Lifetimes and Smart Pointers

Lifetimes are Rust's way of ensuring that references are always valid. Smart pointers provide additional capabilities beyond regular references — like heap allocation, reference counting, and interior mutability.

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What Are Lifetimes?

Every reference in Rust has a lifetime — the scope for which the reference is valid. Most of the time, lifetimes are inferred. Sometimes you need to annotate them explicitly.

// This will not compile — r references x, but x does not live long enough
let r;
{
    let x = 5;
    r = &x; // ERROR: x does not live long enough
}
println!("{r}");

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Lifetime Annotations

Lifetime annotations describe the relationship between the lifetimes of multiple references. They do not change how long values live.

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

The annotation 'a says: "the returned reference will be valid for at least as long as the shorter of x and y."

Syntax