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:

toml [dependencies] smallbox = "0.2"

Next, add this to your crate root:

rust 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:

toml [dependencies] smallbox = { version = "0.2", default-features = false }

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

toml [dependencies.smallbox] version = "0.2" default-features = false features = ["heap"]

Feature Flags

The arraydeque 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 a fixed-capacity allocation, and on stack stores dynamically-sized type. The new method on this type allows creating a instance from a concrete type, returning Err(value) if the instance is too large for the allocated region. So far, the fixed-capcity is about four words (4 * sizeof(usize))

SmallBox<T>: Takes StackBox<T> as an varience, and fallback to Box<T> when type T is too large for StackBox<T>.

Example

The simplest usage can be trait object dynamic-dispatch ```rust use smallbox::StackBox;

let val: StackBox> = StackBox::new(5usize).unwrap();

assert!(*val == 5) ```

One of the most obvious use case is to allow returning capturing closures without having to box them.

```rust use smallbox::StackBox;

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

let closure = makeclosure("world!".toowned()); assert_eq!(closure(), "Hello, world!"); ```

The other use case 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 explicitely pattern matching on SmallBox<T>.

```rust use smallbox::SmallBox;

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

asserteq!(tiny.len(), 2); asserteq!(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!() } ```

Roadmap

• method that convert SmallBox to Box
• conveniently convert bewteen SmallBox<T> and StackBox<T>
• configurable StackBox<T> allocation size
• dowancasting for StackBox<Any> and SmallBox<Any>

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.