Box<T>
- Rust course
- Intermediate
- Lesson 30/115
- 4 min
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 heapUses: 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.
Practice 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.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn main() {
let b = Box::new(41);
let n = *b + 1;
let buyuk = Box::new([0u8; 1000]);
println!("{} {}", n, buyuk.len());
}Expected output
42 1000
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How much stack space does a <code>Box<[u8; 1000]></code> variable take?
- 0
- One pointer (8 bytes on a 64-bit system) Correct answer
- 1000 bytes
- 1008 bytes
Explanation: The data is on the heap; only the address is on the stack. Moving it means copying 8 bytes.
2 Read the content
let b = Box::new(41);
let n = ____b + 1; // 42let b = Box::new(41);
let n = *b + 1; // 42Explanation: * gives the value inside the box.
3 Expression and meaning
| Term | Meaning |
|---|---|
Box::new(5) |
5 on the heap |
*b |
Content |
Box<dyn Display> |
Trait object |
drop(b) |
Early drop |