Rust cannot return value referencing local variable

user_3566441 0
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I guess this is a quite basic Rust question, but I have this piece of code

struct Stock<'a> {
    name: &'a str,
    value: u8,
}
impl<'a> Stock<'a> {
    fn read(file: &mut File) -> Stock<'a> {
        let mut value = vec![0; 1];
        file.read(&mut value);
        let name_size = read_u32(file);
        let mut name = vec![0; name_size as usize];
        file.read(&mut name);
        return Stock {
            name: str::from_utf8(&name).unwrap(),
            value: value[0],
        };
    }
}

and of course, it doesn't work, because I am referencing a local value name. To my limited knowledge, this is because outside the scope of read, name doesn't exist. However wouldn't from_utf8 create a new variable that is not scoped to read?

Also, I've read a bit about other people with this issue, and the suggestion is always to switch to the owned String. But why can't I do something such as name: str::from_utf8(&name).unwrap().to_owned()?

rust
2 Answers

However wouldn't from_utf8 create a new variable that is not scoped to read?

No, it gives you a borrow with the same lifetime as the one you pass.

Even if it created a new variable, you would have to tie it to somewhere, like the scope of read(), so again you would have the same problem.

The issue is that your Stock struct holds a reference. That means whatever it points to it has to live for all the time Stock is alive. But any variables you create in the local scope of read() will die when read() ends.

Instead, what you want is for Stock to own the value so that it keeps alive it itself.

user_93053980

You most likely should change the struct field to name: String and get rid of the 'a lifetime parameter.

user_11348850
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