No need to preinitialize the list. When allocating an element you have two cases: a) the free list is non empty: you pop an element by making the head point to its successor; b) the free list is empty: you increase the size of the vector by one.

To deallocate an element you simply set the elemnt next to the whatever is the current head, then make head point to this element.

You start with an empty vector.

An element is either allocated or in a free list, that's why the next pointer can be kept in an union.