Box<T>

Box<T> owns heap-allocated T with single ownership - like Vec but single element.

Recipe

enum List { Cons(i32, Box<List>), Nil }
use List::{Cons, Nil};
 
fn main() {
    let list = Cons(1, Box::new(Cons(2, Box::new(Nil))));
}

When to reach for this: Recursive types, large values on stack avoidance, trait objects Box<dyn Trait>.

Working Example

trait Draw { fn draw(&self); }
struct Circle;
impl Draw for Circle { fn draw(&self) { println!("circle"); } }
 
fn main() {
    let shapes: Vec<Box<dyn Draw>> = vec![Box::new(Circle)];
    for s in shapes { s.draw(); }
}

What this demonstrates:

• Recursive enum with Box for indirection
• Trait object on heap via Box<dyn Draw>
• Single owner per box

Deep Dive

Box implements Deref, Drop frees heap. Box<[T]> from Vec via into_boxed_slice. No refcount overhead.

Gotchas

• Box everywhere unnecessarily - Prefer owned struct on stack if small. Fix: Profile stack size.
• Box common - Ok for apps; libraries prefer typed errors.
• Moving out of box - *box or Box::into_inner (1.66+).
• Pin<Box<T>> - For self-referential async - advanced.
• Box in no_std - alloc::boxed::Box with allocator.

Alternatives

FAQs

Related

• Smart Pointers Basics
• Trait Objects
• Deref & Deref Coercion
• Custom Smart Pointers

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.