by A Friend function is a function defined outside the class, but it has access to all private and protected members of the class. To declare a friend function, it’s prototype must be declared inside the class, preceding it with keyword “friend”. For eg:
class Demo
{
private:
int m;
int y;
public:
friend int sum (Demo d);
void print_val ();
Demo (int m, int y);
};
Let’s have a look at an example of friend function and it’s usage.
#include <iostream>
class Demo
{
private:
int m;
int y;
public:
friend int sum (Demo d);
void print_val ();
Demo (int m, int y);
};
Demo::Demo (int x, int y): m (x), y (y)
{}
int sum (Demo d)
{
return d.m + d.y;
}
void Demo::print_val ()
{
std::cout << "Values are, x: " << m << " y: " << y << std::endl;
}
int main ()
{
Demo d (10,20);
d.print_val ();
std::cout << "SUM: " << sum (d) << std::endl;
}
Let’s analyze the output of the above function.
Values are, x: 10 y: 20
SUM: 30
Important Points
- These friend functions can be very useful in operator overloading.
- These friend functions make the I/O operations easier.
- These functions may be desirable in case of inter-related classes.
- Derived class doesn’t inherit friend function.
- Friend function can’t be defined as Static functions.
Friend Class
It is possible to define a whole class as a friend to other class. When a class is defined as friend to another class then it got access to all private data members and member functions. For eg:
#include <iostream>
class Demo
{
private:
int m;
int y;
public:
friend int sum (Demo d);
void print_val ();
Demo (int m, int y);
friend class DoubleDemo;
};
class DoubleDemo
{
public:
void invoke ();
};
void DoubleDemo::invoke ()
{
Demo d (10,20);
d.print_val ();
std::cout << "SUM: " << (d.m + d.y) << std::endl;
}
Demo::Demo (int x, int y): m (x), y (y)
{}
int sum (Demo d)
{
return d.m + d.y;
}
void Demo::print_val ()
{
std::cout << "Values are, x: " << m << " y: " << y << std::endl;
}
int main ()
{
DoubleDemo d;
d.invoke ();
}
Let’s analyze the output of above program.
Values are, x: 10 y: 20
SUM: 30
