Containers VS STC

This technique is often used to implement RAII in C. See example in Standard Template Containers. The library delivers STL-like functionality to C.

I'd say it's evil, but quite understandable very commonly used because there are no built-in alternatives in C. I basically never use void* in user-code, simply because there are no need for it when using a templating technique, like in my STC library. Even in the implementation of STC itself, void* is hardly used, if at all.

Study Other Peoples C Code and here's one that is easy to read: https://github.com/stclib/STC/releases

STC

This is more or less how C-ish templates are implemented in STC library.

As the author of a STL-like templated C container library, I had many of the exact same thoughts when implementing the unordered map. In fact, I also changed to many of the suggestions here, rather than consistently following the C++ umap API. E.g.

Sure it is. C misses a proper efficient generic standard/container library, like my https://github.com/stclib/STC, but that is irrelevant.

Smart Template Containers (STC)

Some data structure and algorithm library in C enable the (optional) separation between the interface of the container (which is expanded in your header) and its implementation (which is expanded in your source), like STC.