C + +: Friend class

First, the article for the reason

The last one wrote a friend function, this time write a sister chapter, continue to delve into the friend class.

Second, the definition

All member functions of a friend class are friend functions that also have a class. Have access to the hidden information in a class that contains private members and protected members.

When you want a class to be able to access a private member of a class, you can declare the class as a class of friend. The statement format that defines the friend class is as follows:

Friend class name;

Here: friend and class are keywords, and the class name must be a defined class in the program.

Iii. examples

#include <iostream>using namespace STD;classradius{friend classCircle;//Declare a friend class of circle as Radius    friend voidShow_r (Radius &n);//Declare show_r as friend function Public: Radius (intX) {r = x;} ~radius () {}Private:intR;};voidShow_r (Radius &n) {cout<<"The radius of the circle is:"<<n.r<<endl;//Call the RADIUS object's private member variable R}classcircle{ Public: Circle () {} ~circle () {}DoubleArea (Radius a) {s = A.R * A.R *3.1415926;//Call the RADIUS object's private member variable R    returns;}Private:Doubles;};intMainintargcChar*argv[]) {Radius Objradius (9);    Circle objcircle; Show_r (Objradius);cout<<"area is:"<<objcircle.area (Objradius) <<endl;return 0;}

C + +: Friend class