book/nostarch/chapter05.md

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[TOC]

# Using Structs to Structure Related Data

A *struct*, or *structure*, is a custom data type that lets you package
together and name multiple related values that make up a meaningful group. If
you’re familiar with an object-oriented language, a *struct* is like an
object’s data attributes. In this chapter, we’ll compare and contrast tuples
with structs to build on what you already know and demonstrate when structs are
a better way to group data.

We’ll demonstrate how to define and instantiate structs. We’ll discuss how to
define associated functions, especially the kind of associated functions called
*methods*, to specify behavior associated with a struct type. Structs and enums
(discussed in Chapter 6) are the building blocks for creating new types in your
program’s domain to take full advantage of Rust’s compile time type checking.

## Defining and Instantiating Structs

Structs are similar to tuples, discussed in “The Tuple Type” section, in that
both hold multiple related values. Like tuples, the pieces of a struct can be
different types. Unlike with tuples, in a struct you’ll name each piece of data
so it’s clear what the values mean. Adding these names means that structs are
more flexible than tuples: you don’t have to rely on the order of the data to
specify or access the values of an instance.

To define a struct, we enter the keyword `struct` and name the entire struct. A
struct’s name should describe the significance of the pieces of data being
grouped together. Then, inside curly brackets, we define the names and types of
the pieces of data, which we call *fields*. For example, Listing 5-1 shows a
struct that stores information about a user account.

```
struct User {
    active: bool,
    username: String,
    email: String,
    sign_in_count: u64,
}
```

Listing 5-1: A `User` struct definition

To use a struct after we’ve defined it, we create an *instance* of that struct
by specifying concrete values for each of the fields. We create an instance by
stating the name of the struct and then add curly brackets containing `key:
value` pairs, where the keys are the names of the fields and the values are the
data we want to store in those fields. We don’t have to specify the fields in
the same order in which we declared them in the struct. In other words, the
struct definition is like a general template for the type, and instances fill
in that template with particular data to create values of the type. For
example, we can declare a particular user as shown in Listing 5-2.

```
fn main() {
    let user1 = User {
        email: String::from("someone@example.com"),
        username: String::from("someusername123"),
        active: true,
        sign_in_count: 1,
    };
}
```

Listing 5-2: Creating an instance of the `User` struct

To get a specific value from a struct, we use dot notation. If we wanted
just this user’s email address, we could use `user1.email` wherever we wanted
to use this value. If the instance is mutable, we can change a value by using
the dot notation and assigning into a particular field. Listing 5-3 shows how
to change the value in the `email` field of a mutable `User` instance.

<!--- Do we want to mention that `user1.email` will move the field? We can't
just use `user1.email` multiple times re: "wherever we wanted
to use this value"
/JT --->

```
fn main() {
    let mut user1 = User {
        email: String::from("someone@example.com"),
        username: String::from("someusername123"),
        active: true,
        sign_in_count: 1,
    };

    user1.email = String::from("anotheremail@example.com");
}
```

Listing 5-3: Changing the value in the `email` field of a `User` instance

Note that the entire instance must be mutable; Rust doesn’t allow us to mark
only certain fields as mutable. As with any expression, we can construct a new
instance of the struct as the last expression in the function body to
implicitly return that new instance.

Listing 5-4 shows a `build_user` function that returns a `User` instance with
the given email and username. The `active` field gets the value of `true`, and
the `sign_in_count` gets a value of `1`.

```
fn build_user(email: String, username: String) -> User {
    User {
        email: email,
        username: username,
        active: true,
        sign_in_count: 1,
    }
}
```

Listing 5-4: A `build_user` function that takes an email and username and
returns a `User` instance

It makes sense to name the function parameters with the same name as the struct
fields, but having to repeat the `email` and `username` field names and
variables is a bit tedious. If the struct had more fields, repeating each name
would get even more annoying. Luckily, there’s a convenient shorthand!

