Lifetimes and References

Longest Reference

Returning a Borrow

Lifetime annotations connect the returned reference to the input references it may point at.

Program

Play the program to compare two borrowed strings and return the longer one.

suffix
longest_reference.rs
Replay: real traced execution (multi-file project)
fn longest<'a>(left: &'a str, right: &'a str) -> &'a str {
    if left.len() >= right.len() { left } else { right }
}

fn main() {
    let left = "rust";
    let suffix = "book";
    let winner = longest(left, suffix);
    println!("{winner}");
}

fn longest<'a>(left: &'a str, right: &'a str) -> &'a str {
    if left.len() >= right.len() { left } else { right }
}

fn main() {
    let left = "rust";
    let suffix = "guide";
    let winner = longest(left, suffix);
    println!("{winner}");
}

fn longest<'a>(left: &'a str, right: &'a str) -> &'a str {
    if left.len() >= right.len() { left } else { right }
}

fn main() {
    let left = "rust";
    let suffix = "rs";
    let winner = longest(left, suffix);
    println!("{winner}");
}

  1. left ← "rust", suffix ← "book"

    5fn main() {6    let lef→ "rust"t = "rust";7    let suffi→ "book"x = "book"; //@suffix="guide", "rs"8    let winner = longest(lef"rust"t, suffi"book"x);9    println!("{winner}");

  2. if left.len() >= right.len()

    1fn longest<'a>(left: &'a str, right: &'a str) -> &'a str {2    if left.len() >= right.len() { lef"rust"t } else { right }3}

  3. winner ← "rust"

    7    let suffix = "book"; //@suffix="guide", "rs"8    let winne→ "rust"r = longest(lef"rust"t, suffi"book"x);9    println!("{winner}");10}
    outputrust

  1. left ← "rust", suffix ← "guide"

    5fn main() {6    let lef→ "rust"t = "rust";7    let suffi→ "guide"x = "guide";8    let winner = longest(lef"rust"t, suffi"guide"x);9    println!("{winner}");

  2. else

    1fn longest<'a>(left: &'a str, right: &'a str) -> &'a str {2    if left.len() >= right.len() { left } else { righ"guide"t }3}

  3. winner ← "guide"

    7    let suffix = "guide";8    let winne→ "guide"r = longest(lef"rust"t, suffi"guide"x);9    println!("{winner}");10}
    outputguide

  1. left ← "rust", suffix ← "rs"

    5fn main() {6    let lef→ "rust"t = "rust";7    let suffi→ "rs"x = "rs";8    let winner = longest(lef"rust"t, suffi"rs"x);9    println!("{winner}");

  2. if left.len() >= right.len()

    1fn longest<'a>(left: &'a str, right: &'a str) -> &'a str {2    if left.len() >= right.len() { lef"rust"t } else { right }3}

  3. winner ← "rust"

    7    let suffix = "rs";8    let winne→ "rust"r = longest(lef"rust"t, suffi"rs"x);9    println!("{winner}");10}
    outputrust
lifetime parameter `'a` names the relationship between input and output references.
borrow return The function returns one of the borrowed inputs, not a new string.
comparison The branch decides which borrowed value is returned.