友元

• 定义：类的特点是私有成员无法在作用域外访问，而友元函数是特权函数，允许访问私有成员。

• 语法：在函数或类前加friend。

• 例子：在message中，published每个人都可访问，secret只有自己可以访问。但有时也需要秘密接头人访问secret。

• 注意事项：

  1．友元关系不能被继承。

  2．友元关系是单向的，类A是类B的朋友，但类B不一定是类A的朋友。

  3．友元关系不具有传递性。类B是类A的朋友，类C是类B的朋友，但类C不一定是类A的朋友

14.1 普通成员函数作为类的友元

#include
using namespace std;
class message {
private:string secret;
public:string published;
public:message(string _secret, string _published) {secret = _secret;published = _published;}
private:friend void visiting(message &m) {cout <<"secret: "<< m.secret << endl;cout << "published: " << m.published << endl;}
};
int main()
{message paper("aaa","bbb");visiting(paper);
}

14.2 类的成员函数作为另一类的友元

#include
using namespace std;
class massege; //需要引用的类要在前面声明
class spy {
public:void visiting_1(massege& paper);//不做为友元void visiting_2(massege& paper);//做为友元
};
class massege {
public:string publishing;
private:string secret;
public:massege(string _publishing, string _secret) {publishing = _publishing;secret = _secret;}
private:friend void spy::visiting_2(massege &paper);
};
​
void spy::visiting_1(massege& paper) {cout << "visiting_1 publishing:" << paper.publishing << endl;cout << "visiting_1 can't visit secret!" << endl;
}
​
void spy::visiting_2(massege& paper) {cout << "visiting_2 publishing:" << paper.publishing << endl;cout << "visiting_2 secret:" << paper.secret << endl;
}
​
int main()
{massege paper("ppppp","sssss");spy YY;YY.visiting_1(paper);YY.visiting_2(paper);
}

14.3 整个类作为另一个类的友元

#include
using namespace std;
class massege; //需要引用的类要在前面声明
class spy {
public:void visiting_1(massege& paper);//不做为友元void visiting_2(massege& paper);//做为友元
};
class massege {
public:string publishing;
private:string secret;
public:massege(string _publishing, string _secret) {publishing = _publishing;secret = _secret;}
private:friend spy;
};
​
void spy::visiting_1(massege& paper) {cout << "visiting_1 publishing:" << paper.publishing << endl;cout << "visiting_2 secret:" << paper.secret << endl;
}
​
void spy::visiting_2(massege& paper) {cout << "visiting_2 publishing:" << paper.publishing << endl;cout << "visiting_2 secret:" << paper.secret << endl;
}
​
int main()
{massege paper("ppppp","sssss");spy YY;YY.visiting_1(paper);YY.visiting_2(paper);
}