### Using the Field Init Shorthand

Because the parameter names and the struct field names are exactly the same in
Listing 5-4, we can use the *field init shorthand* syntax to rewrite
`build_user` so that it behaves exactly the same but doesn’t have the
repetition of `email` and `username`, as shown in Listing 5-5.

```
fn build_user(email: String, username: String) -> User {
    User {
        email,
        username,
        active: true,
        sign_in_count: 1,
    }
}
```

Listing 5-5: A `build_user` function that uses field init shorthand because the
`email` and `username` parameters have the same name as struct fields

Here, we’re creating a new instance of the `User` struct, which has a field
named `email`. We want to set the `email` field’s value to the value in the
`email` parameter of the `build_user` function. Because the `email` field and
the `email` parameter have the same name, we only need to write `email` rather
than `email: email`.

### Creating Instances From Other Instances With Struct Update Syntax

It’s often useful to create a new instance of a struct that includes most of
the values from another instance, but changes some. You can do this using
*struct update syntax*.

First, in Listing 5-6 we show how to create a new `User` instance in `user2`
regularly, without the update syntax. We set a new value for `email` but
otherwise use the same values from `user1` that we created in Listing 5-2.

```
fn main() {
    // --snip--

    let user2 = User {
        active: user1.active,
        username: user1.username,
        email: String::from("another@example.com"),
        sign_in_count: user1.sign_in_count,
    };
}
```

Listing 5-6: Creating a new `User` instance using one of the values from `user1`

Using struct update syntax, we can achieve the same effect with less code, as
shown in Listing 5-7. The syntax `..` specifies that the remaining fields not
explicitly set should have the same value as the fields in the given instance.

```
fn main() {
    // --snip--

    let user2 = User {
        email: String::from("another@example.com"),
        ..user1
    };
}
```

Listing 5-7: Using struct update syntax to set a new `email` value for a `User`
instance but use the rest of the values from `user1`

The code in Listing 5-7 also creates an instance in `user2` that has a
different value for `email` but has the same values for the `username`,
`active`, and `sign_in_count` fields from `user1`. The `..user1` must come last
to specify that any remaining fields should get their values from the
corresponding fields in `user1`, but we can choose to specify values for as
many fields as we want in any order, regardless of the order of the fields in
the struct’s definition.

Note that the struct update syntax uses `=` like an assignement; this is
because it moves the data, just as we saw in the “Ways Variables and Data
Interact: Move” section. In this example, we can no longer use `user1` after
creating `user2` because the `String` in the `username` field of `user1` was
moved into `user2`. If we had given `user2` new `String` values for both
`email` and `username`, and thus only used the `active` and `sign_in_count`
values from `user1`, then `user1` would still be valid after creating `user2`.
The types of `active` and `sign_in_count` are types that implement the `Copy`
trait, so the behavior we discussed in the “Stack-Only Data: Copy” section
would apply.

<!--- Misspelled "assignment" above.
/JT --->

### Using Tuple Structs without Named Fields to Create Different Types

Rust also supports structs that look similar to tuples, called *tuple
structs*. Tuple structs have the added meaning the struct name provides but
don’t have names associated with their fields; rather, they just have the types
of the fields. Tuple structs are useful when you want to give the whole tuple a
name and make the tuple a different type from other tuples, and when naming each
field as in a regular struct would be verbose or redundant.

To define a tuple struct, start with the `struct` keyword and the struct name
followed by the types in the tuple. For example, here we define and use
two tuple structs named `Color` and `Point`:

```
struct Color(i32, i32, i32);
struct Point(i32, i32, i32);

fn main() {
    let black = Color(0, 0, 0);
    let origin = Point(0, 0, 0);
}
```

Note that the `black` and `origin` values are different types, because they’re
instances of different tuple structs. Each struct you define is its own type,
even though the fields within the struct have the same types. For example, a
function that takes a parameter of type `Color` cannot take a `Point` as an
argument, even though both types are made up of three `i32` values. Otherwise,
tuple struct instances behave like tuples: you can destructure them into their
individual pieces, you can use a `.` followed by the index to access an
individual value, and so on.

<!--- The last line above feels a bit misleading. There are related restrictions on
tuple structs that don't apply to tuples.

