I think the C++ library https://github.com/skypjack/entt https://github.com/skypjack/entt gets pretty close to the ideal for a purely compile time implementatio

I think the C++ library https://github.com/skypjack/entt https://github.com/skypjack/entt gets pretty close to the ideal for a purely compile time implementation. It is implemented with modern C++ template meta-programming - which is to say it is basically a DSL encoded into C++ and can generate arbitrary code at compile time - while better than it used to be to read C++ template meta-programming is still very difficult to read. It also comes with a number of utility libraries for using it efficiently and correctly, including a signal library, service locator, and scheduler (among others). It still has it's limitations though. Because it is a compile time library, it can't do any runtime optimizations like it could with a JIT. Also, because C++ reflection is atrocious, it doesn't provide the best support for runtime type manipulation (reflection features are often paired with the component pattern implementation, especially in general purpose game engines). This comes back to dynamically meta-programmed languages (e.g. python, lisp) that could theoretically out perform even ENTT using a JIT - with the appropriate reflection features - and meta-programming. However an ECS only really makes sense in a non-garbage collected language (or if the language exposes a non-GC meta-programming interface). Since there aren't really any viable JIT (+ reflection) + meta-programmed + non-GC languages out there, I don't think a perfectly correct implementation exists. I think an ideally implemented example will eventually be Johnathan Blow's experimental language Jai (or a library for it) given what has been stated publicly about it (metaprogramming, JITted, reflection, not garbage collected, has built in support for SoA types). But that's not a viable example to look at at the moment.