···

       
10
       
 
       
    - borrowing

     

       
11
       
 
       
---

     

       
12
       
 
       


     

       
13
       
-
       
One of the biggest Rust's pros is unique ownership system. Unfortunately, it is

     

       
14
       
-
       
also one of the hardest thing to learn. In this article I will try to explain

     

       
15
       
-
       
it the same way I had learnt it and how I introduce people to one.

     

       
16
       
 
       


     

       
17
       
-
       
**Disclaimer**: If you do not find this article helpful try to search another.

     

       
18
       
 
       
People are different and different things *zing* them.

     

       
19
       
 
       


     

       
20
       
 
       
## Let's have a book

     

       
21
       
 
       


     

       
22
       
 
       
Ownership becomes simple and natural if you just acknowledge it as an

     

       
23
       
-
       
application of real world relationship. For example imagine any type in Rust as

     

       
24
       
-
       
a form of text note. We have different type of notes and basing on that, each

     

       
25
       
-
       
of them we will handle 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
       
 
       


     

       
31
       
-
       
Using this analogy let my try to introduce you, dear reader, to the amazing

     

       
32
       
 
       
world of Rust's ownership.

     

       
33
       
 
       


     

       
34
       
 
       
## One can own the book

     
···

       
36
       
 
       
Each note, no matter what size it is, can have one owner. Me, you, anyone, it

     

       
37
       
 
       
doesn't matter, but there will be only one owner. You can do whatever you want

     

       
38
       
 
       
with such note but with that power comes, not so great, responsibility: after

     

       
39
       
-
       
you are done with this book you will need to destroy it. As you are a law

     

       
40
       
-
       
abiding citizen you will recycle it in appreciate institution, but it is your

     

       
41
       
-
       
responsibility to do it. Of course it is not the only way to deal with book. You

     

       
42
       
-
       
can also give it to someone and then it will be her or his responsibility.

     

       
43
       
 
       


     

       
44
       
-
       
To rephrase it in Rust way, it would look like that:

     

       
45
       
 
       


     

       
46
       
 
       
```rust

     

       
47
       
 
       
struct Note;

     

       
48
       
 
       


     

       
49
       
-
       
fn someone() {

     

       
50
       
-
       
    let book = Note; // he 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 he'll destroy all his belongings

     

       
54
       
 
       


     

       
55
       
-
       
fn someone_with_testament() {

     

       
56
       
 
       
    let book = Note; // new book

     

       
57
       
 
       


     

       
58
       
 
       
    // he can do whatever he wants to do with his book

     

       
59
       
 
       


     

       
60
       
-
       
    someone_else(book);

     

       
61
       
-
       
    // he gives her book to `someone_else`,

     

       
62
       
-
       
    // altogether with responsibility to destroy it

     

       
63
       
 
       


     

       
64
       
-
       
    // now he 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
       
 
       


     

       
71
       
-
       
When we don't want to give someone a book (we like that one) we can also lend

     

       
72
       
-
       
him 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 one 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 whole universe. It will be **immutable

     

       
80
       
 
       
  borrow**.

     

       
81
       
 
       


     

       
82
       
 
       
Of course if we borrow something from someone else, then we can lend it further

     

       
83
       
-
       
on the same rules that was applied to us.

     

       
84
       
 
       


     

       
85
       
 
       
Rust also ensures that **mutable borrow** is unique. There will never be more

     

       
86
       
-
       
than one person that will be allowed to edit the book. We still can create chain

     

       
87
       
 
       
of trust - like when I find someone who is better at English than me, I would

     

       
88
       
 
       
allow this person to correct an article written by me or my friend who has

     

       
89
       
 
       
entrusted me with correcting his text.

     
···

       
100
       
 
       


     

       
101
       
 
       
    spelling_corrector(&mut book);

     

       
102
       
 
       
    // we must explicitly mention that we lend the book

     

       
103
       
-
       
    // and we not give it away

     

       
104
       
 
       


     

       
105
       
 
       
    reader(&book);

     

       
106
       
 
       
}

     
···

       
118
       
 
       


     

       
119
       
 
       
Sometimes this whole process of lending and then receiving a note back is much

     

       
120
       
 
       
more complicated then just cloning the whole note for someone else. Imagine that

     

       
121
       
-
       
you are in school and friend wants to copy your homework. What you do is lending

     

       
122
       
-
       
your homework to him, with caution he can clone it on his own. This is what

     

       
123
       
-
       
Rust's `Clone` trait provides - method to clone content of struct without moving

     

       
124
       
 
       
its ownership.

     

       
125
       
 
       


     

       
126
       
 
       
```rust

     
···

       
139
       
 
       
}

     

       
140
       
 
       
```

     

       
141
       
 
       


     

       
142
       
-
       
But some notes are even shorter than that. They are so short and easy to copy

     

       
143
       
-
       
that it is much easier to copy them every time you need, instead of explicitly

     

       
144
       
-
       
mentioning that we are making a copy - like when you give your phone number to a

     

       
145
       
-
       
hot girl at the bar. This is `Copy`. Small type that can be mechanically copied

     

       
146
       
-
       
each time when needed.

     

       
147
       
 
       


     

       
148
       
 
       
```rust

     

       
149
       
 
       
#[derive(Copy, Clone)]

     

       
150
       
-
       
// everything what is `Copy` must be also `Clone`

     

       
151
       
 
       
struct PhoneNo;

     

       
152
       
 
       


     

       
153
       
 
       
fn my() {

     
···

       
163
       
 
       


     

       
164
       
 
       
## Conclusion

     

       
165
       
 
       


     

       
166
       
-
       
There is more stuff to learn, but these are the basic laws of ownership in Rust.

