Friend

• Friend is a mechanism provided by C + + to destroy data encapsulation and data hiding.
• By declaring a module as a friend of another module, one module can refer to the hidden information in another module.
• You can use friend functions and friend classes.
• In order to ensure the integrity of data and the principle of data encapsulation and hiding, it is recommended to use as few friends as possible.

friend function

• If you define a friend function (it can be a member function of another class or a non member function that doesn't belong to any class) elsewhere in this class, you can declare it with friend in the body of this class, and this function will become a friend function of this class, and you can access the private members of this class.
• Friend function is a non class member function described by the keyword friend in class declaration. In its function body, it can access private member, protected member and public member by object name.
• Function: increase flexibility, so that programmers can make reasonable choices in encapsulation and rapidity.
• Members in an access object must pass through the object name.

1. Declare a normal function as a friend function of this type:

   #include <iostream>
   using namespace std;
   class Box{
   public:
   	Box(){}
   	Box(int h ,int w):height(h),width(w){}
   	void volume();
   	static void show();
   	friend void display(Box &b);
   	static int length;
   private:
   	int height;
   	int width;
   	static int c;
   };
   void Box::volume() {
   	cout<<"The volume is:"<<height<<"*"<<width<<"*"<<length<<"="<<height * width*length<<endl;
   }
   void Box::show() {
   	cout << "Static private member c: " << c << "   Static public member length: " <<length<< endl;
   }
   int Box::length = 5;
   int Box::c = 6;
   void display(Box &b) {
   	b.width = 3;
   	b.height = 3;
   	b.c = 3;
   	b.length = 3;
   	b.volume();
   	b.show();
   }
   void main() {
   	Box b;   //Call normal constructor 
   	display(b);
   	/*
   	   Volume: 3 * 3 * 3 = 27
         Static private member c: 3 static public member length: 3
   	*/
   }
   

2. Friend member functions (member functions of other classes)

   #include <iostream>
   using namespace std;
   class Display;  //Declaration of Box class
   class Box {
   public:
   	Box() {}
   	Box(int h, int w) :height(h), width(w) {}
   	void volume(Display &);                        //Member arguments must be Display class objects
   	static int length;
   private:
   	int height;
   	int width;
   	static int c;
   };
   class Display {
   public:
   	Display() { a = 10; b = 5; }
   	void display();
   	friend void Box::volume(Display &);//Take volume in the Box class as the friend function of this class
   private:
   	int a;
   	int b;
   };
   void Display::display() {
   	cout << "a=" << a << "  b=" << b << endl;
   }
   
   void Box::volume(Display &d) {
   	d.display();    //Reference to Display private variable must be added with object name
   	d.a = 20;
   	d.b = 20;
   	d.display();
   	cout << "The volume is:" << height << "*" << width << "*" << length << "=" << height * width*length << endl;
   }
   
   int Box::length = 5;
   int Box::c = 6;
   
   void main() {
   
   
   	Display d;
   	Box b(7,7);
   	b.volume(d);
   
   	/*
   	   a=10  b=5
          a=20  b=20
          Volume: 7 * 7 * 5 = 245
   	*/
   }
   

Friends

1. Class is a friend of another class, all members of this class can access private members of the other class.
2. Declaration syntax: use friend modifier to describe friend class name in another class
3. If class B is declared as a friend of class A, the member functions of class B can access the private and protected data of class A, but the member functions of class a cannot access the private and protected data of class B.