beef

Alternative implementation of Cow that's more compact in memory.

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use beef::Cow;

let borrowed = Cow::borrowed("Hello");
let owned = Cow::from(String::from("World"));

assert_eq!(
    format!("{} {}!", borrowed, owned),
    "Hello World!",
);

// beef::Cow is 3 word sized, while std::borrow::Cow is 4 word sized
assert!(std::mem::size_of::<Cow<str>>() < std::mem::size_of::<std::borrow::Cow<str>>());

How does it work?

The standard library Cow is an enum with two variants:

pub enum Cow<'a, B> where
    B: 'a + ToOwned + ?Sized,
{
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}

For the most common pairs of values - &str and String, or &[u8] and Vec<u8> - this means that the entire enum is 4 words wide:

                                             Padding
                                                |
                                                v
          +----------+----------+----------+----------+
Borrowed: | Tag      | Pointer  | Length   | XXXXXXXX |
          +----------+----------+----------+----------+

          +----------+----------+----------+----------+
Owned:    | Tag      | Pointer  | Length   | Capacity |
          +----------+----------+----------+----------+

Instead of being an enum with a tag, beef::Cow uses capacity to determine whether the value it's holding is owned (capacity is greater than 0), or borrowed (capacity is 0).

License

This crate is distributed under the terms of both the MIT license and the Apache License (Version 2.0). Choose whichever one works best for you.

See LICENSE-APACHE and LICENSE-MIT for details.