smallbox 0.8.7

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

smallbox

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Small Box optimization: store small items on the stack and fall back to heap allocation for large items. Requires Rust 1.56+.

Documentation

Usage

First, add the following to your Cargo.toml:

[dependencies]

smallbox = "0.8"

Next, add this to your crate root:

extern crate smallbox;

If you want this crate to work with dynamically-sized types, you can enable it via:

[dependencies]

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:

[dependencies.smallbox]

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 the std feature flag is disabled, the alloc crate will be linked, which requires nightly Rust.

  • coerce

    • Optional
    • Requires nightly Rust
    • Allow automatic coercion from sized SmallBox to unsized SmallBox.

Unsized Types

There are two ways to create an unsized SmallBox: using the 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 the given value and the target type T. For any invalid type coercions, this macro will trigger a compile-time error.

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

Example

Eliminate heap allocation for small items with 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_eq!(small.is_heap(), false);
assert_eq!(large.is_heap(), true);

Unsized Types

Construct with the smallbox!() macro:

#[macro_use]
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 the 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:

#[macro_use]
extern crate smallbox;

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

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

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

Capacity

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

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

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

Benchmark

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

Contribution

All kinds of contributions are welcome.

  • Issues Feel free to open an issue when you find typos, bugs, or have any questions.
  • Pull requests Better implementations, more tests, more documentation, and typo fixes are all welcome.

License

Licensed under either of

at your option.