Box

Box is an owned pointer to data on the heap:

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

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

Recursive data types or data types with dynamic sizes cannot be stored inline without a pointer indirection. Box accomplishes that indirection:

#[derive(Debug)]
enum List<T> {
    /// A non-empty list: first element and the rest of the list.
    Element(T, Box<List<T>>),
    /// An empty list.
    Nil,
}
fn main() {
    let list: List<i32> =
        List::Element(1, Box::new(List::Element(2, Box::new(List::Nil))));
    println!("{list:?}");
}

This slide should take about 8 minutes.

Box is like std::unique_ptr in C++, except that it’s guaranteed to be not null.
A Box can be useful when you:
- have a type whose size 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 be able to compute a fixed size for the struct in memory (the 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. We get the message “recursive without indirection”, because for data recursion, we have to use indirection, a Box or reference of some kind, instead of storing the value directly.
Though Box looks like std::unique_ptr in C++, it cannot be empty/null. This makes Box one of the types that allow the compiler to optimize storage of some enums (the “niche optimization”).

20.1. Box<T>

Box