The free Member

The free member must destroy the elements and then deallocate the space this StrVec allocated. The for loop calls the allocator member destroy in reverse order, starting with the last constructed element and finishing with the first:

void StrVec::free()
{
    // may not pass deallocate a 0 pointer; if elements is 0, there's no work to do
    if (elements) {
        // destroy the old elements in reverse order
        for (auto p = first_free; p != elements; /* empty */)
            alloc.destroy(--p);
        alloc.deallocate(elements, cap - elements);
    }
}

The destroy function runs the string destructor. The string destructor frees whatever storage was allocated by the strings themselves.

Once the elements have been destroyed, we free the space that this StrVec allocated by calling deallocate. The pointer we pass to deallocate must be one that was previously generated by a call to allocate. Therefore, we first check that elements is not null before calling deallocate.