How does max_len attribute work?

Question

I have this field on the data account of my pda:

#[max_len(0)]
    pub users: Vec<Pubkey>,

But I'm still able to push a value to it, even though it should have 0 space allocated. Why is that so? Also, when I have the init attribute for the #[accounts()] attribute macro, does the fields of the account being initialized are initialized using Default e.g. Option is initialized to None and Vec to empty?

Answer 1

You shouldn't be able to push elements if you set max_len to 0. You might have space elsewhere as a buffer, or you might have allocated more space when you init the struct.

Option and Vec would both init to all zeroes. Vec is some overhead (about 4 bytes) and a then zeroes for whatever space your data would take. Option is 1 byte plus whatever the data is, where 0 is None. See https://book.anchor-lang.com/anchor_references/space.html