smallbox

Box dynamically-sized types on stack. Requires nightly rust.

Store or return trait-object and closure without heap allocation, and fallback to heap when thing goes too large.

Documentation

Usage

First, add the following to your Cargo.toml:

[dependencies]
smallbox = "0.4"

Next, add this to your crate root:

extern crate smallbox;

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, and want to opt out heap dependency and SmallBox<T> type, you can request this via:

[dependencies]
smallbox = { version = "0.4", default-features = false }

Enable heap feature for #![no_std] build to link to alloc crate and bring SmallBox<T> back.

[dependencies.smallbox]
version = "0.4"
default-features = false
features = ["heap"]

Feature Flags

The stackbox crate has the following cargo feature flags:

• std

  • Optional, enabled by default
  • Use libstd

• heap

  • Optional
  • Use heap fallback and include SmallBox<T> type, and link to alloc crate if std feature flag is opted out.

Overview

This crate delivers two core type:

StackBox<T>: Represents as a fixed-capacity allocation, and on stack stores dynamically-sized type. The new method creates returns Err(value) if the instance larger then Space. Default capacity is two words (2 * sizeof(usize)), more details on custom capacity are at the following sections.

SmallBox<T>: Takes StackBox<T> as an varience, and fallback to heap-alloc Box<T> when type T is larger then Space.

Example

The simplest usage is trait object dynamic-dispatch

use smallbox::StackBox;
 
let val: StackBox<PartialEq<usize>> = StackBox::new(5usize).unwrap();
 
assert!(*val == 5)

Any downcasting is also quite a good use.

use std::any::Any;

let num: StackBox<Any> = StackBox::new(1234u32).unwrap();

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

Another use case is to allow returning capturing closures without having to box them.

use smallbox::StackBox;

fn make_closure(s: String) -> StackBox<Fn()->String> {
    StackBox::new(move || format!("Hello, {}", s)).ok().unwrap()
}

let closure = make_closure("world!".to_owned());
assert_eq!(closure(), "Hello, world!");

SmallBox<T> is to eliminate heap alloction for small things, except that the object is large enough to allocte. In addition, the inner StackBox<T> or Box<T> can be moved out by explicit pattern match.

use smallbox::SmallBox;

let tiny: SmallBox<[u64]> = SmallBox::new([0; 2]);
let big: SmallBox<[u64]> = SmallBox::new([1; 8]);

assert_eq!(tiny.len(), 2);
assert_eq!(big[7], 1);

match tiny {
    SmallBox::Stack(val) => assert_eq!(*val, [0; 2]),
    _ => unreachable!()
}

match big {
    SmallBox::Box(val) => assert_eq!(*val, [1; 8]),
    _ => unreachable!()
}

Capacity

The custom capacity of SmallBox<T, Space> and StackBox<T，Space> is expressed by the size of type Space, which default to space::S2 representing as 2 words space (2 * usize). There are some default options in module smallbox::space from S2 to S64. Anyway, you can defind your own space type, or just use some array.

The resize() method on StackBox<T, Space> and SmallBox<T, Space> is used to transforms themselves to the one of larger capacity.

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

let s = SmallBox::<[usize], S8>::new([0usize; 8]);
assert!(s.resize::<S16>().is_ok());

Roadmap

• Static size check (since mem::size_of() has become const fn)

Contribution

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.

License

Licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.