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String & &str

By · Reviewed & last updated · Part of Rust Tutorial

Rust has two string types: String (owned, heap, growable) and &str (borrowed slice). Both are UTF-8. Indexing by byte position can split a codepoint — the API uses methods instead.

String vs &str, slicing, iteration

EXAMPLE
fn main() {
    // 1) Create
    let owned: String = String::from("hello");
    let borrowed: &str = "hello";              // string literal — &'static str
    let from_lit: String = "hi".to_string();
    let pushed = format!("{} world", owned);

    // 2) Convert between
    let s: String = borrowed.to_owned();
    let view: &str = &s;

    // 3) Grow
    let mut name = String::from("Ada");
    name.push(' ');
    name.push_str("Lovelace");
    name += "!";

    // 4) UTF-8 reality — slicing by byte CAN split a codepoint
    let café = String::from("café");
    println!("{}", café.len());                  // 5 — BYTES, not chars
    // let bad = &café[0..3];                    // OK — 'caf'
    // let bad = &café[0..4];                    // PANIC — splits 'é'

    // Iterate properly
    for c in café.chars() { print!("{c} "); }
    for (i, c) in café.char_indices() { println!("{i} {c}"); }
    for b in café.bytes() { print!("{b} "); }

    // 5) Find + slice safely
    if let Some(idx) = café.find('é') {
        let (before, rest) = café.split_at(idx);
        println!("{before} | {rest}");
    }

    // 6) String operations
    let s = "  Hello, World  ";
    s.trim();
    s.to_lowercase();
    s.replace(',', " -");
    s.split(',').collect::<Vec<_>>();
    s.contains("World");
    s.starts_with("  Hello");

    // 7) Parse + format
    let n: i32 = "42".parse().expect("not a number");
    let f: f64 = "3.14".parse().unwrap();
    let msg = format!("got n={n} f={f:.2}");
}

Why it matters

&str is what you accept as a parameter; String is what you return when you own the data. Forcing callers to allocate just because your signature says String is the most common Rust API smell.

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
let owned: String = String::from("hi");
let borrowed: &str = &owned;
println!("{} ({} chars)", borrowed, owned.chars().count());
Try it Yourself »

Discussion

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