- Polymorphism
https://www.guru99.com/cpp-polymorphism.html https://www.geeksforgeeks.org/cpp-polymorphism/ https://www.mygreatlearning.com/blog/polymorphism-in-cpp/#:~:text=Polymorphism%20in%20C%2B%2B%20means%2C%20the,in%20numbers%2C%20it%20performs%20addition.
- Why Virtual Destructors
https://www.quantstart.com/articles/C-Virtual-Destructors-How-to-Avoid-Memory-Leaks/
- Pure Virtual Functions and Abstract Classes
https://www.geeksforgeeks.org/pure-virtual-functions-and-abstract-classes/