A friend function is a function that is not a member of a class but has access to the class’s private and protected members. Friend functions are not considered class members; they are normal external functions that are given special access privileges.

Accessors are instructions that generally give a property of method permission to be accessible in different ways. There are three types:

public: This property or permission is accessible from anywhere in the application.

protected: This property or permission is accessible only from a class that extends the parent class.

private: This property or permission is accessible only within the current class you are working with.

The really neat thing about access is that it protects your code should other developers be working on the same project. If you don’t want them to be able to touch something, don’t give him access to it.

In programming languages and type theory, polymorphism is the provision of a single interface to entities of different types.
A polymorphic type is a type whose operations can also be applied to values of some other type, or types.