this repo has no description
hauleth.dev
1---
2date: 2019-07-14T17:38:48+01:00
3description: "Simple introduction to Rust's ownership system"
4title: "Here be (owned) books"
5tags:
6 - rust
7 - programming
8 - ownership
9 - eli5
10 - borrowing
11---
12
13One of Rust's biggest pros is its unique ownership system. Unfortunately, it is
14also one of the hardest things to learn. In this article I will try to explain
15it the same way I had learned it and how I introduce it to people.
16
17**Disclaimer**: If you do not find this article helpful try to search for another.
18People are different and different things *zing* them.
19
20## Let's have a book
21
22Ownership becomes simple and natural if you just acknowledge it as an
23application of real world relationships. For example, imagine types in Rust as
24a kind of written note. We have different types of notes and based on that, each
25of them will be handled differently.
26
27- short ones, like phone no. of the hot waiter/waitress
28- longer ones, like this article
29- longest ones, like a *Lord of the Rings*
30
31Using this analogy let me try to introduce you, dear reader, to the amazing
32world of Rust's ownership.
33
34## One can own the book
35
36Each note, no matter what size it is, can have one owner. Me, you, anyone, it
37doesn't matter, but there will be only one owner. You can do whatever you want
38with such note but with that power comes, not so great, responsibility: after
39you are done with this book you will need to get rid of it. Since you are a law
40abiding citizen you will recycle the note in the appropriate receptacle, but it is your
41responsibility to do it. Of course this is not the only way to deal with a note. You
42can also give it to someone and then it will be hers or his responsibility.
43
44To rephrase it in the Rust way, it would look like this:
45
46```rust
47struct Note;
48
49fn john() {
50 let book = Note; // john creates the book and he owns it
51
52 // here he can do whatever he want with our `book`
53} // at the end of his life john will destroy all his belongings
54
55fn steve() {
56 let book = Note; // new book
57
58 // he can do whatever he wants to do with his book
59
60 sally(book);
61 // steve gives the book to `sally`,
62 // Sally has the responsibility to destroy it
63
64 // now steve cannot do anything with this book,
65 // as it is not his personal belonging anymore
66}
67```
68
69## One can borrow the book
70
71When we don't want to give someone a book (we like that one), we can also lend
72them one. And there are two ways to borrow one book:
73
74- We can edit that book (ex. it is our personal dairy) and we lend it to someone
75 to check our spelling. We trust that person and we explicitly allow her to
76 edit our notes in place. We call it **mutable borrow**.
77- We do not trust someone and we lend our beloved book with no permission to edit
78 it. Even more, that person knows, that writing something in that book will
79 make us go rampage and destroy the whole universe. It will be an **immutable
80 borrow**.
81
82Of course if we borrow something from someone else, then we can lend it further
83with the same rules that were applied to us.
84
85Rust also ensures that **mutable borrow** is unique. There will never be more
86than one person that will be allowed to edit the book. We can still create a chain
87of trust - like when I find someone who is better at English than me, I would
88allow this person to correct an article written by me or my friend who has
89entrusted me with correcting his text.
90
91**Immutable borrows** aren't exclusive. I can lend my books as many times as I
92want with one exception: I cannot lend a book that is still borrowed by someone
93who can change its content.
94
95In Rust it would look like that:
96
97```rust
98fn my() {
99 let mut book = Note;
100
101 spelling_corrector(&mut book);
102 // we must explicitly mention that we lend the book
103 // and we don't give it away
104
105 reader(&book);
106}
107
108fn spelling_corrector(book: &mut Note) {
109 // correct spelling in place
110}
111
112fn reader(book: &Note) {
113 // read a book
114}
115```
116
117## Not all notes are worth borrowing
118
119Sometimes this whole process of lending and then receiving a note back is much
120more complicated then just cloning the whole note for someone else. Imagine that
121you are in school and friend wants to copy your homework. What you do is lend
122your homework to him, and with caution he can clone it on his own. This is what
123Rust's `Clone` trait provides - a method to clone content of struct without moving
124its ownership.
125
126```rust
127#[derive(Clone)]
128struct Homework;
129
130fn my() {
131 let homework = Homework;
132
133 friend(&homework);
134}
135
136fn friend(work: &Homework) { // we lend it immutably
137 let mut homework: Homework = work.clone();
138 // your friend now has his own modifiable copy
139}
140```
141
142But some notes are even shorter than that. They are so short and easy to clone
143that it is much easier to clone them every time, instead of explicitly
144calling the method. Like when you give your phone number to a hot girl at the
145bar, the `Copy` trait automatically clones your note so the other has their own copy.
146Again, this is for small types that can be mechanically copied each time when needed.
147
148```rust
149#[derive(Copy, Clone)]
150// everything that is `Copy` must be also `Clone`
151struct PhoneNo;
152
153fn my() {
154 let no = PhoneNo;
155
156 hot_stuff(no);
157}
158
159fn hot_stuff(no: PhoneNo) {
160 // fingers crossed
161}
162```
163
164## Conclusion
165
166There is more to learn, but these are the basic laws of ownership in Rust.
167Everything else is based on this. If you understand this, it will become much
168easier for you to understand how other types behave and, more importantly, why
169they work the way they do.