::cout;

04	using std::ostream;

05	using std::istream;

06	using std::ostream;

07	class cylinder

08

{

09	friend istream operator>>(istream& is,cylinder &cy);

10

public:

11	inline double square()

12	{       return length*(width+height)*2+width*height*2;    }

13	inline double volume()

14	{      return length*width*height;      }

15

private:

16	double length;

17	double width;

18	double height;

19

};

20	istream operator>>(istream is,cylinder &cy)

21

{

22	cout<<"input length:"<<endl;

23	is>>cy.length;

24	cout<<"input width:"<<endl;

25	is>>cy.width;

26	cout<<"input height:"<<endl;

27	is>>cy.height;

28	return is;

29

}

30	int main()

31

{

32	cylinder first;

33	cin>>first;

34	cout<<first.square()<<endl;

35	cout<<first.volume()<<endl;

36

return 0;

37

}

原因:

這據說是VC的一個經典BUG。和namespace也有關.

只要含有using namespace std; 就會提示友員函數沒有訪問私有成員的權限。

解決方法：去掉using namespace std;換成更小的名字空間。

例如:
含有#include <string>就要加上using std::string
含有#include <fstream>就要加上using std::fstream
含有#include <iostream>就要加上using std::cin; using std::cout; using std::ostream; using std::istream; using std::endl; 等等,需要什麼即可通過using聲明什麼.

下面給出流浪給的解決辦法:

 

view sourceprint?

01

//方法一：

02

//提前聲明

03	class cylinder;

04	istream &operator>>(istream& is,cylinder &cy);

05

06

//方法二：

07	//不用命名空間 或者 像晨雨那樣寫

08	#include<iostream.h>

09

10

//方法三：

11

12	class cylinder

13

{

14	friend istream &operator>>(istream& is,cylinder &cy)//寫在類裏面

15

{

16	cout<<"input length:"<<endl;

17	is>>cy.length;

18	cout<<"input width:"<<endl;

19	is>>cy.width;

20	cout<<"input height:"<<endl;

21	is>>cy.height;

22	return is;

23

24

}

25	..........

26

27	//方法四：打SP6補丁，貌似不好使。。。(呵呵,是貌似也沒用)

28

29	//方法五：換別的對標準C++支持好的編譯器，如DEV C++/。。。(呵呵)