Rust ownership explained with books

One of the biggest Rust’s pros is unique ownership system. Unfortunately, it is also one of the hardest thing to learn. In this article I will try to explain it the same way I had learnt it and how I introduce people to one.

Disclaimer: If you do not find this article helpful try to search another. People are different and different things zing them.

Let’s have a book

Ownership becomes simple and natural if you just acknowledge it as an application of real world relationship. For example imagine any type in Rust as a form of text note. We have different type of notes and basing on that, each of them we will handle differently.

• short ones, like phone no. of the hot waiter/waitress
• longer ones, like this article
• longest ones, like a Lord of the Rings

Using this analogy let my try to introduce you, dear reader, to the amazing world of Rust’s ownership.

One can own the book

Each note, no matter what size it is, can have one owner. Me, you, anyone, it doesn’t matter, but there will be only one owner. You can do whatever you want with such note but with that power comes, not so great, responsibility: after you are done with this book you will need to destroy it. As you are a law abiding citizen you will recycle it in appreciate institution, but it is your responsibility to do it. Of course it is not the only way to deal with book. You can also give it to someone and then it will be her or his responsibility.

To rephrase it in Rust way, it would look like that:

struct Note;

fn someone() {
    let book = Note; // he creates the book and he owns it

    // here he can do whatever he want with our `book`
} // at the end of his life he'll destroy all his belongings

fn someone_with_testament() {
    let book = Note; // new book

    // he can do whatever he wants to do with his book

    someone_else(book);
    // he gives her book to `someone_else`,
    // altogether with responsibility to destroy it

    // now he cannot do anything with this book,
    // as it is not his personal belonging anymore
}

One can borrow the book

When we don’t want to give someone a book (we like that one) we can also lend him one. And there are two ways to borrow one book:

• We can edit that book (ex. it is our personal dairy) and we lend it to someone to check our spelling. We trust that person and we explicitly allow her to edit our notes in place. We call it mutable borrow .
• We do not trust one and we lend our beloved book with no permission to edit it. Even more, that person knows, that writing something in that book will make us go rampage and destroy whole universe. It will be immutable borrow .

Of course if we borrow something from someone else, then we can lend it further on the same rules that was applied to us.

Rust also ensures that mutable borrow is unique. There will never be more than one person that will be allowed to edit the book. We still can create chain of trust - like when I find someone who is better at English than me, I would allow this person to correct an article written by me or my friend who has entrusted me with correcting his text.

Immutable borrowsaren’t exclusive. I can lend my books as many times as I want with one exception: I cannot lend a book that is still borrowed by someone who can change its content.

In Rust it would look like that:

fn my() {
    let mut book = Note;

    spelling_corrector(&mut book);
    // we must explicitly mention that we lend the book
    // and we not give it away

    reader(&book);
}

fn spelling_corrector(book: &mut Note) {
    // correct spelling in place
}

fn reader(book: &Note) {
    // read a book
}

Not all notes are worth borrowing

Sometimes this whole process of lending and then receiving a note back is much more complicated then just cloning the whole note for someone else. Imagine that you are in school and friend wants to copy your homework. What you do is lending your homework to him, with caution he can clone it on his own. This is what Rust’s Clone trait provides - method to clone content of struct without moving its ownership.

#[derive(Clone)]
struct Homework;

fn my() {
    let homework = Homework;

    friend(&homework);
}

fn friend(work: &Homework) { // we lend it immutably
    let mut homework: Homework = work.clone();
    // your friend now has his own modifiable copy
}

But some notes are even shorter than that. They are so short and easy to copy that it is much easier to copy them every time you need, instead of explicitly mentioning that we are making a copy - like when you give your phone number to a hot girl at the bar. This is Copy . Small type that can be mechanically copied each time when needed.

#[derive(Copy, Clone)]
// everything what is `Copy` must be also `Clone`
struct PhoneNo;

fn my() {
    let no = PhoneNo;

    hot_stuff(no);
}

fn hot_stuff(no: PhoneNo) {
    // fingers crossed
}

Conclusion

There is more stuff to learn, but these are the basic laws of ownership in Rust. Everything else is based on those and if you learn that, it will became much easier for you to understand how other types behave and, more importantly, why they work the way they do.