11 releases

0.3.0	May 26, 2026
0.2.6	Aug 4, 2025
0.2.5	Apr 24, 2025
0.2.3	Oct 30, 2024
0.1.0	Mar 1, 2023

#29 in Data structures

482,205 downloads per month
Used in 364 crates (82 directly)

MIT/Apache

97KB
2K SLoC

ecow

Compact, clone-on-write vector and string.

Types

An EcoVec is a reference-counted clone-on-write vector. It takes up two words of space (= 2 usize) and has the same memory layout as a &[T] slice. Within its allocation, it stores a reference count, its capacity and its elements.
An EcoString is a reference-counted clone-on-write string with inline storage. It takes up 16 bytes of space. It has 15 bytes of inline storage and starting from 16 bytes it becomes an EcoVec<u8>.

Example

// This is stored inline.
let small = ecow::EcoString::from("Welcome");

// This spills to the heap, but only once: `big` and `third` share the
// same underlying allocation. Vectors and spilled strings are only
// really cloned upon mutation.
let big = small + " to earth! 🌱";
let mut third = big.clone();

// This allocates again to mutate `third` without affecting `big`.
assert_eq!(third.pop(), Some('🌱'));
assert_eq!(third, "Welcome to earth! ");

Why should I use this instead of ...

Type	Details
`Vec<T>` / `String`	Normal vectors are a great general purpose data structure. But they have a quite big footprint (3 machine words) and are expensive to clone. The `EcoVec` has a bit of overhead for mutation, but is cheap to clone and only takes two words.
`Arc<Vec<T>>` / `Arc<String>`	These require two allocations instead of one and are less convenient to mutate.
`Arc<[T]>` / `Arc<str>`	While these require only one allocation, they aren't mutable.
Small vector	Different trade-off. Great when there are few, small `T`s, but expensive to clone when spilled to the heap.
Small string	The `EcoString` combines different small string qualities into a very practical package: It has inline storage, a smaller footprint than a normal `String`, is efficient to clone even when spilled, and at the same time mutable.

License

This crate is dual-licensed under the MIT and Apache 2.0 licenses.

Dependencies

~0–2MB
~24K SLoC