References

References in Rust are different from pointers in languages such as C and C++. In Rust, references are always valid and cannot be null. Use & to reference a variable, and * to dereference.

fn main() {
    let my_vec: Vec<usize> = vec![1, 2, 3, 4, 5];

    println!("{:?}", &my_vec); // Pass my_vec as a reference to the println! macro.
}

References are useful for passing variables to other functions without needing to clone the data. In the following example below, we'll create a Vec and then pass it by reference to a function. Note the syntax here, we actually don't pass a reference &Vec<usize>. We could, but a more idiomatic approach (in my opinion) is to pass a slice instead.

fn print_a_vec(x: &[usize]) {
    println!("{:?}", x);
}

fn main() {
    let my_vec: Vec<usize> = vec![1, 2, 3, 4, 5];

    print_a_vec(&my_vec[..]);
}

There is an important rule when it comes to references, which I'll quote from the official Rust book:

At any given time, you can have either one mutable reference or any number of immutable references.

This makes perfect sense when you think about it. If we are able to mutate a variable, we do not want a bunch of read-only references with unpredictable values when read.