友元

友元函数为类

class A{
private:
    int a;
public:
    A() { a = 55; }
    friend class B;
};

class B{
private:
    int b;
public:
    void showA(A &x) {
        cout << x.a << endl;
    }
};

int main() {
    A a1;
    B b1;
    b1.showA(a1);
    return 0;
}

输出
55

友元函数为类的函数

class B;

class A
{
public:
    void showB(B& );
};

class B
{
private:
    int b;
public:
    B()  {  b = 0; }
    friend void A::showB(B& x); // Friend function
};

void A::showB(B &x)
{
    // Since show() is friend of B, it can
    // access private members of B
    std::cout << "B::b = " << x.b;
}

int main()
{
    A a;
    B x;
    a.showB(x);
    return 0;
}

输出
B::b = 0

友元函数为全局函数

class A{
private:
    int a;
public:
    A(int x) { a = x; }
    friend void showA(A &x);
};

void showA(A &x) {
    cout << x.a << endl;
}

int main()
{
    A a(44);
    showA(a);
    return 0;
}

输出
44

---

以下讨论继承与友元的关系：

1. 友元函数在基函数中

class base{
protected:
    int x;
public:
    base(int a=0):x(a){}
    friend void show();
};

class base2:public base{
protected:
    int y;
public:
    base2(int b):y(b){}
};

void show() {
    base2 base_2(2);
    cout << base_2.x << endl;
    cout << base_2.y << endl;//出问题了
}

int main()
{
    show();
    return 0;
}

1. 友元函数在继承函数中

class base{
private:
    int x1;
protected:
    int x;
public:
    base(int a=0):x(a){}
};

class base2:public base{
private:
    int y;
public:
    base2(int b):y(b){}
    friend void show();
};

void show() {
    base2 base_2(2);
    cout << base_2.x1 << endl;
    cout << base_2.x << endl;
    cout << base_2.y << endl;
}

总结：
友元函数在基函数里面a，一定可以访问基函数a的private的数据，但是不能访问继承函数B的protected的数据：
友元函数在继承函数B里面，能够访问B的private数据，同时能够访问基函数A的protected的数据，但是不能访问A的private的数据。