nomicon/vec-layout.md

% Layout

First off, we need to come up with the struct layout. Naively we want this design:

pub struct Vec<T> {
    ptr: *mut T,
    cap: usize,
    len: usize,
}

# fn main() {}

And indeed this would compile. Unfortunately, it would be incorrect. The compiler will give us too strict variance, so e.g. an &Vec<&'static str> couldn't be used where an &Vec<&'a str> was expected. More importantly, it will give incorrect ownership information to dropck, as it will conservatively assume we don't own any values of type T. See [the chapter on ownership and lifetimes] (lifetimes.html) for details.

As we saw in the lifetimes chapter, we should use Unique<T> in place of *mut T when we have a raw pointer to an allocation we own:

#![feature(unique)]

use std::ptr::{Unique, self};

pub struct Vec<T> {
    ptr: Unique<T>,
    cap: usize,
    len: usize,
}

# fn main() {}

As a recap, Unique is a wrapper around a raw pointer that declares that:

• We own at least one value of type T
• We are Send/Sync iff T is Send/Sync
• Our pointer is never null (and therefore Option<Vec> is null-pointer-optimized)

That last point is subtle. First, it makes Unique::new unsafe to call, because putting null inside of it is Undefined Behaviour. It also throws a wrench in an important feature of Vec (and indeed all of the std collections): an empty Vec doesn't actually allocate at all. So if we can't allocate, but also can't put a null pointer in ptr, what do we do in Vec::new? Well, we just put some other garbage in there!

This is perfectly fine because we already have cap == 0 as our sentinel for no allocation. We don't even need to handle it specially in almost any code because we usually need to check if cap > len or len > 0 anyway. The traditional Rust value to put here is 0x01. The standard library actually exposes this as std::rt::heap::EMPTY. There are quite a few places where we'll want to use heap::EMPTY because there's no real allocation to talk about but null would make the compiler angry.

All of the heap API is totally unstable under the heap_api feature, though. We could trivially define heap::EMPTY ourselves, but we'll want the rest of the heap API anyway, so let's just get that dependency over with.