C++ Friend Function

A function is a block of code that performs a certain task and provides the output. It is mainly used to eliminate repetitive code. In this tutorial, we will look into the friend function in C++ and explain its concept with working examples.

What is Friend Function?

Friend function is a non-member function that can access the private and protected members of a class. “Friend” is a keyword used to indicate that a function is the friend of a class. This allows the compiler to know that the particular function is a friend of the given class. The friend function then should be able to access the private and protected member of a given class. Without the friend keyword, a non-member outside function can only access the public members of a class.

Key Features of Friend Function:

Here are the key features of the friend function:

1. A friend function is not in the scope of the class.
2. The friend function cannot be invoked using an instance of a class.
3. It can access the members using the object and dot operator.

Syntax of Friend Function:

Here is the syntax of the friend function:

class Class_Name_Demo
{
   ………………………………………
   ………………………………………

    friend return_Type function_Name(arg_1, arg_2, …);
};

Example of Friend Function:

Now, let us look into an example program to understand the concept of the friend function in C++. In the below example program, we have the “Friend_Demo” class. It has three different types of data members, i.e., private, protected, and public.

We have defined another function, i.e., “friendDemo_Func()” outside the scope of the “Friend_Demo” class and tried to access the members (private, protected, and public) of the “Friend_Demo” class.

But, as you can see in the output below when we compile the program, it throws compilation errors. The friend function is going to exactly solve this problem.

#include <iostream>
using namespace std;

class Friend_Demo
{
   private:
    int i_private;
   protected:
    int i_protected;
   public:
    int i_public;
};

void friendDemo_Func()
{
    Friend_Demo fd;
   
    fd.i_private    = 10;
    fd.i_protected  = 15;
    fd.i_public     = 20;
   
    cout << fd.i_private << endl;
    cout << fd.i_protected << endl;
    cout << fd.i_public << endl;
}

int main()
{
    friendDemo_Func();

    return 0;
}

In the previous program, we were getting compilation errors while trying to access the private, protected, and public members of a class from a non-member function. This is because a non-member function is not allowed to access the private and protected members of a class from outside the scope of the class.

Now, in this example, we have declared “friendDemo_Func()” function as a friend inside the scope of the class, i.e., “Friend_Demo”:

friend void friendDemo_Func();

We have created an object, i.e., “fd” of the “Friend_Demo” class inside the “friendDemo_Func()” function. Now, we can access the private, protected, and public members of the “Friend_Demo” class using the dot operator. We have assigned 10, 15, and 20 to i_private, i_protected, and i_public, respectively.

As you can see in the output below, this program is now compiled and executed without any errors and print the output as expected.

#include <iostream>
using namespace std;

class Friend_Demo
{
   private:
    int i_private;
   protected:
    int i_protected;
   public:
    int i_public;
    friend void friendDemo_Func();
};

void friendDemo_Func()
{
    Friend_Demo fd;
   
    fd.i_private    = 10;
    fd.i_protected  = 15;
    fd.i_public = 20;
   
    cout << fd.i_private << endl;
    cout << fd.i_protected << endl;
    cout << fd.i_public << endl;
}

int main()
{
    friendDemo_Func();

    return 0;
}

Conclusion:

In this article, I have explained the concept of the friend function in C++. I have also shown two working examples to explain how the friend function behaves in C++. Sometimes, the friend function can be very useful in a complex programming environment. However, a programmer should be cautious about overusing it and compromising its OOP features.

from Linux Hint https://ift.tt/300E7Go