Struct with immutable reference to other struct

Question

First of all I want to apologise for this noob question with an unspecific title, I'm quite new to Rust.

Anyway, here is some (working) code:

struct A {
    data: i32
}

struct B<'s> {
    a: &'s A
}

impl<'s> B<'s> {
    fn new(reference: &'s A) -> B<'s> {
        B {
            a: reference
        }
    }
}

fn main() {
    let a1 = A{data: 0};
    let b1 = B::new(&a1);
    let b2 = B::new(&a1);
}

There is a struct A with some data and a struct B that contains an immutable reference to A. In the main method several B-objects are created with references to a single A-object.

Now I only want to change one thing: In the B::new() method I want to modify the data of 'reference' before using it as the immutable member of B. I tried it like this:

struct A {
    data: i32
}

struct B<'s> {
    a: &'s A
}

impl<'s> B<'s> {
    fn new(reference: &'s mut A) -> B<'s> {

        // Modify data
        reference.data += 1;

        B {
            a: reference
        }
    }
}

fn main() {
    let mut a1 = A{data: 0};
    let b1 = B::new(&mut a1);
    let b2 = B::new(&mut a1);
}

But the compiler won't let me, error: cannot borrow a1 as mutable more than once at a time. Why isn't the mutable borrow over once new() has finished? What would be the proper way to achieve what I'm trying?

tafia tafia · Accepted Answer · 2015-07-20T11:44:48

As @AndreaP said, the issue is because of your reference with the same lifetime.

Usually you can work around these issues by simply splitting your functions (one & and one &mut).

If you really need this kind on pattern, then I think what you are looking for is Cell/RefCell: http://doc.rust-lang.org/std/cell/index.html

EDIT: Thanks to @Ker comment, I updated the code so B indeed holds a reference to A instead of its copy. Old code was just totally wrong.

use std::cell::{Cell};

#[derive(Debug)]
struct A {
    data: Cell<i32>
}

#[derive(Debug)]
struct B<'a> {
    a: &'a A
}

impl<'a> B<'a> {
    fn new(reference: &'a A) -> B<'a> {
        // Modify data
        reference.data.set(reference.data.get() + 1);
        B {
            a: reference
        }
    }
}

fn main() {
    let a1 = A{data: Cell::new(0)};
    let b1 = B::new(&a1);
    let b2 = B::new(&a1);
    println!("{}", a1.data.get());
    println!("{:?}", b1);
    println!("{:?}", b2);
}

Struct with immutable reference to other struct

2 Answers