One example is you can't create a tuple struct with a tuple. 
```
struct Color(i32, i32, i32);

fn main() {
    let x: Color = (1, 2, 3);
}
```

You can't pass a tuple struct to something that expects a tuple, either.
/JT --->

### Unit-Like Structs Without Any Fields

You can also define structs that don’t have any fields! These are called
*unit-like structs* because they behave similarly to `()`, the unit type that
we mentioned in “The Tuple Type” section. Unit-like structs can be useful when
you need to implement a trait on some type but don’t have any data that you
want to store in the type itself. We’ll discuss traits in Chapter 10. Here’s an
example of declaring and instantiating a unit struct named `AlwaysEqual`:

```
struct AlwaysEqual;

fn main() {
    let subject = AlwaysEqual;
}
```

To define `AlwaysEqual`, we use the `struct` keyword, the name we want, then a
semicolon. No need for curly brackets or parentheses! Then we can get an
instance of `AlwaysEqual` in the `subject` variable in a similar way: using the
name we defined, without any curly brackets or parentheses. Imagine that later
we’ll implement behavior for this type such that every instance of
`AlwaysEqual` is always equal to every instance of any other type, perhaps to
have a known result for testing purposes. We wouldn’t need any data to
implement that behavior! You’ll see in Chapter 10 how to define traits and
implement them on any type, including unit-like structs.

> ### Ownership of Struct Data
>
> In the `User` struct definition in Listing 5-1, we used the owned `String`
> type rather than the `&str` string slice type. This is a deliberate choice
> because we want each instance of this struct to own all of its data and for
> that data to be valid for as long as the entire struct is valid.
>
> It’s also possible for structs to store references to data owned by something
> else, but to do so requires the use of *lifetimes*, a Rust feature that we’ll
> discuss in Chapter 10. Lifetimes ensure that the data referenced by a struct
> is valid for as long as the struct is. Let’s say you try to store a reference
> in a struct without specifying lifetimes, like the following; this won’t work:
>
> Filename: src/main.rs
>
> ```
> struct User {
>     username: &str,
>     email: &str,
>     sign_in_count: u64,
>     active: bool,
> }
>
> fn main() {
>     let user1 = User {
>         email: "someone@example.com",
>         username: "someusername123",
>         active: true,
>         sign_in_count: 1,
>     };
> }
> ```
>
> The compiler will complain that it needs lifetime specifiers:
>
> ```
> $ cargo run
>    Compiling structs v0.1.0 (file:///projects/structs)
> error[E0106]: missing lifetime specifier
>  --> src/main.rs:3:15
>   |
> 3 |     username: &str,
>   |               ^ expected named lifetime parameter
>   |
> help: consider introducing a named lifetime parameter
>   |
> 1 ~ struct User<'a> {
> 2 |     active: bool,
> 3 ~     username: &'a str,
>   |
>
> error[E0106]: missing lifetime specifier
>  --> src/main.rs:4:12
>   |
> 4 |     email: &str,
>   |            ^ expected named lifetime parameter
>   |
> help: consider introducing a named lifetime parameter
>   |
> 1 ~ struct User<'a> {
> 2 |     active: bool,
> 3 |     username: &str,
> 4 ~     email: &'a str,
>   |
> ```
>
> In Chapter 10, we’ll discuss how to fix these errors so you can store
> references in structs, but for now, we’ll fix errors like these using owned
> types like `String` instead of references like `&str`.

## An Example Program Using Structs

To understand when we might want to use structs, let’s write a program that
calculates the area of a rectangle. We’ll start by using single variables, and
then refactor the program until we’re using structs instead.

Let’s make a new binary project with Cargo called *rectangles* that will take
the width and height of a rectangle specified in pixels and calculate the area
of the rectangle. Listing 5-8 shows a short program with one way of doing
exactly that in our project’s *src/main.rs*.