     

       
167
       
-
       
Everything else is based on those and if you learn that, it will became much

     

       
168
       
 
       
easier for you to understand how other types behave and, more importantly, why

     

       
169
       
 
       
they work the way they do.

     

···

       
10
       
 
       
    - borrowing

     

       
11
       
 
       
---

     

       
12
       
 
       


     

       
13
       
+
       
One of Rust's biggest pros is its unique ownership system. Unfortunately, it is

     

       
14
       
+
       
also one of the hardest things to learn. In this article I will try to explain

     

       
15
       
+
       
it 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.

     

       
18
       
 
       
People are different and different things *zing* them.

     

       
19
       
 
       


     

       
20
       
 
       
## Let's have a book

     

       
21
       
 
       


     

       
22
       
 
       
Ownership becomes simple and natural if you just acknowledge it as an

     

       
23
       
+
       
application of real world relationships. For example, imagine types in Rust as

     

       
24
       
+
       
a kind of written note. We have different types of notes and based on that, each

     

       
25
       
+
       
of 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
       
 
       


     

       
31
       
+
       
Using this analogy let me try to introduce you, dear reader, to the amazing

     

       
32
       
 
       
world of Rust's ownership.

     

       
33
       
 
       


     

       
34
       
 
       
## One can own the book

     
···

       
36
       
 
       
Each note, no matter what size it is, can have one owner. Me, you, anyone, it

     

       
37
       
 
       
doesn't matter, but there will be only one owner. You can do whatever you want

     

       
38
       
 
       
with such note but with that power comes, not so great, responsibility: after

     

       
39
       
+
       
you are done with this book you will need to get rid of it. Since you are a law

     

       
40
       
+
       
abiding citizen you will recycle the note in the appropriate receptacle, but it is your

     

       
41
       
+
       
responsibility to do it. Of course this is not the only way to deal with a note. You

     

       
42
       
+
       
can also give it to someone and then it will be hers or his responsibility.

     

       
43
       
 
       


     

       
44
       
+
       
To rephrase it in the Rust way, it would look like this:

     

       
45
       
 
       


     

       
46
       
 
       
```rust

     

       
47
       
 
       
struct Note;

     

       
48
       
 
       


     

       
49
       
+
       
fn 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
       
 
       


     

       
55
       
+
       
fn 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
       
 
       


     

       
71
       
+
       
When we don't want to give someone a book (we like that one), we can also lend

     

       
72
       
+
       
them 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
       
 
       


     

       
82
       
 
       
Of course if we borrow something from someone else, then we can lend it further

     

       
83
       
+
       
with the same rules that were applied to us.

     

       
84
       
 
       


     

       
85
       
 
       
Rust also ensures that **mutable borrow** is unique. There will never be more

     

       
86
       
+
       
than one person that will be allowed to edit the book. We can still create a chain

     

       
87
       
 
       
of trust - like when I find someone who is better at English than me, I would

     

       
88
       
 
       
allow this person to correct an article written by me or my friend who has

     

       
89
       
 
       
entrusted me with correcting his text.

     
···

       
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
       
 
       
}

     
···

       
118
       
 
       


     

       
119
       
 
       
Sometimes this whole process of lending and then receiving a note back is much

     

       
120
       
 
       
more complicated then just cloning the whole note for someone else. Imagine that

     

       
121
       
+
       
you are in school and friend wants to copy your homework. What you do is lend

     

       
122
       
+
       
your homework to him, and with caution he can clone it on his own. This is what

     

       
123
       
+
       
Rust's `Clone` trait provides - a method to clone content of struct without moving

     

       
124
       
 
       
its ownership.

     

       
125
       
 
       


     

       
126
       
 
       
```rust

     
···

       
139
       
 
       
}

     

       
140
       
 
       
```

     

       
141
       
 
       


     

       
142
       
+
       
But some notes are even shorter than that. They are so short and easy to clone

     

       
143
       
+
       
that it is much easier to clone them every time, instead of explicitly

     

       
144
       
+
       
calling the method. Like when you give your phone number to a hot girl at the 

     

       
145
       
+
       
bar, the `Copy` trait automatically clones your note so the other has their own copy. 

     

       
146
       
+
       
Again, 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`

     

       
151
       
 
       
struct PhoneNo;

     

       
152
       
 
       


     

       
153
       
 
       
fn my() {

     
···

       
163
       
 
       


     

       
164
       
 
       
## Conclusion

     

       
165
       
 
       


     

       
166
       
+
       
There is more to learn, but these are the basic laws of ownership in Rust.

     

       
167
       
+
       
Everything else is based on this. If you understand this, it will become much

     

       
168
       
 
       
easier for you to understand how other types behave and, more importantly, why

     

       
169
       
 
       
they work the way they do.