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copied!<p>For my soon-to-be personal game project, I use a component-based entity system.</p> <p>You can read more about it by searching "component based game development". A famous article is <a href="http://cowboyprogramming.com/2007/01/05/evolve-your-heirachy/" rel="noreferrer">Evolve Your Hierarchy</a> from the Cowboy programming blog.</p> <p>In my system, entities are just ids - unsigned long, a bit like in a relational database. All the data and the logic associated to my entities are written into Components. I have Systems that link entity ids with their respective components. Something like that:</p> <pre><code>typedef unsigned long EntityId; class Component { Component(EntityId id) : owner(id) {} EntityId owner; }; template <typename C> class System { std::map<EntityId, C * > components; }; </code></pre> <p>Then for each kind of functionality, I write a special component. All entities don't have the same components. For example you could have a static rock object that has the WorldPositionComponent and the ShapeComponent, and a moving enemy that has the same components plus the VelocityComponent. Here's an example:</p> <pre><code>class WorldPositionComponent : public Component { float x, y, z; WorldPositionComponent(EntityId id) : Component(id) {} }; class RenderComponent : public Component { WorldPositionComponent * position; 3DModel * model; RenderComponent(EntityId id, System<WorldPositionComponent> & wpSys) : Component(id), position(wpSys.components[owner]) {} void render() { model->draw(position); } }; class Game { System<WorldPositionComponent> wpSys; System<RenderComponent> rSys; void init() { EntityId visibleObject = 1; // Watch out for memory leaks. wpSys.components[visibleObject] = new WorldPositionComponent(visibleObject); rSys.components[visibleObject] = new RenderComponent(visibleObject, wpSys); EntityId invisibleObject = 2; wpSys.components[invisibleObject] = new WorldPositionComponent(invisibleObject); // No RenderComponent for invisibleObject. } void gameLoop() { std::map<EntityId, RenderComponent *>::iterator it; for (it = rSys.components.iterator(); it != rSys.components.end(); ++it) { (*it).second->render(); } } }; </code></pre> <p>Here you have 2 components, WorldPosition and Render. The Game class holds the 2 systems. The Render component has an access to the position of the object. If the entity doesn't have a WorldPosition component, you can choose default values, or ignore the entity. The Game::gameLoop() method will only render visibleObject. There is no waste of processing for non-renderable components.</p> <p>You can also split my Game class into two or three, to separate display and input systems from the logic. Something like Model, View and Controller.</p> <p>I find it neat to define my game logic in term of components, and to have entities that only have the functionality that they need - no more empty render() or useless collision detection checks.</p>

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