Filename: src/main.rs

```
fn main() {
    let width1 = 30;
    let height1 = 50;

    println!(
        "The area of the rectangle is {} square pixels.",
        area(width1, height1)
    );
}

fn area(width: u32, height: u32) -> u32 {
    width * height
}
```

Listing 5-8: Calculating the area of a rectangle specified by separate width
and height variables

Now, run this program using `cargo run`:

```
$ cargo run
   Compiling rectangles v0.1.0 (file:///projects/rectangles)
    Finished dev [unoptimized + debuginfo] target(s) in 0.42s
     Running `target/debug/rectangles`
The area of the rectangle is 1500 square pixels.
```

This code succeeds in figuring out the area of the rectangle by calling the
`area` function with each dimension, but we can do more to make this code clear
and readable.

The issue with this code is evident in the signature of `area`:

```
fn area(width: u32, height: u32) -> u32 {
```

The `area` function is supposed to calculate the area of one rectangle, but the
function we wrote has two parameters, and it's not clear anywhere in our
program that the parameters are related. It would be more readable and more
manageable to group width and height together. We’ve already discussed one way
we might do that in “The Tuple Type” section of Chapter 3: by using tuples.

### Refactoring with Tuples

Listing 5-9 shows another version of our program that uses tuples.

Filename: src/main.rs

```
fn main() {
    let rect1 = (30, 50);

    println!(
        "The area of the rectangle is {} square pixels.",
        area(rect1)
    );
}

fn area(dimensions: (u32, u32)) -> u32 {
    dimensions.0 * dimensions.1
}
```

Listing 5-9: Specifying the width and height of the rectangle with a tuple

In one way, this program is better. Tuples let us add a bit of structure, and
we’re now passing just one argument. But in another way, this version is less
clear: tuples don’t name their elements, so we have to index into the parts of
the tuple, making our calculation less obvious.

Mixing up the width and height wouldn’t matter for the area calculation, but if
we want to draw the rectangle on the screen, it would matter! We would have to
keep in mind that `width` is the tuple index `0` and `height` is the tuple
index `1`. This would be even harder for someone else to figure out and keep in
mind if they were to use our code. Because we haven’t conveyed the meaning of
our data in our code, it’s now easier to introduce errors.

### Refactoring with Structs: Adding More Meaning

We use structs to add meaning by labeling the data. We can transform the tuple
we’re using into a struct with a name for the whole as well as names for the
parts, as shown in Listing 5-10.

Filename: src/main.rs

```
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!(
        "The area of the rectangle is {} square pixels.",
        area(&rect1)
    );
}

fn area(rectangle: &Rectangle) -> u32 {
    rectangle.width * rectangle.height
}
```

Listing 5-10: Defining a `Rectangle` struct

Here we’ve defined a struct and named it `Rectangle`. Inside the curly
brackets, we defined the fields as `width` and `height`, both of which have
type `u32`. Then in `main`, we created a particular instance of `Rectangle`
that has a width of 30 and a height of 50.

Our `area` function is now defined with one parameter, which we’ve named
`rectangle`, whose type is an immutable borrow of a struct `Rectangle`
instance. As mentioned in Chapter 4, we want to borrow the struct rather than
take ownership of it. This way, `main` retains its ownership and can continue
using `rect1`, which is the reason we use the `&` in the function signature and
where we call the function.

The `area` function accesses the `width` and `height` fields of the `Rectangle`
instance. Our function signature for `area` now says exactly what we mean:
calculate the area of `Rectangle`, using its `width` and `height` fields. This
conveys that the width and height are related to each other, and it gives
descriptive names to the values rather than using the tuple index values of `0`
and `1`. This is a win for clarity.

<!--- Tying to my comment above about `user1.email` moving that field: we should
take a minute here and explain that accessing fields on a borrowed struct does
not move them, and why you often see borrows of structs.
/JT --->

### Adding Useful Functionality with Derived Traits

It’d be useful to be able to print an instance of `Rectangle` while we’re
debugging our program and see the values for all its fields. Listing 5-11 tries
using the `println!` macro as we have used in previous chapters. This won’t
work, however.

