rust-book-cn/nostarch/chapter05.md

[TOC]

Structs

A struct, short for "structure", gives us the ability to name and package together multiple related values that make up a meaningful group. If you come from an object-oriented language, a struct is like an object's data attributes. struct and enum (that we will talk about in the next chapter) are the building blocks you can use in Rust to create new types in your program's domain in order to take full advantage of Rust's compile-time type checking.

Let’s write a program which calculates the distance between two points. We’ll start off with single variable bindings, and then refactor it to use structs instead.

Let’s make a new project with Cargo:

$ cargo new --bin points
$ cd points

Here’s a short program which calculates the distance between two points. Put it into your src/main.rs:

Filename: src/main.rs

fn main() {
    let x1 = 0.0;
    let y1 = 5.0;

    let x2 = 12.0;
    let y2 = 0.0;

    let answer = distance(x1, y1, x2, y2);

    println!("Point 1: ({}, {})", x1, y1);
    println!("Point 2: ({}, {})", x2, y2);
    println!("Distance: {}", answer);
}

fn distance(x1: f64, y1: f64, x2: f64, y2: f64) -> f64 {
    let x_squared = f64::powi(x2 - x1, 2);
    let y_squared = f64::powi(y2 - y1, 2);

    f64::sqrt(x_squared + y_squared)
}

Let's try running this program with cargo run:

$ cargo run
   Compiling points v0.1.0 (file:///projects/points)
     Running `target/debug/points`
Point 1: (0, 5)
Point 2: (12, 0)
Distance: 13

Let's take a quick look at distance() before we move forward. To find the distance between two points, we can use the Pythagorean Theorem. The theorem is named after Pythagoras, who was the first person to mathematically prove this formula. The details aren't that important; just know the theorem says that the formula for the distance between two points is equal to:

• squaring the distance between the points horizontally (the "x" direction)
• squaring the distance between the points vertically (the "y" direction)
• adding those together
• and taking the square root of that.

So that's what we're implementing here.

f64::powi(2.0, 3)

The double colon (::) here is a namespace operator. We haven’t talked about modules and namespaces in depth yet, but you can think of the powi() function as being scoped inside of another name. In this case, the name is f64, the same as the type. The powi() function takes two arguments: the first is a number, and the second is the power that it raises that number to. In this case, the second number is an integer, hence the i in its name. Similarly, sqrt() is a function under the f64 module, which takes the square root of its argument.

Why structs?

Our little program is okay, but we can do better. The key to seeing this is in the signature of distance():

fn distance(x1: f64, y1: f64, x2: f64, y2: f64) -> f64 {

The distance function is supposed to calculate the distance between two points. But our distance function calculates some distance between four numbers. The first two and last two arguments are related, but that’s not expressed anywhere in our program itself. It would be nicer if we had a way to group (x1, y1) and (x2, y2) together.

We’ve already discussed one way to do that: tuples. Here’s a version of our program which uses tuples:

Filename: src/main.rs

fn main() {
    let p1 = (0.0, 5.0);

    let p2 = (12.0, 0.0);

    let answer = distance(p1, p2);

    println!("Point 1: {:?}", p1);
    println!("Point 2: {:?}", p2);
    println!("Distance: {}", answer);
}

fn distance(p1: (f64, f64), p2: (f64, f64)) -> f64 {
    let x_squared = f64::powi(p2.0 - p1.0, 2);
    let y_squared = f64::powi(p2.1 - p1.1, 2);

    f64::sqrt(x_squared + y_squared)
}

This is a little better, for sure. Tuples let us add a little bit of structure. We’re now passing two arguments, so that’s more clear. But it’s also worse: tuples don’t give names to their elements, so our calculation has gotten more confusing:

p2.0 - p1.0
p2.1 - p1.1

When writing this example, your authors almost got it wrong themselves! Distance is all about x and y points, but our code is talking about 0 and 1. This isn’t great.

Enter structs. We can transform our tuples into something with a name for the whole as well as names for the parts:

let p1 = (0.0, 5.0);

struct Point {
    x: f64,
    y: f64,
}

let p1 = Point { x: 0.0, y: 5.0 };

Here we've defined a struct and given it the name Point. The parts inside {} are defining the fields of the struct. We can have as many or as few of them as we'd like, and we give them a name and specify their type. Here we have two fields named x and y, and they both hold f64s.

We can access the field of a struct in the same way we access an element of a tuple, except we use its name:

let p1 = (0.0, 5.0);
let x = p1.0;

struct Point {
    x: f64,
    y: f64,
}

let p1 = Point { x: 0.0, y: 5.0 };
let x = p1.x;

Let’s convert our program to use our Point struct. Here’s what it looks like now:

Filename: src/main.rs

#[derive(Debug,Copy,Clone)]
struct Point {
    x: f64,
    y: f64,
}

fn main() {
    let p1 = Point { x: 0.0, y: 5.0};

    let p2 = Point { x: 12.0, y: 0.0};

    let answer = distance(p1, p2);

    println!("Point 1: {:?}", p1);
    println!("Point 2: {:?}", p2);
    println!("Distance: {}", answer);
}

fn distance(p1: Point, p2: Point) -> f64 {
    let x_squared = f64::powi(p2.x - p1.x, 2);
    let y_squared = f64::powi(p2.y - p1.y, 2);

    f64::sqrt(x_squared + y_squared)
}

Our function signature for distance() now says exactly what we mean: it calculates the distance between two Points. And rather than 0 and 1, we’ve got back our x and y. This is a win for clarity.

