created_at2020-08-02 22:06:09.914714
updated_at2023-04-30 04:04:43.746513
descriptionFreezeBox: a deref'able lazy-initialized container
Eric Seppanen (ericseppanen)



FreezeBox: atomic lazy-initialized ref-able containers.

This crate contains two similar container types: FreezeBox and MaybeBox. Both containers can be late-initialized using only a shared reference, and both allow a caller to get a reference to the value inside.

let x = FreezeBox::<String>::default();
assert_eq!(x.len(), 5);

This is useful for data structures that are shared first, but some members of that data structure gets initialized later.

let x = FreezeBox::<String>::default();
let shared_x = Arc::new(x);
assert_eq!(shared_x.len(), 5);

FreezeBox and MaybeBox share some behavior: they can only be initialized once; initialization is atomic; and the initialized value may never be removed, except by consuming the container with into_inner().

The main difference between FreezeBox and MaybeBox is that FreezeBox implements Deref. FreezeBox is intended to be used in situations where the inner value is always expected to be present before an attempted use. In this scenario, trying to read an uninitialized FreezeBox is a bug, so the attempted read will cause a panic.

MaybeBox is meant to be used in a situation where the inner value may sometimes be missing. MaybeBox does not implement Deref; instead we need to call get, which returns Option<&T>.

let x = MaybeBox::<String>::default();
if some_runtime_config {
if let Some(val) = x.get() {
    println!("{}", val);


This example creates a shared data structure, then initializes a member variable later.

use freezebox::FreezeBox;
use std::sync::Arc;

/// A data structure that we will initialize late.
struct Resources {
    name: FreezeBox<String>

// Create an instance of the `Resources` struct, which contains an
// uninitialized `name` field.
let resources = Arc::new(Resources::default());

// Clone the Arc to emulate sharing with other threads, contexts,
// or data structures.
let res2 = resources.clone();

// Here we emulate another thread accessing the shared data structure.
// NOTE: it's still our responsibility to ensure that the FreezeBox
// is initialized before anyone dereferences it.
let func = move || {
    // explicit deref
    assert_eq!(*, "Hello!");
    // implicit deref allows transparent access to inner methods
    assert_eq!(, 6);

Comparison to other approaches

  1. Option<T>

Late initialization requires mutable access to the Option. This is fine unless the parent struct is already shared.

  1. Mutex<Option<T>>

This solves the problem of late-initialization, but requires every caller to lock the Mutex and unwrap the Option. The added code and runtime overhead might not be desirable, paticularly if all we need is a shared reference to the inner T.

  1. lazy_static

lazy_static! declares a hidden static variable, so it's not suitable for lazy-initialized struct members. It also requires the initialization code to be placed at the point of declaration, and uses a spinlock internally.

  1. once_cell

once_cell is generally preferable to lazy_static in new Rust code, and would be a good choice in the case where multiple threads are racing to initialize the inner value.

OnceCell doesn't implement Deref, and requires explicit calls to get() or get_or_init(). This is similar to MaybeBox, but is more verbose in the use case FreezeBox was designed for, where readers expect the value to be already initialized.

OnceCell does not Box the internal value, but this makes the atomic initialization more complicated, so once_cell::sync::OnceCell is not available in no_std contexts.

Safety and Compatibility

FreezeBox is compatible with no_std projects (no feature flags needed). It may be used in any environment with a memory allocator.

FreezeBox uses unsafe code internally. To ensure soundness, the unit tests pass under Miri, and the unsafe code is simple and easy to understand.

The minimum supported Rust version is 1.48.

Commit count: 32

cargo fmt