Layout

Let's start by making the layout for our implementation of Arc.

An Arc<T> provides thread-safe shared ownership of a value of type T, allocated in the heap. Sharing implies immutability in Rust, so we don't need to design anything that manages access to that value, right? Although interior mutability types like Mutex allow Arc's users to create shared mutability, Arc itself doesn't need to concern itself with these issues.

However there is one place where Arc needs to concern itself with mutation: destruction. When all the owners of the Arc go away, we need to be able to drop its contents and free its allocation. So we need a way for an owner to know if it's the last owner, and the simplest way to do that is with a count of the owners -- Reference Counting.

Unfortunately, this reference count is inherently shared mutable state, so Arc does need to think about synchronization. We could use a Mutex for this, but that's overkill. Instead, we'll use atomics. And since everyone already needs a pointer to the T's allocation, we might as well put the reference count in that same allocation.

Naively, it would look something like this:

use std::sync::atomic;

pub struct Arc<T> {
    ptr: *mut ArcInner<T>,
}

pub struct ArcInner<T> {
    rc: atomic::AtomicUsize,
    data: T
}

This would compile, however it would be incorrect. First of all, the compiler will give us too strict variance. For example, an Arc<&'static str> couldn't be used where an Arc<&'a str> was expected. More importantly, it will give incorrect ownership information to the drop checker, as it will assume we don't own any values of type T. As this is a structure providing shared ownership of a value, at some point there will be an instance of this structure that entirely owns its data. See the chapter on ownership and lifetimes for all the details on variance and drop check.

To fix the first problem, we can use NonNull<T>. Note that NonNull<T> is a wrapper around a raw pointer that declares that:

  • We are variant over T
  • Our pointer is never null

To fix the second problem, we can include a PhantomData marker containing an ArcInner<T>. This will tell the drop checker that we have some notion of ownership of a value of ArcInner<T> (which itself contains some T).

With these changes we get our final structure:

use std::marker::PhantomData;
use std::ptr::NonNull;
use std::sync::atomic::AtomicUsize;

pub struct Arc<T> {
    ptr: NonNull<ArcInner<T>>,
    phantom: PhantomData<ArcInner<T>>
}

pub struct ArcInner<T> {
    rc: AtomicUsize,
    data: T
}