Vec::into_parts() returns a NonNull pointer, but isn’t the pointer allowed to be null if the vector is empty? I would expect that this method should be able to return a null pointer, but I’m probably missing something…

[–] SorteKanin@feddit.dk 3 points 6 days ago* (last edited 6 days ago) (1 children)

It's a good question. I believe Vec uses a near-zero pointer but not actually a null pointer for an empty Vec. I think part of the reason is that it makes for better optimisation of Option<Vec> which can then have the same size as Vec with the pointer being null signifying None.

Another big reason is that references are guaranteed not to be null and you can get a &[T] from an empty vec and that reference can't be null. Even in a non empty vec you need to consider this if T is zero sized.

[–] arendjr@programming.dev 2 points 6 days ago (2 children)

Ah yes, that makes sense. Curious what it points to then when empty…

[–] SorteKanin@feddit.dk 4 points 6 days ago

I looked it up, it's a pointer equal to the alignment of the type. So usually some small power of two.

[–] fil@programming.dev 2 points 6 days ago* (last edited 6 days ago)

Vec::<u8>::new().as_ptr() = 0x0000000000000001

playground

[–] BB_C@programming.dev 2 points 6 days ago

To add to the other answers. Usually, the easiest way to answer such questions is to click on source. Then in this case, you would immediately see this comment:

// SAFETY: A `Vec` always has a non-null pointer.