How can I borrow the item in an Option or create a new item when it's None?

Question

When I have an Option and want a reference to what's inside or create something if it's a None I get an error.

Example Code:

fn main() {
    let my_opt: Option<String> = None;

    let ref_to_thing = match my_opt {
        Some(ref t) => t,
        None => &"new thing created".to_owned(),
    };

    println!("{:?}", ref_to_thing);
}

playground

Error:

error[E0597]: borrowed value does not live long enough
  --> src/main.rs:6:18
   |
6  |         None => &"new thing created".to_owned(),
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
   |                  |                            |
   |                  |                            temporary value dropped here while still borrowed
   |                  temporary value does not live long enough
...
10 | }
   | - temporary value needs to live until here

Basically the created value doesn't live long enough. What is the best way to get a reference to the value in a Some or create a value if it's a None and use the reference?

See also [Converting from Option to Option<&str>](https://stackoverflow.com/q/31233938/155423). — Shepmaster, Aug 12 '18 at 13:33

ljedrz · Accepted Answer · 2018-08-12T13:28:34.347

6

You can also just write:

None => "new thing created"

With this adjustment your initial variant of the code will compile without the need of an extra variable binding.

An alternative could also be:

let ref_to_thing = my_opt.unwrap_or("new thing created".to_string());

edited Aug 12 '18 at 13:28

answered Aug 12 '18 at 04:10

ljedrz

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Well my attempt to simplify my problem to an example failed. I wasn't using &str or String. In my original problem it was a &DatabaseConnection which wasn't easily solved that way as well as having the option be part of &mut of a struct which made things interesting. But yes for my example you are correct. – sbditto85 Sep 06 '18 at 04:30
Going to mark this as the correct answer as in my lame example it is. – sbditto85 Sep 06 '18 at 04:34
Worth noting that `unwrap_or` takes ownership of the Option which is not always possible. Like if the Option is owned by a struct and you only have an `&mut` of the struct. [unwrap_or](https://doc.rust-lang.org/std/option/enum.Option.html#method.unwrap_or) – sbditto85 Sep 06 '18 at 04:40

score 4 · Answer 2 · edited Aug 12 '18 at 12:52

The only way I've found is to create a "dummy variable" to hold the created item and give it a lifetime:

fn main() {
    let my_opt: Option<String> = None;

    let value_holder;
    let ref_to_thing = match my_opt {
        Some(ref t) => t,
        None => {
            value_holder = "new thing created".to_owned();
            &value_holder
        }
    };

    println!("{:?}", ref_to_thing);
}

playground

score 3 · Answer 3 · answered Aug 12 '18 at 12:56

3

If you don't mind mutating your Option in place, you can use Option::method.get_or_insert_with:

fn main() {
    let mut my_opt: Option<String> = None;

    let ref_to_thing = my_opt.get_or_insert_with(|| "new thing created".to_owned());

    println!("{:?}", ref_to_thing);
}

answered Aug 12 '18 at 12:56

Shepmaster

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Interesting, learned something new. For my original problem that wouldn't work, but for the example I suppose it could. Thanks! – sbditto85 Sep 06 '18 at 04:32

How can I borrow the item in an Option or create a new item when it's None?

3 Answers3