CODEWITHSUNDEEP
rust
May
May

Reputation: 121

rust lifetime with string slice on the stack

take the code from rustbook for example

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() {
        x
    } else {
        y
    }
}

we have the above function, now suppose we have main:

// can't compile , i understand this part, just post here for reference
fn main1(){
       let string1 = String::from("abcd");
        let result;
        {
            let string2 = String::from("xyz"); // we will change this to &str in next example
            result = longest(string1.as_str(), string2.as_str());
                                                ^^^^^^ doesn't compile , string2 doesn't live long enough
        }
        println!("The longest string is {}", result);
}

but if we slight change it to the code below, changing the string2 to be a string slice, this code actually compiles , and i dont quite know what's going on, does the "xyz" still count as valid memory?

//This compiles. but why? shouldn't ```longest()``` return a smaller lifetime and refuses to compile?
fn main2(){
    let string1 = String::from("abcd");
    let result;
    {
        let string2 = "xyz"; // <------ changed
        result = longest(string1.as_str(), string2);
    }
    println!("The longest string is {}", result);

}

Upvotes: 0

Views: 76

Answers (3)

Miiao
Miiao

Reputation: 998

string2 will always be dropped, but in first example string2 owns your data, in second it’s just a reference to ’static data, so the data won’t be invalidated after string2’s drop.

Upvotes: 1

Mikdore
Mikdore

Reputation: 799

The lifetime of a string literal is 'static (the complete type is &'static str), which means it will live until the end of the program. That's because strings are put as data into a special section of the compiled binary, so as long as the program is running, that data is also accessible.

Upvotes: 1

Chayim Friedman
Chayim Friedman

Reputation: 70860

"xyz" is &'static str, it is valid for the entire duration of the program.

Upvotes: 1

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