Rust consume a vector inside a struct

April 5, 2023

I have a Container struct containing a vector, which I’d like to consume while doing some work on it. For a standalone vector the following works:

fn empty(vec : &mut Vec<f32>){
    vec.into_iter().for_each(|x|println!("value: {:?}", x));
}

fn main() {
    let mut vec = vec![1.0,2.0,3.0,4.0];
    empty(&mut vec);
    assert!(0 == vec.len());
}

However, trying this with the Container structure introduce fails:

struct Container{
    vec : Vec<f32>,
}

fn empty(c : &mut Container){
    c.vec.into_iter().for_each(|x|println!("value: {:?}", x));
}

fn main() {
    let mut container = Container{
        vec: vec![1.0,2.0,3.0,4.0]
    };
    empty(&mut container);
}

One Solution I think would work is to iterate over the vector without consuming it, and then assign an empty vector.

fn empty(c : &mut Container){
    c.vec.iter().for_each(|x|println!("value: {:?}", x));
    c.vec = Vec::new();
}

But is there a better way to do it?

>Solution :

You can use Vec::drain to remove elements from your vector and create an iterator from the removed elements:

struct Container{
    vec : Vec<f32>,
}

fn empty(c : &mut Container){
    c.vec.drain(..).for_each(|x|println!("value: {:?}", x));
}

fn main() {
    let mut container = Container{
        vec: vec![1.0,2.0,3.0,4.0]
    };
    empty(&mut container);
    assert!(container.vec.is_empty());
}

Playground.