> Reserve will not initialize, but then you have to keep track of the real vector size on the side

You already do that when you use push_back. It tracks the size for you, overallocates to amortize the cost of growing and most importantly does not initialize the overallocated memory before it is used, meaning pages will not be touched / mapped by the OS unless you actually end up using it. Giving you the benefit of amortizing vector growth without paying for the uninitialized memory it allocates behind the scenes for future use.

Directly accessing reserved memory instead of using resize was to check if the allocator zero initialized that overallocated memory. That the parts that are used end up initialized at a later point is entirely irrelevant to my point.

So your previous point:

> They double in size, right? Well, the default allocator for vectors will actually zero-initialize the new elements.

They double in capacity, not size when used with push_back. Which means exactly one new element will be initialized no matter how much uninitialized/unused/unmapped capacity the vector allocates for future use.