If you were to implement your own realloc for the purpose of this data structure, you could do that. However, a generic realloc implementation needs to know how

If you were to implement your own realloc for the purpose of this data structure, you could do that. However, a generic realloc implementation needs to know how many bytes to copy, and generally that means storing the capacity just in front of the allocated region. I don't think there is another feasible way. On e.g. glibc, you can use `malloc_usable_size` to extract this information (modulo certain caveats).