Store reference of struct in other struct

Question

I have two structs. App and Item.

What I want to achieve is to store an Item in the items vector of the App struct by passing a mutable reference to the Items constructor.

pub struct App<'a> { items: Vec<&'a Item> } impl<'a> App<'a> { pub fn new() -> App<'a> { App { items: Vec::new() } } pub fn register_item(&mut self, item: &'a Item) { self.items.push(item); } } pub struct Item; impl Item { pub fn new(app: &mut App) -> Item { let item = Item; app.register_item(&item); item } } fn main() { let mut app = App::new(); let item = Item::new(&mut app);; }

The code thows the following error:

test.rs:8:28: 8:32 error: `item` does not live long enough test.rs:8 app.register_item(&item);

Is there any way to do this?

This already has an answer in stackoverflow.com/questions/23135409/… and in stackoverflow.com/questions/28608823/… — oli_obk
– oli_obk, Commented May 29, 2015 at 20:53
Apart from the immediate error, storing pointers in the Vec seems wrong. You almost certainly want a Vec<Item> in App. — user395760
– user395760, Commented May 29, 2015 at 21:01

Shepmaster · Accepted Answer · 2017-08-15 12:48:35Z

While Rc might be correct for your use case, it's good to understand why you are getting the error you are. Please read Why can't I store a value and a reference to that value in the same struct? as it has a much more in-depth discussion about why your code cannot work as-is. A simplified explanation follows.

Let's look at your constructor:

fn new(app: &mut App) -> Item { let item = Item; app.register_item(&item); item }

Here, we create a new Item on the stack at some address. Let's pretend that address is 0x1000. We then take the address of item (0x1000) and store it into the Vec inside the App. We then return item to the calling function, which resides in a different stack frame. That means that the address of item will change, which means that 0x1000 is no longer guaranteed to point to a valid Item! This is how Rust prevents you from making whole classes of memory errors!

I'd say that you'd normally see this written as:

fn main() { let item = Item; let mut app = App::new(); app.register_item(&item); }

This will have the same problem if you tried to return app or item from this function, as the address will change.

If you have a straight-forward tree structure, I'd advocate for simply letting the parent nodes own the children:

struct App { items: Vec<Item> } impl App { fn new() -> App { App { items: Vec::new() } } fn register_item(&mut self, item: Item) { self.items.push(item); } } pub struct Item; fn main() { let mut app = App::new(); app.register_item(Item); }

The last piece of crucial information for me was that if you want to later access or modify an Item, you must do so via the app's Vec, instead of directly. See my playground that helped me understand here: play.rust-lang.org/…

lschmierer · Accepted Answer · 2015-05-29 23:33:51Z

Simple solution is to use Rc.

use std::rc::Rc; pub struct Item; impl Item { pub fn new(app: &mut App) -> Rc<Item> { let item = Rc::new(Item); app.register_item(item.clone()); item } } pub struct App { items: Vec<Rc<Item>> } ...

Collectives™ on Stack Overflow

Store reference of struct in other struct

2 Answers 2

1 Comment

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Linked

Related