Box<T>

Box is an owned pointer to data on the heap:

fn main() {
    let five = Box::new(5);
    println!("five: {}", *five);
}
5StackHeapfive

Box<T> implements Deref<Target = T>, which means that you can call methods from T directly on a Box<T>.

Recursive data types or data types with dynamic sizes need to use a Box:

#[derive(Debug)]
enum List<T> {
    Cons(T, Box<List<T>>),
    Nil,
}

fn main() {
    let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
    println!("{list:?}");
}
StackHeaplistCons1Cons2Nil
This slide should take about 10 minutes.

  • Box is like std::unique_ptr in C++, except that it’s guaranteed to be not null.

  • In the above example, you can even leave out the * in the println! statement thanks to Deref.

  • A Box can be useful when you:

    • have a type whose size that can’t be known at compile time, but the Rust compiler wants to know an exact size.
    • want to transfer ownership of a large amount of data. To avoid copying large amounts of data on the stack, instead store the data on the heap in a Box so only the pointer is moved.

  • If Box was not used and we attempted to embed a List directly into the List, the compiler would not compute a fixed size of the struct in memory (List would be of infinite size).

  • Box solves this problem as it has the same size as a regular pointer and just points at the next element of the List in the heap.

  • Remove the Box in the List definition and show the compiler error. “Recursive with indirection” is a hint you might want to use a Box or reference of some kind, instead of storing a value directly.

More to Explore

Niche Optimization

#[derive(Debug)]
enum List<T> {
    Cons(T, Box<List<T>>),
    Nil,
}

fn main() {
    let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
    println!("{list:?}");
}

A Box cannot be empty, so the pointer is always valid and non-null. This allows the compiler to optimize the memory layout:

StackHeaplist12null