Explain push a struct into a vec as reference?

[zoombinis]

Hi,

I am also having the same issue documented here:

Push a struct into a vec and pass its reference within a loop

I'm struggling to figure out how to iterate over a collection of structs, and, within the loop, push the struct into a new vec and pass its reference to another struct. It's a hard problem to put words to, so here's some example code that demonstrates the problem: fn main() { works(); does_not_work(); } fn works() { let original = vec![TestObj::new("a"), TestObj::new("b"), TestObj::new("c")]; let mut new = Vec::new(); let mut holder = Holder::new(); for object in orig…

Can anyone offer an explanation why you must iterate twice? I am new and cannot reason about this, thank you!

[alice]

It's because push could reallocate the vector, which would make any existing references point at the wrong thing. However, once everything is pushed, you know the locations in memory wont change anymore.

[alice]

Note that you can still do stuff like this:

fn main() {
    let mut vec = vec![0; 1024];
    let mut refs: Vec<&i32> = Vec::new();
    
    let mut counter = 0;
    for mut_ref in vec.iter_mut() {
        *mut_ref = counter;
        counter += 1;
        
        refs.push(&*mut_ref);
    }
    
    println!("{:?}", refs);
}

[zoombinis]

but wasn't everything already "pushed" into the original? I don't see the difference between original and new I guess. THanks for the quick answer! I'm sure it's right I'm just missing the understanding still.

[alice]

Sure, but they wanted references that point into new, and new did not have everything pushed yet.