CRTP-based Singleton with private construction token — looking for feedback

I experimented with a CRTP-based Singleton that enforces construction via a private token. Curious to hear thoughts.

So, I wanted to implement a singleton in my ECS crtp engine for design and architectural reasons, and I sat down to think about an efficient and crtp-friendly way to do this kind of pattern without necessarily having to alter the original Singleton class contract. The solution is a crtp-based Singleton in which the Derived (the original singleton) inherits from the base Singleton, which exposes the methods required for instantiation and the single exposure of the object. Simply put, instead of boilerplating the class with the classic Singleton code (op = delete), we move this logic and transform it into a proxy that returns a static instance of the derivative without the derivative even being aware of it.

In this way, we manage private instantiation with a struct token which serves as a specific specialization for the constructor and which allows, among other things, making the construction exclusive to objects that have this token.

This keeps the singleton type-safe, zero-cost, CRTP-friendly, and easy to integrate with proxy-based or ECS-style architectures.

Link to the GitHub repo

https://redd.it/1psmcaf
@r_cpp