References in Rust

This is part of the ownership lesson in Rust.
References is a cheap way to use other variable on other function without passing the whole value.

fn main() {
    let s1 = String::from("hello");

    let len = calculate_length(&s1);

    println!("The length of '{}' is {}.", s1, len);
}

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

This program will run. len and s1 will be printed on main function.

BUT! we’ve learned that, a copied or a function that passed to a function will be gone

The answer is the “ampersands sign” &.
This ampersand called references

What references do?

Reference will point/refer to a value without taking their ownership. So the variable will still be valid after that.

To use reference, just use & + variable name

and to accept it in a function as argument, use ampersand sign on the data type

(s: &String)

Since the function didn’t own it, so no value will be dropped.

You can also imagine 2nd function is “borrowing” the value of the 1st function.

You can’t modify references

If you try to run this function

fn main() {
    let s = String::from("hello");

    change(&s);
}

fn change(some_string: &String) {
    some_string.push_str(", world");
}

It will return an error. We can’t modify any reference, since we just borrowing it.

Mutable Reference

But can we change it though? Yes we can. Using mutable references.

fn main() {
    let mut s = String::from("hello");

    change(&mut s);
}

fn change(some_string: &mut String) {
    some_string.push_str(", world");
}

Make sure the variable itself is mutable.
Then create a mutable reference with &mut variableName

There is a restriction. We can only have one!
This is not valid:

fn main() {
    let mut s = String::from("hello");

    let r1 = &mut s;
    let r2 = &mut s;

    println!("{}, {}", r1, r2);
}

Rust did this, to prevent “a data race” at compile time. It’s a problem when they try and change the same data at the same time.

If you really need to reference and change the variable, we can just create a new scope for one of the variable

fn main() {
    let mut s = String::from("hello");

    {
        let r1 = &mut s;
    } // r1 goes out of scope here, so we can make a new reference with no problems.

    let r2 = &mut s;
}