What is a friend function? What are the merits and demerit of using friend function. Explain with example.

A friend function in C++ is a function that has the right to access all private and protected members (variables and functions) of the class, even though it is not a member of that class. It is defined using the keyword 'friend' before the function name.

Here is an example of a friend function:

class MyClass {
private:
    int myVar;

public:
    MyClass() : myVar(0) {}

    friend void changeMyVar(MyClass& obj, int value);
};

void changeMyVar(MyClass& obj, int value) {
    obj.myVar = value;  // Directly accessing private member
}

In this example, changeMyVar is a friend function of the class MyClass. It can directly access and modify the private member myVar of MyClass.

Merits of using friend function:

1. It can access the private and protected members of the class, which provides more flexibility.
2. It can be used when a certain functionality cannot be achieved by the member functions of the class.
3. It can be used to overload certain operators for the class.

Demerits of using friend function:

1. It violates the encapsulation principle of Object Oriented Programming, as it accesses the private and protected members of the class directly.
2. The use of friend functions increases the risk of data being accidentally modified, as it has access to the private members of the class.
3. If not used carefully, it can make the code more complex and harder to debug.