Borrowing and references

Rather than transferring ownership to a function, you can let the function borrow the variable

fn main() {
    let str = String::from("Harkirat");
    let len = get_length(&str);
    println!("{} {}", str, len);
}

fn get_length(str: &String) -> usize {
    let len = str.len();
    return len
}

When you pass a variable by reference, the variable is still owned by the first function. It is only borrowed by the get_length function.

Rules of borrowing -

1. You can only have one immutable reference. If there is an immutable reference, there cant be other immutable or mutable references
2. You can have multiple immutable references

Quiz

1. Will this code compile

fn main() {
    let str = String::from("Harkirat");
    let ref1 = &str;
    let ref2 = &str;

    println!("{} {}", ref1, ref2);
}

1. Will this code compile

fn main() {
    let mut str = String::from("Harkirat");
    let ref1 = &mut str;
    let ref2 = &str;

    println!("{} {}", ref1, ref2);
}

1. Will this code compile

fn main() {
    let mut str = String::from("Harkirat");
    let ref1 = &mut str;
    ref1.push_str("Singh");
    let ref2 = &str;

    println!("{}", ref2);
}

Assignments

1. Borrowing an Immutable Reference

Goal: Write a function calculate_length that takes an immutable reference to a String and returns its length. Then call this function from main and print both the original String and its length.

• Hint

  fn calculate_length(s: &String) -> usize {
      // ...
  }

2. Borrowing a Mutable Reference

Goal: Write a function append_text that takes a mutable reference to a String and appends some text to it. For example, if the string is "Hello", the function could append ", World!".

• Hint

  fn append_text(s: &mut String) {
      // ...
  }