Vector index vs reference

help
1

Hello there, I want to have a reference inside a vector, but the borrow checker for obvious reasons gives errors. Firstly I've solved it by using index instead of ref like in snippet below:

pub struct RenderSystem {
    current_pass_id: Option<usize>,
    passes: Vec<Pass>,
}

pub fn query(&mut self, command: RenderCommand) {
    assert!(self.current_pass_id.is_some(), "Can't query command without render pass");
    let id = self.current_pass_id.unwrap();
    self.passes[id].commands.push(command);
}

It works, but it looks not idiomatically for the rust. I could use lifetime for ref, but I don't want element to live while lives RenderSystem, every frame vector passes is being cleared, so lifetime is render function scope, but add this element to pass I can before enter to render function.

There is another solution I've come up with - use Rc and RefCell, but It looks pretty ugly and actually I think it's less idiomatic than the previous example:

pub struct RenderSystem {
    current_pass: Option<Rc<RefCell<Pass>>>,
    passes: Vec<Rc<RefCell<Pass>>>,
}

pub fn query(&mut self, command: RenderCommand) {
    assert!(
        self.current_pass.is_some(),
        "Can't query command without render pass"
    );
    let pass = self.current_pass.as_ref().unwrap();
    pass.borrow_mut().commands.push(command);
}

Actually, in my particular case I've solved this problem just by using the last element of the vector:

pub fn query(&mut self, command: RenderCommand) {
    assert!(
        !self.passes.is_empty(),
        "Can't query command without render pass"
    );
    let current_pass = self.passes.last_mut().unwrap();
    current_pass.commands.push(command);
}

So the problem has solved, but I still curious if there are any other solutions without RefCell and vector index. Or maybe it’s actually okay to have an index that refers to the desired item.

2

Storing an index into a Vec to emulate self-borrowing is fairly common, and probably the most idiomatic solution.

5 Likes
3

I would say the idiomatic solution is to use an index.

1 Like
4

I see, thanks!

5

Using indexes as pseudo-pointers is common. See, for example, Modeling graphs in Rust using vector indices · baby steps

6

petgraph is a good example using indexes.

7

… as is the ECS paradigm. (Google "Rust ECS" for a choice of relevant material.)

8

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