[][src]Crate smallbox

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


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.

Using the smallbox!() macro is the only option on stable rust. This macro will check the types of the expression and the expected type T. For any invalid type coersions, this macro invokes a compiler error.

Once the feature coerce is enabled, sized SmallBox<T> can be 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].



Space types that is used to define capacity



Box value on stack or on heap depending on its size



An optimized box that store value on stack or on heap depending on its size