VC6.0中友元函数无法访问类私有成员的解决办法

举个例子:

#include<iostream>
using namespace std;
class cylinder
{
friend istream operator>>(istream& is,cylinder &cy);
public:
inline double square()
{       return length*(width+height)*2+width*height*2;    }
inline double volume()
{      return length*width*height;      }
private:
double length;
double width;
double height;
};
istream operator>>(istream is,cylinder &cy)
{
cout<<"input length:"<<endl;
is>>cy.length;
cout<<"input width:"<<endl;
is>>cy.width;
cout<<"input height:"<<endl;
is>>cy.height;
return is;
}
int main()
{
cylinder first;
cin>>first;
cout<<first.square()<<endl;
cout<<first.volume()<<endl;
return 0;
}

这些代码在VC6.0中不能被编译通过:提示不能访问私有成员,没有这个访问权限

改成这样就可以了,代码如下:

#include<iostream>
using std::cin;
using std::endl; using std::cout;
using std::ostream;
using std::istream;
using std::ostream;
class cylinder
{
friend istream operator>>(istream& is,cylinder &cy);
public:
inline double square()
{       return length*(width+height)*2+width*height*2;    }
inline double volume()
{      return length*width*height;      }
private:
double length;
double width;
double height;
};
istream operator>>(istream is,cylinder &cy)
{
cout<<"input length:"<<endl;
is>>cy.length;
cout<<"input width:"<<endl;
is>>cy.width;
cout<<"input height:"<<endl;
is>>cy.height;
return is;
}
int main()
{
cylinder first;
cin>>first;
cout<<first.square()<<endl;
cout<<first.volume()<<endl;
return 0;
}

原因:

这据说是VC的一个经典BUG。和namespace也有关.

只要含有using namespace std; 就会提示友员函数没有访问私有成员的权限。

解决方法：去掉using namespace std;换成更小的名字空间。

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,需要什么即可通过using声明什么.

下面给出流浪给的解决办法:

//方法一：
//提前声明
class cylinder;
istream &operator>>(istream& is,cylinder &cy);

//方法二：
//不用命名空间 或者 像晨雨那样写
#include<iostream.h>

//方法三：

class cylinder
{
friend istream &operator>>(istream& is,cylinder &cy)//写在类里面
{
cout<<"input length:"<<endl;
is>>cy.length;
cout<<"input width:"<<endl;
is>>cy.width;
cout<<"input height:"<<endl;
is>>cy.height;
return is;

}
..........

//方法四：打SP6补丁，貌似不好使。。。(呵呵,是貌似也没用)

//方法五：换别的对标准C++支持好的编译器，如DEV C++/。。。(呵呵)