Filename: src/main.rs

```
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!("rect1 is {}", rect1);
}
```

Listing 5-11: Attempting to print a `Rectangle` instance

When we compile this code, we get an error with this core message:

```
error[E0277]: `Rectangle` doesn't implement `std::fmt::Display`
```

The `println!` macro can do many kinds of formatting, and by default, the curly
brackets tell `println!` to use formatting known as `Display`: output intended
for direct end user consumption. The primitive types we’ve seen so far
implement `Display` by default, because there’s only one way you’d want to show
a `1` or any other primitive type to a user. But with structs, the way
`println!` should format the output is less clear because there are more
display possibilities: Do you want commas or not? Do you want to print the
curly brackets? Should all the fields be shown? Due to this ambiguity, Rust
doesn’t try to guess what we want, and structs don’t have a provided
implementation of `Display` to use with `println!` and the `{}` placeholder.

If we continue reading the errors, we’ll find this helpful note:

```
= help: the trait `std::fmt::Display` is not implemented for `Rectangle`
= note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
```

Let’s try it! The `println!` macro call will now look like `println!("rect1 is
{:?}", rect1);`. Putting the specifier `:?` inside the curly brackets tells
`println!` we want to use an output format called `Debug`. The `Debug` trait
enables us to print our struct in a way that is useful for developers so we can
see its value while we’re debugging our code.

Compile the code with this change. Drat! We still get an error:

```
error[E0277]: `Rectangle` doesn't implement `Debug`
```

But again, the compiler gives us a helpful note:

```
= help: the trait `Debug` is not implemented for `Rectangle`
= note: add `#[derive(Debug)]` or manually implement `Debug`
```

Rust *does* include functionality to print out debugging information, but we
have to explicitly opt in to make that functionality available for our struct.
To do that, we add the outer attribute `#[derive(Debug)]` just before the
struct definition, as shown in Listing 5-12.

Filename: src/main.rs

```
#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!("rect1 is {:?}", rect1);
}
```

Listing 5-12: Adding the attribute to derive the `Debug` trait and printing the
`Rectangle` instance using debug formatting

Now when we run the program, we won’t get any errors, and we’ll see the
following output:

```
$ cargo run
   Compiling rectangles v0.1.0 (file:///projects/rectangles)
    Finished dev [unoptimized + debuginfo] target(s) in 0.48s
     Running `target/debug/rectangles`
rect1 is Rectangle { width: 30, height: 50 }
```

Nice! It’s not the prettiest output, but it shows the values of all the fields
for this instance, which would definitely help during debugging. When we have
larger structs, it’s useful to have output that’s a bit easier to read; in
those cases, we can use `{:#?}` instead of `{:?}` in the `println!` string.
In this example, using the `{:#?}` style will output:

```
$ cargo run
   Compiling rectangles v0.1.0 (file:///projects/rectangles)
    Finished dev [unoptimized + debuginfo] target(s) in 0.48s
     Running `target/debug/rectangles`
rect1 is Rectangle {
    width: 30,
    height: 50,
}
```

Another way to print out a value using the `Debug` format is to use the `dbg!`
macro, which takes ownership of an expression, prints the file and line number
of where that `dbg!` macro call occurs in your code along with the resulting
value of that expression, and returns ownership of the value.

<!--- I've forgotten how to format a note in markdown... I suggest making this
a note since we don't explote what it means for our program here /LC --->
<!-- No problem -- we've been abusing blockquotes in markdown for notes. I've
fixed! Good call! /Carol -->
> Note: Calling the `dbg!` macro prints to the standard error console stream
> (`stderr`), as opposed to `println!` which prints to the standard output
> console stream (`stdout`). We’ll talk more about `stderr` and `stdout` in the
> “Writing Error Messages to Standard Error Instead of Standard Output” section
> in Chapter 12.

