Rs
Ownership & Borrowing
Module 2Borrowing & ReferencesLesson 4 of 920 min

Immutable Borrows (&T)

Read data without taking ownership — the default for helpers and metrics.

Borrow with &

rust
fn mid(bid: &f64, ask: &f64) -> f64 {
    (*bid + *ask) / 2.0
}
// or simply take f64 by Copy for numbers — but the idea matters for String/struct

For non-Copy types:

rust
fn print_ticker(t: &String) {
    println!("{t}");
}
// better API: fn print_ticker(t: &str)

fn main() {
    let ticker = String::from("AAPL");
    print_ticker(&ticker);
    println!("still own: {ticker}");
}
Output
AAPL
still own: AAPL

Many readers OK

rust
fn main() {
    let s = String::from("IBM");
    let r1 = &s;
    let r2 = &s;
    println!("{r1} {r2}");
}
Output
IBM IBM

Exercise

Compute mid and spread from a borrowed quote struct.

Challenge

Add fn is_locked(q: &Quote, max_spread: f64) -> bool.

Tips
  • Field access through &Quote does not need explicit *
  • Prefer &str over &String in APIs
  • Borrowing keeps the owner valid

Try it live

Immutable Borrows (&T)Rust 2021
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Output
Press Run to compile and execute.