What is Box?

In rust Box is a smart pointer that gives access to allocate values in heap. It is a smart pointer because the compiler will automatically decide when to free that value in the heap. Also Box values has a single owner which is the reference.

let x: i32 = 5;
let box_x: i32 = Box::new(x); // allocates the value x in the heap
// box_x is an value of type Box(smart pointer) which has the address of the allocated value

Just like any other value in Rust the value of type Box also has a single owner, in the above example box_x is the owner. When the variable box_x goes out of scope the value allocated in the heap will be freed. Box is not Copy so if you pass it as an argument to a function or assign it to a new variable it will obey the Move semantics.

Accessing values in Box

// Reading values
let a: Box<i32> = Box::new(13);
print!("{}", a);

// Mutation example
struct Point {
    x: i32,
    y: i32,
}
let mut b: Box<Point> = Box::new(Point{x: 12, y: -4});
b.x = 25;

Box is part of the safe Rust meaning, it will always point to a valid memory location, it can never point to a NULL and it works in a well defined manner unless the allocation in heap itself fails because of no free memory in the RAM.

If there is no NULL supported then how to represent missing values?

Here comes Option with Box to the rescue.

struct Person {
    name: String,
    age: u8,
}
fn may_exist() -> Option<Box<Person>> {
    // Do some computation
    if computation_succeeded() {
        Some(Box::new(Person{
            name: "name",
            age: 42,
        }))
    } else {
        None
    }
}

None variant of the Option can be used to denote missing values. unwarpping a None results in panic which is still a well defined, reproducible behaviour.

But Why do we need to allocate memory in Heap?

1. To simplify usage of recursive non cyclic data types like linked list, tree etc.,

2. You need to transfer the ownership of a value without copying it

3. Trait objects, that is values that implement a specific trait can be owned using Box.

To read more about the above three points refer rust book.