Here’s an example where we’re interested in the value that gets assigned to the
`width` field, as well as the value of the whole struct in `rect1`:

```
#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let scale = 2;
    let rect1 = Rectangle {
        width: dbg!(30 * scale),
        height: 50,
    };

    dbg!(&rect1);
}
```

We can put `dbg!` around the expression `30 * scale` and, because `dbg!`
returns ownership of the expression’s value, the `width` field will get the
same value as if we didn’t have the `dbg!` call there. We don’t want `dbg!` to
take ownership of `rect1`, so we use a reference to `dbg!` in the next call.
Here’s what the output of this example looks like:

<!--- is it worth calling out that println! doesn't have the dbg! shortcoming
of taking ownership?
/JT --->

```
$ cargo run
   Compiling rectangles v0.1.0 (file:///projects/rectangles)
    Finished dev [unoptimized + debuginfo] target(s) in 0.61s
     Running `target/debug/rectangles`
[src/main.rs:10] 30 * scale = 60
[src/main.rs:14] &rect1 = Rectangle {
    width: 60,
    height: 50,
}
```

We can see the first bit of output came from *src/main.rs* line 10, where we’re
debugging the expression `30 * scale`, and its resulting value is 60 (the
`Debug` formatting implemented for integers is to print only their value). The
`dbg!` call on line 14 of *src/main.rs* outputs the value of `&rect1`, which is
the `Rectangle` struct. This output uses the pretty `Debug` formatting of the
`Rectangle` type. The `dbg!` macro can be really helpful when you’re trying to
figure out what your code is doing!

In addition to the `Debug` trait, Rust has provided a number of traits for us
to use with the `derive` attribute that can add useful behavior to our custom
types. Those traits and their behaviors are listed in Appendix C. We’ll cover
how to implement these traits with custom behavior as well as how to create
your own traits in Chapter 10. There are also many attributes other than
`derive`; for more information, see the “Attributes” section of the Rust
Reference at *https://doc.rust-lang.org/reference/attributes.html*.

Our `area` function is very specific: it only computes the area of rectangles.
It would be helpful to tie this behavior more closely to our `Rectangle`
struct, because it won’t work with any other type. Let’s look at how we can
continue to refactor this code by turning the `area` function into an `area`
*method* defined on our `Rectangle` type.

## Method Syntax

*Methods* are similar to functions: we declare them with the `fn` keyword and a
name, they can have parameters and a return value, and they contain some code
that’s run when the method is called from somewhere else. Unlike functions,
methods are defined within the context of a struct (or an enum or a trait
object, which we cover in Chapters 6 and 17, respectively), and their first
parameter is always `self`, which represents the instance of the struct the
method is being called on.

<!--- minor nit: some folks call the non-self functions in an `impl` 
"static methods" as a nod to OO languages that do the same. For folks
from that background, we may want to call out that instance methods always
have `self` and methods on the type do not.
/JT --->

### Defining Methods

Let’s change the `area` function that has a `Rectangle` instance as a parameter
and instead make an `area` method defined on the `Rectangle` struct, as shown
in Listing 5-13.

Filename: src/main.rs

```
#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!(
        "The area of the rectangle is {} square pixels.",
        rect1.area()
    );
}
```

Listing 5-13: Defining an `area` method on the `Rectangle` struct

To define the function within the context of `Rectangle`, we start an `impl`
(implementation) block for `Rectangle`. Everything within this `impl` block
will be associated with the `Rectangle` type. Then we move the `area` function
within the `impl` curly brackets and change the first (and in this case, only)
parameter to be `self` in the signature and everywhere within the body. In
`main`, where we called the `area` function and passed `rect1` as an argument,
we can instead use *method syntax* to call the `area` method on our `Rectangle`
instance. The method syntax goes after an instance: we add a dot followed by
the method name, parentheses, and any arguments.

In the signature for `area`, we use `&self` instead of `rectangle: &Rectangle`.
The `&self` is actually short for `self: &Self`. Within an `impl` block, the
type `Self` is an alias for the type that the `impl` block is for. Methods must
have a parameter named `self` of type `Self` for their first parameter, so Rust
lets you abbreviate this with only the name `self` in the first parameter spot.
Note that we still need to use the `&` in front of the `self` shorthand to
indicate this method borrows the `Self` instance, just as we did in `rectangle:
&Rectangle`. Methods can take ownership of `self`, borrow `self` immutably as
we’ve done here, or borrow `self` mutably, just as they can any other parameter.

