fn main() {
    // Create a Box to store an i32 on the heap.
    // Box<T> ensures memory is freed when the Box goes out of scope.
    let b1 = Box::new(5);
    println!("Heap allocated value: {}", b1);

    // Boxes can be used for recursive data structures, like a simple Cons list.
    enum List {
        Cons(i32, Box<List>),
        Nil,
    }

    use List::{Cons, Nil};

    let list = Cons(1, Box::new(Cons(2, Box::new(Cons(3, Box::new(Nil))))));

    // When `list` goes out of scope, all allocated memory for the list elements
    // will be automatically deallocated by Box's Drop implementation.
    println!("List created on heap.");
}