An entity denotes a game element in the world. In our game, possible entities are friendly and enemy airplanes, bullets, missiles, or pickups that increase the player's strength. Entities interact with each other: enemy airplanes can fire missiles, the player's airplane may evade them, and the missiles may explode if they hit the player's plane, dealing damage to it. The player's aircraft can touch a pickup to collect it, as a result of which it gets a new ability. The possibilities are nearly unlimited, and they may occur between almost any pair of entity types.
In our code, we represent entities using the traditional approach of an entity hierarchy. We have a base class called Entity, which contains the data and functionality that all different kinds of entities have in common. We have multiple classes that derive from Entity, and that implement specific functionality. These derived classes could represent airplanes, projectiles (such as missiles), or pickups. One commonality between...