Abstract classes do not necessarily have: \( \begin{array}{l}\text { Aero instances of their class in their vtable } \\ \text { Zero references to their class. } \\ \text { A virtual function prototype with the notation = } \\ 0 .\end{array} \)

Abstract classes, by definition, are classes that cannot be instantiated directly. They serve as blueprints for other classes and must include at least one pure virtual function, which is a function declared with `= 0`. This allows derived classes to implement these functions, ensuring that they provide specific functionality while enforcing a common interface. In practical applications, abstract classes are widely used in frameworks and libraries. For example, in game development, you might have an abstract class called `GameObject` with pure virtual functions like `update()` and `render()`. This allows you to create specific game objects like `Player` and `Enemy` that inherit from `GameObject`, each implementing the functions in ways that suit their unique needs. This pattern promotes clean, reusable, and organized code.