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Understanding private and protected member access is crucial when working with classes in C++.

In a class, members can either be declared as public, private, or protected. Public members are accessible from anywhere outside the class. Private members, on the other hand, can be accessed only within the class itself. Protected members are similar to private members, but they can also be accessed in derived classes, which are classes that inherit from another class.

Normally, non-member functions cannot access private or protected members of a class due to encapsulation, which is a core principle of object-oriented programming that restricts access to an object's internal state. This is where friend functions offer an exception. They allow function access typically forbidden by C++'s access control. A friend function, being external to the class, can access the class's private and protected members as if it were a member of the class.

A C++ class structure defines a blueprint for creating objects that encapsulate data and functions.

A class typically includes the following components:

• Member Variables: Variables to hold data. They are often private or protected to enforce encapsulation.
• Member Functions: Functions that can operate on the class's data members.
• Constructors and Destructors: Special member functions for initializing and cleaning up objects.
• Accessor and Mutator Methods: Functions for getting and setting the value of private data members.
• Friend Declarations: Optionally declares external functions or classes as friends, granting them access to private and protected members.

Each element within a class serves a specific role, keeping the data and the operations on that data neatly packaged together, which is the essence of object-oriented design.

In C++, non-member functions are functions that are not defined within the scope of a class. They could be standalone functions or part of another class. Non-member functions cannot access the private and protected members of a class, as their scope is outside of the class.

However, there are instances where non-member functions need to interact closely with a class, manipulating its state in ways that regular public interfaces do not allow. The friend function, a non-member function, can access a class's private and protected members. Declaring a function as a friend within a class is a way to grant special access rights. Such functions are useful in operator overloading and when functionality needs to be added without altering the class's public interface.