In C++ object oriented programming, what is a friend function of a class?

Introduction / Context:
Friend functions are a unique feature of C++ that relate to the encapsulation and access control system. Normally, private and protected members of a class cannot be accessed from outside the class except by member functions and friends. Declaring a function as a friend of a class is a deliberate step taken by the class designer to allow that function to bypass normal access restrictions. This question checks whether you can correctly describe what a friend function is and how it relates to class membership.

Given Data / Assumptions:

• The language is C++.
• We are dealing with the concept of friend functions and not friend classes.
• We assume standard access rules: private and protected members are not visible to unrelated external functions.
• Friendship is explicitly granted inside the class definition using the friend keyword.

Concept / Approach:
A friend function is not a member of the class, but it is declared inside the class with the keyword friend. This declaration tells the compiler that the function should be allowed to access the class's private and protected data members, just like a member function can. The function itself is usually defined outside the class, and it is called like any other regular function, not with member access syntax. The key point is that friendship provides a controlled way to break encapsulation when needed, for example for operator overloading or helper functions that need intimate knowledge of a class's internals.

Step-by-Step Solution:
Step 1: Recall that class members can be private, protected, or public, and that private and protected members are normally hidden from non-member functions. Step 2: Remember that the keyword friend can appear in a class definition to declare that a particular external function is a friend of the class. Step 3: Understand that a friend function does not become a member of the class; it remains a normal function, but with special access rights. Step 4: Note that the friend function is allowed to read or modify private and protected members of the class, which is the main reason for granting friendship. Step 5: Compare the options and choose the one that states that a friend function is a non-member function with access to private and protected members.

Verification / Alternative check:
A typical example is overloading the stream insertion operator for a class. You might write a function like ostream and operator<<(ostream and os, const MyClass and obj), and declare it as a friend inside MyClass. This function is not a member of MyClass, but it needs access to private fields for printing. Declaring it as a friend gives it that access. This common pattern confirms that a friend function is external to the class but granted special access privileges.

Why Other Options Are Wrong:
Option A member function that can only be called by objects of the same class: This describes neither friend functions nor any standard access control concept. Option A function defined inside main that can access all global variables: Any function can access global variables in scope, and this has nothing to do with friendship. Option Any function that is overloaded more than once: Overloading has no inherent relationship with the friend keyword or access rights.

Common Pitfalls:
One common mistake is to think that friend functions are members of the class, which leads to confusion about how they are called. Another pitfall is to overuse friendship and break encapsulation unnecessarily, which can make code harder to maintain. A good design uses friend functions sparingly and only when there is a clear need for an external function to access private implementation details. Understanding this balance is important for writing clean C++ code.

Final Answer:
A friend function in C++ is A non-member function that is granted access to the private and protected members of a class.