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The latest developer blog post for Druidstone: The Secret of the Menhir Forest, the upcoming fantasy RPG from the people who brought us the Legend of Grimrock games, gets really technical, really fast. It goes over a variety of approaches to programming and then talks about the use of components that build Druidstone's game objects. I'll be honest – it's all Greek to me, but if you're more technically minded, you might find this post interesting. An excerpt:
Modern games tend to create new game objects by compositing them from separate reusable components. This is a very powerful concept as complex behavior can be built from relatively simple building blocks. Components can be things such as models, lights, animations, sound emitters and gameplay related components such as health and item components, just to give you some examples. In this blog post I’ll talk about how we use components to build the game objects in Druidstone.
The Dark Ages
Let’s start with some attempts that don’t work. Back in the 90s when C++ was hot and object-oriented programming was still considered a good idea, somebody had a great idea: hey, let’s build game objects using inheritance! Base-classes would be something like Weapon, Enemy, Player, Light or Vehicle. So if you needed an enemy that had a weapon and carried a light source you would create a new class and inherit from the necessary base-classes. Your typical textbook OOP solution. Needless to say this quickly led to a huge mess and to an explosion with the number of different classes.
Ok, what if you don’t use inheritance and just make the different components of the object direct data members of your class? Nope, this still doesn’t work because you end up with a new class for every combination of components in your game. Fast forward ten years. What you really need is some sort of entity-component-system where the components are decoupled from objects and game objects are no longer classes. What you have instead are “entities” which are usually just numeric IDs and components which are linked to these entity ID. This way components are maximally decoupled.
This kind of component system is how many game engines work. This is also basically how my previous codebase powering Legend of Grimrock 2 was structured (although the components were written in Lua, not C++). However, there are some problems. Inter-component communication and dependencies are kind of hard. With the dark age inheritance model you could at least call the methods in the same class without a fuzz. Not so with a component-entity system. If component A wants to talk to component B, or just do a simple thing such as look up a value in the other component, first it has to know the entity ID and look up the other component using the entity ID before it can do anything. This has to be done for every component look up. And believe me, there are many, many of those look-ups in a game! If this sounds too slow or cumbersome you can start adding coupling between the components by storing direct references to other components in your components… and you’re back to square 1. This direct coupling is what you wanted to avoid in the first place.