In C++ object oriented programming, what is a friend function and how does it relate to the class that declares it as a friend?

Introduction / Context:
C++ supports the concept of friend functions as part of its access control mechanisms. Normally, only member functions of a class can access its private and protected data. However, in some designs it is convenient to allow certain non member functions or functions of other classes to access those members directly. Friend declarations provide a controlled way to grant such access without making the members public. This question tests whether you understand what a friend function is and how it is related to the class that declares it.

Given Data / Assumptions:

• The programming language is C++.
• We are discussing access control keywords such as private, protected and public.
• Friendship is declared inside the class whose members are to be accessed.

Concept / Approach:
A friend function is specified using the friend keyword in the class definition. For example, friend void display(const MyClass and obj);. This line inside MyClass allows the non member function display to access private and protected members of MyClass. Friend functions are not class members, so they are called like normal functions, but they enjoy special access rights. Friendship is not symmetric or transitive; it must be explicitly granted by each class. The correct option must mention that a friend function is granted access to private and protected members through an explicit friend declaration.

Step-by-Step Solution:
Step 1: Recall that without friend, only member functions and classes derived in certain ways can access non public members.Step 2: Remember that friend functions are declared using the friend keyword inside the class.Step 3: Option A states that a non member or member function is granted access to private and protected members of a class by being declared as a friend inside that class.Step 4: Options B, C and D describe behaviours that are unrelated to friend functions in C++.Step 5: Therefore, option A accurately captures the definition and relationship.

Verification / Alternative check:
If you implement an operator overload like operator<< for output streams, it is common to make it a friend of the class so that it can print private data. Example code shows friend std::ostream and operator<<(std::ostream and os, const MyClass and obj); in the class definition. This real world usage demonstrates that friend functions are normal non member functions with special access granted. It also confirms that friend status must be explicitly declared inside the class.

Why Other Options Are Wrong:
Option B invents a behaviour unrelated to friendship and describes automatic copying that is actually handled by constructors or assignment operators. Option C claims that friend functions can only access public members, which is the opposite of their purpose, since any function can already access public members without being a friend. Option D suggests that friend functions belong to every class and require no declaration, which contradicts the requirement for explicit friend declarations.

Common Pitfalls:
Some learners think that friend functions are member functions, but in fact they are usually non members that are granted special access. Others assume that friendship is inherited or global, which it is not. Each class decides exactly which functions or other classes are friends. For exam answers, emphasise explicit friend declarations and extended access to private and protected members as the key characteristics.

Final Answer:
A non member or member function that is granted access to the private and protected members of a class by being explicitly declared as a friend inside that class.