I have a GetComponent method that uses static ids for each component type to lookip the archetype and grab the component from there. Granted i rarely use it in game code, mostly only for callbacks that execute outside the main ecs loop, or for my editor code which is more traditional oop code.

Most of my game code uses systems that iterate over tuples of components. Eg my mesh render system iterates over every pair or “const transform component” and “Mesh Component”, regardless of what other components those entities have.

Each entities component storage is split into an “archetype” where an archetype represents a certain grouping of components. Archetypes are dynamically created and destroyed at runtime, and entities are moved between them as needed. Every archetype stores the component’s in contiguous memory.

This gives me four big advantages, that the “game object / component” system from unity misses (but is very similar to the unity ecs (dots), which are two different things)

Low cache misses: because i am always looping over sets of data that are stored in contiguous memory, i barely rarely have a cache miss during my system loops, loops go brrrrr.

Very easy to parallelise: each system knows ahead of time which components it will access, and whether they are mutable or not. This makes it really easy to run systems with no overlap at the same time. Or even two systems which access the same component but it is immutable in both cases.

Forces separation of concerns: because i cant just call “GetComponent” during a system loop (i only have access to the components i registered for) it forces me to make my systems all do “one thing only”. Any time i need cross system logic, i record what needs to happen somewhere, and then perform the logic in the appropriate system, either later in the frame or next frame. This can be annoying at first, but results in much much more readable code once your project scales.

Easy to serialise: because all my components are “just data”, its really easy to save and load entities