Friend Class A friend class can access private and protected members of other class in which it is declared as friend. It is sometimes useful to allow a particular class to access private members of other class. For example, a LinkedList class may be allowed to access private members of Node.
class Node {private: int key;Node* next;/* Other members of Node Class */ // Now class LinkedList can// access private members of Nodefriend class LinkedList;}; |
Friend function
If a function is defined as a friend function in C++, then the protected and private data of a class can be accessed using the function.
By using the keyword friend compiler knows the given function is a friend function.
For accessing the data, the declaration of a friend function should be done inside the body of a class starting with the keyword friend.
Declaration of friend function
class class_name
{
friend data_type function_name(argument/s); // syntax of friend function.
};
In the above declaration, the friend function is preceded by the keyword friend. The function can be defined anywhere in the program like a normal C++ function. The function definition does not use either the keyword friend or scope resolution operator.
Characteristics of a Friend function:
- The function is not in the scope of the class to which it has been declared as a friend.
- It cannot be called using the object as it is not in the scope of that class.
- It can be invoked like a normal function without using the object.
- It cannot access the member names directly and has to use an object name and dot membership operator with the member name.
- It can be declared either in the private or the public part.
C++ friend function Example
Let’s see the simple example of C++ friend function used to print the length of a box.
#include <iostream>
using namespace std;
class Box
{
private:
int length;
public:
Box(): length(0) { }
friend int printLength(Box); //friend function
};
int printLength(Box b)
{
b.length += 10;
return b.length;
}
int main()
{
Box b;
cout<<“Length of box: “<< printLength(b)<<endl;
return 0;
}
Output:
Length of box: 10