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Rust Lifetime

Nov 05, 20232 min read

Lifetime annotations don’t change how long any of the references live. Rather, they describe the relationships of the lifetimes of multiple references to each other without affecting the lifetimes.

fn main() {
    let string1 = String::from("abcd");
    let string2 = "xyz";
 
    let result = longest(string1.as_str(), string2);
    println!("The longest string is {}", result);
}
 
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() {
        x
    } else {
        y
    }
}
 
struct Book<'a> {
    author: &'a str,
    title: &'a str,
}
 
fn main() {
    let name = String::from("Jill Smith");
    let title = String::from("Fish Flying");
    let book = Book { author: &name, title: &title };
 
    println!("{} by {}", book.title, book.author);
}

Struct Lifetime

struct ImportantExcerpt<'a> {
    part: &'a str,
}
 
fn main() {
    let novel = String::from("Call me Ishmael. Some years ago...");
    let first_sentence = novel.split('.').next().expect("Could not find a '.'");
    let i = ImportantExcerpt {
        part: first_sentence,
    };
}

This annotation means an instance of ImportantExcerpt can’t outlive the reference it holds in its part field.

static Lifetime

The ​'static​ lifetime​​ is the longest​​ possible lifetime​​, and it lasts​​ for the entire​​ duration of the​​ running program.
The text of the string is stored directly in the program’s binary, which is always available.

There are two ways to make a variable with a 'static lifetime, and both are stored in the read-only memory of the binary:

  1. Make a constant with the static declaration.
  2. Make a string literal which has type &'static str.
// A reference with 'static lifetime:
let s: &'static str = "hello world";
 
// 'static as part of a trait bound:
fn generic<T>(x: T) where T: 'static {}

Elision

fn elided_input(x: &i32) {
    println!("`elided_input`: {}", x);
}
 
fn annotated_input<'a>(x: &'a i32) {
    println!("`annotated_input`: {}", x);
}
 
fn elided_pass(x: &i32) -> &i32 { x }
 
fn annotated_pass<'a>(x: &'a i32) -> &'a i32 { x }

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