Friend and Friend Class

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0.Summary

Friends provide a mechanism for ordinary functions or class member functions to access private or protected members in another class.In other words, there are two forms of friends:

（1）Friend Function：Ordinary functions access a private or protected member of a class.

（2）Friend Class：Member functions in class a access private or protected members in class B

Advantages: improve the efficiency of the program.

Disadvantages: it destroys the encapsulation of classes and the transparency of data. Conclusion：

• Access to private members
• Breaking encapsulation
• Friendship is not transitive
• The unidirectionality of friend relationship
• There are no restrictions on the form and number of friend declarations

1.Friend function

It is declared in any region of the class declaration, and the definition is outside the class.

friend <type><name>(<Parameter table>);

Note that the friend function is only a common function, not a class member function of this class. It can be called anywhere. In the friend function, private or protected members of the class can be accessed through the object name.

Code：friend_func.cpp

#include <iostream>

using namespace std;

class A
{
public:
    A(int _a):a(_a){};
    friend int geta(A &ca);  ///< Friend function
private:
    int a;
};

int geta(A &ca) 
{
    return ca.a;
}

int main()
{
    A a(3);    
    cout<<geta(a)<<endl;

    return 0;
}

2.Friend Class

The declaration of a friend class is in the declaration of the class, and the implementation is outside the class.

friend class <friend class name>;

Class B is a friend of class A, so class B can directly access private members of A. Code：friend_class.cpp

#include <iostream>

using namespace std;

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

class B
{
public:
    int getb(A ca) {
        return  ca.a; 
    };
};

int main() 
{
    A a(3);
    B b;
    cout<<b.getb(a)<<endl;
    return 0;
}

3.Attention

• Friendship has no inheritance If class B is a friend of class A and class C inherits from Class A, then friend class B cannot directly access private or protected members of class C.
• Friendship is not transitive If class B is a friend of class A and class C is a friend of class B, then friend class C cannot directly access private or protected members of class A, that is, there is no such relationship as "friend of friend".