We’ve chosen `&self` here for the same reason we used `&Rectangle` in the
function version: we don’t want to take ownership, and we just want to read the
data in the struct, not write to it. If we wanted to change the instance that
we’ve called the method on as part of what the method does, we’d use `&mut
self` as the first parameter. Having a method that takes ownership of the
instance by using just `self` as the first parameter is rare; this technique is
usually used when the method transforms `self` into something else and you want
to prevent the caller from using the original instance after the transformation.

The main reason for using methods instead of functions, in addition to providing
method syntax and not having to repeat the type of `self` in every method’s
signature, is for organization. We’ve put all the things we can do with an
instance of a type in one `impl` block rather than making future users of our
code search for capabilities of `Rectangle` in various places in the library we
provide.

Note that we can choose to give a method the same name as one of the struct’s
fields. For example, we can define a method on `Rectangle` also named `width`:

Filename: src/main.rs

```
impl Rectangle {
    fn width(&self) -> bool {
        self.width > 0
    }
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    if rect1.width() {
        println!("The rectangle has a nonzero width; it is {}", rect1.width);
    }
}
```

Here, we’re choosing to make the `width` method return `true` if the value in
the instance’s `width` field is greater than 0, and `false` if the value is 0:
we can use a field within a method of the same name for any purpose. In `main`,
when we follow `rect1.width` with parentheses, Rust knows we mean the method
`width`. When we don’t use parentheses, Rust knows we mean the field `width`.

Often, but not always, when we give methods with the same name as a field we
want it to only return the value in the field and do nothing else. Methods like
this are called *getters*, and Rust does not implement them automatically for
struct fields as some other languages do. Getters are useful because you can
make the field private but the method public and thus enable read-only access
to that field as part of the type’s public API. We will be discussing what
public and private are and how to designate a field or method as public or
private in Chapter 7.

> ### Where’s the `->` Operator?
>
> In C and C++, two different operators are used for calling methods: you use
> `.` if you’re calling a method on the object directly and `->` if you’re
> calling the method on a pointer to the object and need to dereference the
> pointer first. In other words, if `object` is a pointer,
> `object->something()` is similar to `(*object).something()`.
>
> Rust doesn’t have an equivalent to the `->` operator; instead, Rust has a
> feature called *automatic referencing and dereferencing*. Calling methods is
> one of the few places in Rust that has this behavior.
>
> Here’s how it works: when you call a method with `object.something()`, Rust
> automatically adds in `&`, `&mut`, or `*` so `object` matches the signature of
> the method. In other words, the following are the same:
>
>
> ```
> p1.distance(&p2);
> (&p1).distance(&p2);
> ```
>
> The first one looks much cleaner. This automatic referencing behavior works
> because methods have a clear receiver—the type of `self`. Given the receiver
> and name of a method, Rust can figure out definitively whether the method is
> reading (`&self`), mutating (`&mut self`), or consuming (`self`). The fact
> that Rust makes borrowing implicit for method receivers is a big part of
> making ownership ergonomic in practice.

### Methods with More Parameters

Let’s practice using methods by implementing a second method on the `Rectangle`
struct. This time, we want an instance of `Rectangle` to take another instance
of `Rectangle` and return `true` if the second `Rectangle` can fit completely
within `self` (the first `Rectangle`);
<!--- self being the first instance? /LC --->
<!-- Yup! /Carol -->
otherwise it should return `false`. That is, once we’ve defined the `can_hold`
method, we want to be able to write the program shown in Listing 5-14.

