#smart-pointers #cow #str #user-friendly #slice #path #borrow

no-std dairy

A more compact, user friendly clone-on-write smart pointer

5 releases

0.2.2 Jul 11, 2021
0.2.1 Jul 5, 2021
0.2.0 Jul 4, 2021
0.1.1 Jul 1, 2021
0.1.0 Jun 27, 2021

#337 in Memory management

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1.5K SLoC


Crates.io Version Docs.rs Latest Build Status

A more compact, user friendly clone-on-write smart pointer.

use dairy::Cow;
let borrowed: Cow<str> = Cow::borrowed("Hello World!");
let owned: Cow<str> = Cow::owned(String::from("Hello World!"));

🚀 Getting started

Add the following to your Cargo manifest.

dairy = "0.2"

no_std is also supported by disabling the default std feature. An allocator is required.

dairy = { version = "0.2", default-features = false }

Serde is supported behind the serde feature.

dairy = { version = "0.2", features = ["serde"] }

🤔 Description

dairy::Cow is an improved version of the standard library std::borrow::Cow. Depending on the platform and type this crate provides a better underlying implementation which will be more compact. This crate currently supports the following types: str, [T], CStr, OsStr, and Path.

dairy::Cow is also able to provide many more From implementations; some which are not possible for the standard library to provide due to the alloc, std split. For example Cow<Path> now has the useful From<&str> implementation.

Underlying implementation

  • On 64-bit platforms the compact implementation of Cow is two words wide, storing the length, capacity, and the ownership tag in the same word.
  • On 32-bit platforms the compact implementation of Cow is three words wide, storing the capacity and the ownership tag in the same word.
  • The default implementation simply uses the the standard library implementation which is four words wide. This is typically required in cases where the standard library does not provide an .into_raw_parts() or equivalent method for the owned version of types.

The following table documents how Cow<T> is implemented for each type on depending on the platform.

Cow<T> Unix/WASI Other
Cow<str> compact compact
Cow<[T]> compact compact
Cow<CStr> compact compact
Cow<OsStr> compact default
Cow<Path> compact default


Some implementation details taken from the excellent beef crate.


Licensed under either of

at your option.