Lifetimes and References

Struct Reference

Holding a Borrowed Field

A struct can hold a reference when its lifetime parameter says the reference stays valid long enough.

Program

Play the program to borrow a label and repeat the borrowed text.

repeat
struct_reference.rs
Replay: real traced execution (multi-file project)
struct Label<'a> {
    text: &'a str,
}

fn main() {
    let word = "trace";
    let repeat = 2;
    let label = Label { text: word };
    let output = label.text.repeat(repeat);
    println!("{output}");
}

struct Label<'a> {
    text: &'a str,
}

fn main() {
    let word = "trace";
    let repeat = 3;
    let label = Label { text: word };
    let output = label.text.repeat(repeat);
    println!("{output}");
}

struct Label<'a> {
    text: &'a str,
}

fn main() {
    let word = "trace";
    let repeat = 4;
    let label = Label { text: word };
    let output = label.text.repeat(repeat);
    println!("{output}");
}

  1. word ← "trace", repeat ← 2, label ← (empty), output ← "tracetrace"

    5fn main() {6    let wor→ "trace"d = "trace";7    let repea→ 2t = 2; //@repeat=3, 48    let labe→ (empty)l = Label { text: wor"trace"d };9    let outpu→ "tracetrace"t = label.text.repeat(repeat);10    println!("{output}");11}
    outputtracetrace

  1. word ← "trace", repeat ← 3, label ← (empty), output ← "tracetracetrace"

    5fn main() {6    let wor→ "trace"d = "trace";7    let repea→ 3t = 3;8    let labe→ (empty)l = Label { text: wor"trace"d };9    let outpu→ "tracetracetrace"t = label.text.repeat(repeat);10    println!("{output}");11}
    outputtracetracetrace

  1. word ← "trace", repeat ← 4, label ← (empty), output ← "tracetracetracetrace"

    5fn main() {6    let wor→ "trace"d = "trace";7    let repea→ 4t = 4;8    let labe→ (empty)l = Label { text: wor"trace"d };9    let outpu→ "tracetracetracetrace"t = label.text.repeat(repeat);10    println!("{output}");11}
    outputtracetracetracetrace
lifetime on struct `Label<'a>` says the struct contains a reference valid for `'a`.
borrowed field `text: &'a str` stores a borrowed string slice.
method on borrow `label.text.repeat(repeat)` reads through the borrowed field.