Box

All values in Rust are stack allocated by default. Box is basically a pointer to a heap allocated value. When Box goes out of scope, its destructor is called, freeing up the memory on the heap.

Question: when / why to do box in rust?

We use Box whenever we need to put something on the heap and have a pointer to it. Because by default Rust values live on the stack, and their size must be known at compile time.

Use Box when:

• Recursive types - without Box the type woul dhave inifnite size:

enum List { Cons(i32, Box<List>), Nil }

• Types whose size can’t be known at compile time but you need a sized handle:

Trait objects like Box<dyn Error> or Box<dyn MyTrait> put the unknown‑sized value on the heap and give you a pointer of known size so functions can return or store them easily.

• Moving large data efficiently:

If you have a big struct and want to move ownership around without copying all its bytes on the stack, you can store it in a Box and move just the pointer (cheap copy).

Don’t need Box

For example, Vec, String, HashMap<K, V>, etc., these already have a pointer, so dont need Box for them.

Reference