created_at2017-05-28 17:00:32.383605
updated_at2023-09-26 06:09:21.447222
description`Small Box` optimization: store small item on stack and fallback to heap for large item.
Andy Lok (andylokandy)




Build Status

Small Box optimization: store small item on stack and fallback to heap for large item. Requires Rust 1.56+.



First, add the following to your Cargo.toml:

smallbox = "0.8"

Next, add this to your crate root:

extern crate smallbox;

If you want this crate to work with dynamic-sized type, you can request it via:

smallbox = { version = "0.8", features = ["coerce"] }

Currently smallbox by default links to the standard library, but if you would instead like to use this crate in a #![no_std] situation or crate, you can request this via:

version = "0.8"
features = ["coerce"]
default-features = false

Feature Flags

This crate has the following cargo feature flags:

  • std

    • Optional, enabled by default
    • Use libstd
    • If std feature flag is opted out, alloc crate will be linked, which requires nightly rust.
  • coerce

    • Optional

    • Require nightly rust

    • Allow automatic coersion from sized SmallBox to unsized SmallBox.

Unsized Type

There are two ways to have an unsized SmallBox: Using smallbox!() macro or coercing from a sized SmallBox instance(requires nightly compiler).

Using the smallbox!() macro is the only option on stable rust. This macro will check the type of given value and the target type T. For any invalid type coersions, this macro will invoke a compile-time error.

Once the feature coerce is enabled, sized SmallBox<T> will be automatically coerced into SmallBox<T: ?Sized> if necessary.


Eliminate heap alloction for small items by SmallBox:

use smallbox::SmallBox;
use smallbox::space::S4;

let small: SmallBox<_, S4> = SmallBox::new([0; 2]);
let large: SmallBox<_, S4> = SmallBox::new([0; 32]);

assert_eq!(small.len(), 2);
assert_eq!(large.len(), 32);

assert_eq!(*small, [0; 2]);
assert_eq!(*large, [0; 32]);

assert!(small.is_heap() == false);
assert!(large.is_heap() == true);

Unsized type

Construct with smallbox!() macro:

extern crate smallbox;

use smallbox::SmallBox;
use smallbox::space::*;

let array: SmallBox<[usize], S2> = smallbox!([0usize, 1]);

assert_eq!(array.len(), 2);
assert_eq!(*array, [0, 1]);

With coerce feature:

use smallbox::SmallBox;
use smallbox::space::*;
let array: SmallBox<[usize], S2> = SmallBox::new([0usize, 1]);

assert_eq!(array.len(), 2);
assert_eq!(*array, [0, 1]);

Any downcasting:

extern crate smallbox;

use std::any::Any;
use smallbox::SmallBox;
use smallbox::space::S2;

let num: SmallBox<Any, S2> = smallbox!(1234u32);

if let Some(num) = num.downcast_ref::<u32>() {
    assert_eq!(*num, 1234);
} else {


The capacity is expressed by the size of type parameter Space, regardless of what actually the Space is.

The crate provides some spaces in module smallbox::space, from S1, S2, S4 to S64, representing "n * usize" spaces.

Anyway, you can defind your own space type such as byte array [u8; 64]. Please note that the space alignment is also important. If the alignment of the space is smaller than the alignment of the value, the value will be stored in the heap.


The test platform is Windows 10 on Intel E3 v1230 v3.

running 6 tests
test box_large_item                  ... bench:         104 ns/iter (+/- 14)
test box_small_item                  ... bench:          49 ns/iter (+/- 5)
test smallbox_large_item_large_space ... bench:          52 ns/iter (+/- 6)
test smallbox_large_item_small_space ... bench:         106 ns/iter (+/- 25)
test smallbox_small_item_large_space ... bench:          18 ns/iter (+/- 1)
test smallbox_small_item_small_space ... bench:           2 ns/iter (+/- 0)

test result: ok. 0 passed; 0 failed; 0 ignored; 6 measured; 0 filtered out


All kinds of contribution are welcome.

  • Issue Feel free to open an issue when you find typos, bugs, or have any question.

  • Pull requests. Better implementation, more tests, more documents and typo fixes are all welcome.


Licensed under either of

at your option.

Commit count: 87

cargo fmt