Filename: src/main.rs

```
fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };
    let rect2 = Rectangle {
        width: 10,
        height: 40,
    };
    let rect3 = Rectangle {
        width: 60,
        height: 45,
    };

    println!("Can rect1 hold rect2? {}", rect1.can_hold(&rect2));
    println!("Can rect1 hold rect3? {}", rect1.can_hold(&rect3));
}
```

Listing 5-14: Using the as-yet-unwritten `can_hold` method

And the expected output would look like the following, because both dimensions
of `rect2` are smaller than the dimensions of `rect1` but `rect3` is wider than
`rect1`:

```
Can rect1 hold rect2? true
Can rect1 hold rect3? false
```

We know we want to define a method, so it will be within the `impl Rectangle`
block. The method name will be `can_hold`, and it will take an immutable borrow
of another `Rectangle` as a parameter. We can tell what the type of the
parameter will be by looking at the code that calls the method:
`rect1.can_hold(&rect2)` passes in `&rect2`, which is an immutable borrow to
`rect2`, an instance of `Rectangle`. This makes sense because we only need to
read `rect2` (rather than write, which would mean we’d need a mutable borrow),
and we want `main` to retain ownership of `rect2` so we can use it again after
calling the `can_hold` method. The return value of `can_hold` will be a
Boolean, and the implementation will check whether the width and height of
`self` are both greater than the width and height of the other `Rectangle`,
respectively. Let’s add the new `can_hold` method to the `impl` block from
Listing 5-13, shown in Listing 5-15.

Filename: src/main.rs

```
impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }

    fn can_hold(&self, other: &Rectangle) -> bool {
        self.width > other.width && self.height > other.height
    }
}
```

Listing 5-15: Implementing the `can_hold` method on `Rectangle` that takes
another `Rectangle` instance as a parameter

When we run this code with the `main` function in Listing 5-14, we’ll get our
desired output. Methods can take multiple parameters that we add to the
signature after the `self` parameter, and those parameters work just like
parameters in functions.

### Associated Functions

All functions defined within an `impl` block are called *associated functions*
because they’re associated with the type named after the `impl`. We can define
associated functions that don’t have `self` as their first parameter (and thus
are not methods) because they don’t need an instance of the type to work with.
We’ve already used one function like this: the `String::from` function that’s
defined on the `String` type.

Associated functions that aren’t methods are often used for constructors that
will return a new instance of the struct. For example, we could provide an
associated function that would have one dimension parameter and use that as
both width and height, thus making it easier to create a square `Rectangle`
rather than having to specify the same value twice:

Filename: src/main.rs

```
impl Rectangle {
    fn square(size: u32) -> Rectangle {
        Rectangle {
            width: size,
            height: size,
        }
    }
}
```

To call this associated function, we use the `::` syntax with the struct name;
`let sq = Rectangle::square(3);` is an example. This function is namespaced by
the struct: the `::` syntax is used for both associated functions and
namespaces created by modules. We’ll discuss modules in Chapter 7.

<!--- Should we mention the most common associated function is `new`? And that 
new isn't built into the language.
/JT --->

### Multiple `impl` Blocks

Each struct is allowed to have multiple `impl` blocks. For example, Listing
5-15 is equivalent to the code shown in Listing 5-16, which has each method
in its own `impl` block.

```
impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }
}

impl Rectangle {
    fn can_hold(&self, other: &Rectangle) -> bool {
        self.width > other.width && self.height > other.height
    }
}
```

Listing 5-16: Rewriting Listing 5-15 using multiple `impl` blocks

There’s no reason to separate these methods into multiple `impl` blocks here,
but this is valid syntax. We’ll see a case in which multiple `impl` blocks are
useful in Chapter 10, where we discuss generic types and traits.

## Summary

Structs let you create custom types that are meaningful for your domain. By
using structs, you can keep associated pieces of data connected to each other
and name each piece to make your code clear. In `impl` blocks, you can define
functions that are associated with your type, and methods are a kind of
associated function that let you specify the behavior that instances of your
structs have.

But structs aren’t the only way you can create custom types: let’s turn to
Rust’s enum feature to add another tool to your toolbox.

<!--- We don't mention that you can only use `impl` in the same crate as the
type it's created in, otherwise you could use `impl` and add methods on types
that come from other people (which you can't do, unless you make a trait to
attach them to)

Another thing we may want to mention is that `Self` inside of an `impl` refers
to the type being impl'd. So you might write the above:

```
impl Rectangle {
    fn square(size: u32) -> Self {
        Self {
            width: size,
            height: size,
        }
    }
}
```
which is often a bit more ergonomic.

/JT --->