The parent of all world objects

An entity is essentially just another word for a game object. It's an abstract class that acts as a parent to all of its derivatives, which include the player, enemies, and perhaps even items, depending on how you want to implement that. Having these entirely different concepts share the same roots allows the programmer to define types of behavior that are common to all of them. Moreover, it lets the game engine act upon them in the same manner, as they all share the same interface. For example, the enemy can be pushed, and so can the player. All enemies, items, and the player have to be affected by gravity as well. Having that common ancestry between these different types allows us to offload a lot of redundant code and focus on the aspects that are unique to each entity, instead of re-writing the same code over and over again.

Let's begin by defining what entity types we're going to be dealing with:

enum class EntityType{ Base, Enemy, Player };

The base entity...