yvesdum/refbox

A smart pointer with a single owner and many weak references for the Rust programming language.

RefBox

A Box with weak references.

A RefBox is a smart pointer that owns the data, just like a standard Box. Similarly, a RefBox cannot be cloned cheaply, and when it is dropped, the data it points to is dropped as well. However, a RefBox may have many Ref pointers to the same data. These pointers don’t own the data and are reference counted, comparable to the standard library's Weak. Which means, as long as the RefBox is alive, Refs can be used to access the data from multiple places without lifetime parameters.

A RefBox could be seen as a lighter alternative to the standard library’s Rc, Weak and RefCell combination in cases where there is one Rc with many Weaks to the same data.

A RefBox does not differentiate between strong and weak pointers and immutable and mutable borrows. There is always a single strong pointer, zero, one or many weak pointers and all borrows are mutable. This means there can only be one borrow active at any given time. But in return, RefBox uses less memory, is faster to borrow from, and a Ref does not need to be upgraded to a RefBox in order to access the data.

Note: this crate is currently experimental.

Rc<RefCell> vs RefBox

	Rc<RefCell<T>>	RefBox<T>
Pointer kinds	Many strong pointers and many weak pointers	One strong owner and many weak pointers
Clonable	Both Rc and Weak are cheap to clone	Only Ref can be cheaply cloned
Up-/Downgrading	Rc is downgradable, Weak is upgradable	No up- or downgrading, but RefBox::create_ref
Data access	RefCell::try_borrow_mut	RefBox::try_borrow_mut
Weak data access	1. Weak::upgrade 2. RefCell::try_borrow_mut 3. Rc::drop	Ref::try_borrow_mut
Active borrows	One mutable or many immutable	One (mutable or immutable)
T::drop	When all Rcs are dropped	When owner RefBox is dropped
Max no. Weaks	usize::MAX	u32::MAX
Heap overhead	64-bit: 24 bytes 32-bit: 12 bytes	8 bytes
Performance	Cloning is fast, mutating is slow	Cloning references is a tiny bit slower, mutating is much faster

Examples

use refbox::RefBox;

fn main() {
    // Create a RefBox.
    let owner = RefBox::new(100);

    // Create a weak reference.
    let weak = owner.create_ref();

    // Access the data.
    let borrow = weak.try_borrow_mut().unwrap();
    assert_eq!(*borrow, 100);
}

Performance comparison

A number of benchmarks are included to compare the performance of RefBox vs Rc. Each benchmark follows the same process:

1. An Rc or RefBox is created, with optionally a weak reference;
2. An operation is performed for x number of times;
3. The Rc or RefBox is dropped.

The horizontal axes show the number of times the operation was performed. The vertical axes show the average time it took to complete the entire process described above.

The benchmarks are performed on an HP Intel Core i7-7700HQ CPU @ 2.80GHz, Windows 10 64-bit. You are encouraged to perform the benchmarks yourself as well.

Mutating through the owner takes less time (only ~80%):

Mutating through a weak reference takes much less time (only ~36%):

However, creating, cloning and dropping weak references takes a little bit more time:

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

refbox's Issues

Why only mutable borrows?

Hi! I found this crate when trying to figure out how to implement trees without multiple ownership. This pattern of having a smart pointer with exclusive ownership together with the possibility of creating weak pointers that allow mutability was exactly what was needed!

I was just wondering why it's not possible to access data behind a Ref<_> without getting a mutable borrow? Why isn't there a borrow() method on Ref?

Thanks!