VC6.0中友元函数无法访问类私有成员的解决办法

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#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;

}

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#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;

}

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//方法一：

//提前声明

class cylinder;

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

//方法二：

//不用命名空间

#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;

    }