Value referencing data owned by the current function

Question

Here's my code:

struct Something<'a> {
    val: u32,
    another: &'a AnotherThing,
}

struct AnotherThing {
    val: u32,
}

impl Default for AnotherThing {
    fn default() -> Self {
        Self {
            val: 2,
        }
    }
}

trait Anything {
    fn new(val: u32) -> Self;
}

impl Anything for Something<'_> {
    fn new(val: u32) -> Self {
        Self {
            val,
            another: &AnotherThing::default(),
        }
    }
}

fn main() {
    let _ = Something::new(1);
}

It doesn't compile because:

   Compiling playground v0.0.1 (/playground)
error[E0515]: cannot return value referencing temporary value
  --> src/main.rs:24:9
   |
24 | /         Self {
25 | |             val,
26 | |             another: &AnotherThing::default(),
   | |                       ----------------------- temporary value created here
27 | |         }
   | |_________^ returns a value referencing data owned by the current function

I understand the problem but I don't know how to fix it. If it's not possible to use the Default trait for this case, how can I deal with the function ownership. Below a simpler example:

struct Something<'a> {
    val: u32,
    another: &'a AnotherThing,
}

struct AnotherThing {
    val: u32,
}

trait Anything {
    fn new(val: u32) -> Self;
}

impl Anything for Something<'_> {
    fn new(val: u32) -> Self {
        let at = AnotherThing { val : 2 };
        Self {
            val,
            another: &at,
        }
    }
}

fn main() {
    let _ = Something::new(1);
}

If I had another: &AnotherThing { val : 2 } instead of another: &at it would work. If I want the another attribute to be a reference and get the value from a function, how can I do it?

You cannot implement default for that structure. Instead you can make the "another" field optional -> another: Option<&'a Another>, then your default impl will just fill it with None — Svetlin Zarev
Does this answer your question? Is there any way to return a reference to a variable created in a function? — Svetlin Zarev
Many thanks for your help. I don't want the field to be optional. I could copy the default() code in the new() method and it would work. But I was wondering how I could implement the another: &AnotherThing::default() part. Even if it's not with the Default trait, but how the function can give ownership? I'll write another example. — user1238214
I read your link, there was another link stackoverflow.com/questions/32300132/… but I didn't get everything. — user1238214

Zeppi Zeppi · Accepted Answer · 2021-10-04T11:03:58

You can do like this

#[derive(Default)]
struct Something<'a> {
    val: u32,
    another: &'a AnotherThing,
}

struct AnotherThing {
    val: u32,
}

impl<'a> Default for &'a AnotherThing {
    fn default() -> &'a AnotherThing {
        &AnotherThing {
            val: 3,
        }
    }
}

trait Anything {
    fn new(val: u32) -> Self;
}

impl Anything for Something<'_> {
    fn new(val: u32) -> Self {
        Self {
            val,
            ..Default::default()
        }
    }
}

Value referencing data owned by the current function

1 Answers