Abstract classes: Are defined, but the programmer never intends to instantiate any objects from them. Can have objects instantiated from them if the proper permissions are set. Contain at most one pure virtual function. Cannot have abstract derived classes.

Abstract classes are a fundamental concept in object-oriented programming, serving as blueprints for creating derived classes. They allow for the definition of methods that must be implemented in any derived class, ensuring a consistent interface across different implementations. This promotes code reusability and helps manage complex systems by allowing developers to define shared behaviors while leaving specific implementations to subclasses. When using abstract classes, a common mistake is attempting to instantiate them directly, which results in a compilation error. It’s also crucial to remember that if an abstract class defines a pure virtual function, all derived classes must implement that function; otherwise, those derived classes will also be treated as abstract. Keeping an eye on these details will help prevent frustrating bugs and ensure a smoother coding experience!