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++.
Table of Contents
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