void* is a problem because the caller and callee need to coordinate across the encapsulation boundary, thus breaking it. (Internally it would be fine to use - the author could carefully check that qsort casts to the right type inside the .c file)

> What if the data structure is more complex, such as a binary tree?

I think that's what the author was going with by exposing opaque structs with _new() and _free() methods.

But yeah, his good and bad versions of strclone look more or less the same to me.