Friend function and friend class of C + +

With the Friend keyword, we can declare a function that is not part of the current class in the current class, which can be a friend function of the current class.

#include <iostream>
using namespace Std;

Class Book
{
Public
Book ()
{
cout <<this->price << Endl; Here is a member variable that is not assigned to use, which is undesirable
cout << this->title << Endl;

}
Book (char* A, double p);
friend void display (book &b);
Private
Double Price;
char * title;
};

Book::book (char* A, double p)
{
title = A;
Price = P;
}

void display (book &b)
{
cout<< "The price" <<b.title<< "is $" <<b.price<<endl;
}

int main ()
{
Book Hello;
Display (Hello);
Book Alice ("Alice in Wonderland", 29.9);
Display (Alice);
Book Harry ("Harry Potter", 49.9);
Display (Harry);

return 0;
}

result:6.95314e-310

While we know that we can access the private variables in the class with the meta-function, it is also important to remember that member variables that are not initialized cannot be used, which can cause the program to crash.

In this example, display is a top-level function, in the book class, we declare it as a friend function with the Friend keyword, and as a result, in the display function body, we can access the title and price member variables of the private property. This is the function of the friend function. A friend function can access a private member in this class. If the display function is not a friend function of the book class, the member function of the public property must also be called in the body of the function. In this case, it is important to note that the parameter of the friend function is a reference to the class object, and that you must add the object name when accessing the private member variable.

Except that the top-level function can be defined as a friend function, the member functions of other classes can also be declared as friend functions of this class, as shown in Example 2.

Example 2

#include <iostream>
using namespace Std;

Class time;

Class Date
{
Public
Date (int y,int m,int D);
void display (time &t);
Private
int year;
int month;
int day;
};

Class time
{
Public
Time (int s,int m,int h);
friend void Date::d isplay (Time & T);
Private
int second;
int minute;
int hour;
};

Time::time (int s,int m,int h)
{
Second = s;
minute = m;
hour = h;
}

Date::d ate (int y,int m,int D)
{
Year = y;
month = m;
Day = D;
}

void Date::d isplay (Time &t)
{
cout<< "The time is:" <<endl;
cout<<year<< "/" <<month<< "/" <<day<< "";
cout<<t.hour<< ":" <<t.minute<< ":" <<t.second<<endl;
}

int main ()
{
Date d (2015,1,16);
Time t (20,2,30);
D.display (t);
return 0;
}

Friend function and friend class of C + +