Box<T>

Allocate on the heap with Box<T>.

Box<T>

Box<T> is the simplest smart pointer: it puts a value on the heap and has a single owner. When the box goes out of scope, the heap value is dropped too. The content is read with *:

let b = Box::new(41);
let n = *b + 1;          // 42
let buyuk = Box::new([0u8; 1000]);   // 1000 bytes on the heap

Uses: types whose size isn't known at compile time (recursive types, dyn Trait), moving large values without copying.

Box and trait objects

The most common use is keeping different types behind the same interface. Box<dyn Trait> is a "fat pointer": data address + vtable address:

use std::fmt::Display;

let ogeler: Vec<Box<dyn Display>> = vec![Box::new(3), Box::new("üç"), Box::new(3.0)];
for o in &ogeler {
    print!("{o} ");   // 3 üç 3
}

Box is dereferenced automatically when calling methods; b.len() calls the inner value's method.

Exercise Boxes

Create Box::new(41) and add 1 to its content. Also create Box::new([0u8; 1000]). Print the sum and the array's length separated by a space.

Show example solution

fn main() {
    let b = Box::new(41);
    let n = *b + 1;
    let buyuk = Box::new([0u8; 1000]);
    println!("{} {}", n, buyuk.len());
}

Test yourself

How much stack space does a <code>Box<[u8; 1000]></code> variable take?
- 0
- One pointer (8 bytes on a 64-bit system)
- 1000 bytes
- 1008 bytes
Show answer

One pointer (8 bytes on a 64-bit system) — The data is on the heap; only the address is on the stack. Moving it means copying 8 bytes.

Read the content

let b = Box::new(41);
let n = ____b + 1;   // 42

Show answer

let b = Box::new(41);
let n = *b + 1;   // 42

Expression and meaning

Box::new(5)

5 on the heap
*b

Content
Box<dyn Display>

Trait object
drop(b)

Early drop

Next lesson: Rc<T>

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies