Friend Functions in C++ are a category of functions that can access private and protected members of a class while being a public function/outside the class.
You may wonder; is this even possible? Well, modern C++ says yes!
Let us look at how we can use these functions in C++.
Friend Function in C++
We call a function a friend function if the function definition is prefixed with the friend keyword.
We cannot use the friend prefix outside a class, so it can only be used in the member function declaration.
For example, if we want to use the friend keyword in the declaration, we can use it like this:
#include <iostream>
using namespace std;
class MyClass {
private:
int a, b;
public:
MyClass(int a, int b) {
// Constructor
this->a = a;
this->b = b;
}
void set_a(int val);
void set_b(int val);
int get_a() {
return a;
}
int get_b() {
return b;
}
// Declare a friend function of this class
friend void my_fun(MyClass& my_obj, int val, char option);
};
void my_fun(MyClass& my_obj, int val, char option) {
// Friend function that sets the private variables
// based on the option
if (option == 'a') {
// Set a
my_obj.a = val;
}
else {
// Set b
my_obj.b = val;
}
}
int main() {
MyClass my_obj(1, 2);
cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;
// Friend functions can be accessed from outside the class!
my_fun(my_obj, 20, 'a');
my_fun(my_obj, 40, 'b');
cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;
return 0;
}
Here, my_fun() is a friend function of MyClass, so it can access the private members too!
Notice that we can access a friend function from outside the class as well.
The above friend function sets the value of the private members a or b directly!
Output
my_obj.a = 1 and my_obj.b = 2
my_obj.a = 20 and my_obj.b = 40
A friend function can either be within the same class or even inside another class. But it must be a member function.
Let’s look at these two cases one by one.
Friend Function within the same class in C++
The above example showed us that we can use a friend function inside the same class.
class MyClass {
private:
int a, b;
public:
MyClass(int a, int b) {
// Constructor
this->a = a;
this->b = b;
}
void set_a(int val);
void set_b(int val);
int get_a() {
return a;
}
int get_b() {
return b;
}
// Declare a friend function of this class
// within the same class itself!
friend void my_fun(MyClass& my_obj, int val, char option);
};
We’ll move on to the next case; when a friend function is inside another class!
This is actually inside another topic, called Friend Classes, but they are directly related to each other.
Friend Classes in C++ – Friend Functions in another class
Any class B which has friend functions of another class A is called a Friend Class of A.
Similar to friend functions, any member function inside the friend class B can access private and protected members of A.
Let’s understand this more clearly using an example.
I will use the previous example, but I will add another class Student, and make it as the friend class of MyClass.
#include <iostream>
#include <string>
using namespace std;
class Student;
class MyClass {
private:
int a, b;
public:
MyClass(int a, int b) {
// Constructor
this->a = a;
this->b = b;
}
void set_a(int val);
void set_b(int val);
int get_a() {
return a;
}
int get_b() {
return b;
}
// Declare a friend function of this class
friend void my_fun(MyClass& my_obj, int val, char option);
// Make Student a friend Class of MyClass
friend Student;
};
Now, let’s write the Student class:
class Student {
private:
string name;
int marks;
public:
Student(string s_name, int s_marks) {
// Constructor
name = s_name;
marks = s_marks;
}
int get_marks() {
return marks;
}
string get_name() {
return name;
}
void set_marks(Student& stud, int s_marks) {
stud.marks = s_marks;
}
void set_name(Student& stud, string s_name) {
stud.name = s_name;
}
void change_a(MyClass& my_obj, int val) {
// You need to pass the object by reference.
// Otherwise, it will not reflect the changes on
// the original object
my_obj.a = val;
}
void change_b(MyClass& my_obj, int val) {
// You need to pass the object by reference.
// Otherwise, it will not reflect the changes on
// the original object
my_obj.b = val;
}
};
As you can see, I am using change_a() and change_b() to change the MyClass object attributes directly!
Let’s now run the complete snippet below.
#include <iostream>
#include <string>
using namespace std;
class Student;
class MyClass {
private:
int a, b;
public:
MyClass(int a, int b) {
// Constructor
this->a = a;
this->b = b;
}
void set_a(int val);
void set_b(int val);
int get_a() {
return a;
}
int get_b() {
return b;
}
// Declare a friend function of this class
friend void my_fun(MyClass& my_obj, int val, char option);
// Make Student a friend Class of MyClass
friend Student;
};
class Student {
private:
string name;
int marks;
public:
Student(string s_name, int s_marks) {
// Constructor
name = s_name;
marks = s_marks;
}
int get_marks() {
return marks;
}
string get_name() {
return name;
}
void set_marks(Student& stud, int s_marks) {
stud.marks = s_marks;
}
void set_name(Student& stud, string s_name) {
stud.name = s_name;
}
void change_a(MyClass& my_obj, int val) {
// You need to pass the object by reference.
// Otherwise, it will not reflect the changes on
// the original object
my_obj.a = val;
}
void change_b(MyClass& my_obj, int val) {
// You need to pass the object by reference.
// Otherwise, it will not reflect the changes on
// the original object
my_obj.b = val;
}
};
void my_fun(MyClass& my_obj, int val, char option) {
// Friend function that sets the private variables
// based on the option
if (option == 'a') {
// Set a
my_obj.a = val;
}
else {
// Set b
my_obj.b = val;
}
}
int main() {
MyClass my_obj(1, 2);
cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;
// Friend functions can be accessed from outside the class!
my_fun(my_obj, 20, 'a');
my_fun(my_obj, 40, 'b');
cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;
// Class Student objects
Student stud("Amit", 34);
cout << "stud.name = " << stud.get_name() << " and stud.marks = " << stud.get_marks() << endl;
// Change my_obj.a and my_obj.b using the friend class
stud.change_a(my_obj, 100);
stud.change_b(my_obj, 200);
cout << "After using the Friend Class methods,\n";
cout << "my_obj.a = " << my_obj.get_a() << " and my_obj.b = " << my_obj.get_b() << endl;
return 0;
}
Output
my_obj.a = 1 and my_obj.b = 2
my_obj.a = 20 and my_obj.b = 40
stud.name = Amit and stud.marks = 34
After using the Friend Class methods,
my_obj.a = 100 and my_obj.b = 200
Indeed, the changes made by the friend class methods are indeed reflected in my_obj.
Conclusion
Hopefully, that gives you a good idea of what friend functions in C++ are.
C++ has given us this flexibility to even modify private and protected variables using friend methods. Make sure you use them wisely!
References
- cplusplus.com article on Friend Functions