Derived Traits

There’s one other thing that’s a bit strange here, this stuff above the struct declaration:

#[derive(Debug,Copy,Clone)]
struct Point {

This is an annotation that tells the compiler our struct should get some default behavior for the Debug, Copy, and Clone traits. We talked about marking that types can be Copy and Clone-able in Chapter XX when we discussed ownership. Debug is the trait that enables us to print out our struct so that we can see its value while we are debugging our code.

So far, we’ve been printing values using {} in a println! macro. If we try that with a struct, however, by default, we'll get an error. Say we have the following program:

Filename: src/main.rs

struct Point {
    x: f64,
    y: f64,
}

fn main() {
    let p1 = Point { x: 0.0, y: 5.0};
    println!("Point 1: {}", p1);
}

This code tries to print the p1 point directly, which may seem innocuous. But running it produces the following output:

$ cargo run
   Compiling points v0.1.0 (file:///projects/points)
error: the trait bound `Point: std::fmt::Display` is not satisfied [--explain E0277]
 --> src/main.rs:8:29
8 |>     println!("Point 1: {}", p1);
  |>                             ^^
<std macros>:2:27: 2:58: note: in this expansion of format_args!
<std macros>:3:1: 3:54: note: in this expansion of print! (defined in <std macros>)
src/main.rs:8:5: 8:33: note: in this expansion of println! (defined in <std macros>)
note: `Point` cannot be formatted with the default formatter; try using `:?` instead if you are using a format string
note: required by `std::fmt::Display::fmt`

Whew! The core of the error is this part: the trait bound Point: std::fmt::Display is not satisfied. println! can do many kinds of formatting. By default, {} implements a kind of formatting known as Display: output intended for direct end-user consumption. The primitive types we’ve seen implement Display, as there’s only one way you’d show a 1 to a user. But with structs, the output is less clear. Do you want commas or not? What about the {}s? Should all the fields be shown?

More complex types in the standard library and that are defined by the programmer do not automatically implement Display formatting. Standard library types implement Debug formatting, which is intended for the programmer to see. The #[derive(Debug)] annotation lets us use a default implementation of Debug formatting to easily get this ability for types we've defined. To ask println! to use Debug formatting with our Point, we add the annotation to derive the trait and include :? in the print string, like this:

Filename: src/main.rs

#[derive(Debug)]
struct Point {
    x: f64,
    y: f64,
}

fn main() {
    let p1 = Point { x: 0.0, y: 5.0};
    println!("Point 1: {:?}", p1);
}

If you run this, it should print the values of each field in the Point struct as desired:

$ cargo run
   Compiling points v0.1.0 (file:///projects/points)
     Running `target/debug/points`
Point 1: Point { x: 0, y: 5 }

You’ll see this repeated later with other types. We’ll cover traits fully in Chapter XX.

Method Syntax

In Chapter 4 when we discussed ownership, we made several references to methods. Methods look like this:

let s1 = "hello";

// call a method on s1
let s2 = s1.clone();

println!("{}", s1);

The call to clone() is attached to s1 with a dot. This is called method syntax, and it’s a way to call certain functions with a different style.

Why have two ways to call functions? We’ll talk about some deeper reasons related to ownership in a moment, but one big reason is that methods look nicer when chained together:

// with functions
h(g(f(x)));

// with methods
x.f().g().h();

The nested-functions version reads in reverse: the program executes f(), then g(), then h(), but we read it left-to-right as h(), then g(), then f(). The method syntax is executed in the same order as we would read it.

Before we get into the details, let’s talk about how to define your own methods.

Defining methods

We can define methods with the impl keyword. impl is short for implementation. Doing so looks like this:

#[derive(Debug,Copy,Clone)]
struct Point {
    x: f64,
    y: f64,
}

impl Point {
    fn distance(&self, other: &Point) -> f64 {
        let x_squared = f64::powi(other.x - self.x, 2);
        let y_squared = f64::powi(other.y - self.y, 2);

        f64::sqrt(x_squared + y_squared)
    }
}

let p1 = Point { x: 0.0, y: 0.0 };
let p2 = Point { x: 5.0, y: 6.5 };

assert_eq!(8.200609733428363, p1.distance(&p2));

Let’s break this down. First, we have our Point struct from earlier in the chapter. Next comes our first use of the impl keyword:

impl Point {
    // ...
}

Everything we put inside of the curly braces will be methods implemented on Point. Next is our definition:

fn distance(&self, other: &Point) -> f64 {
    // ...
}

Other than this, the rest of the example is familiar: an implementation of distance() and using the method to find an answer.

Our definition of distance() here as a method looks very similar to our previous definition of distance() as a function, but with two differences. Here's the distance() function again:

fn distance(p1: Point, p2: Point) -> f64 {
    // ...
}

The first difference is in the first argument. Instead of a name and a type, we have written &self. This is what distinguishes a method from a function: using self inside of an impl block means we have a method. Because we already know that we are implementing this method on Point because of the surrounding impl Point block, we don’t need to write the type of self out.

Note that we have written &self, not just self. This is because we want to take a reference to our argument's value rather than taking ownership of it. In other words, these two forms are the same:

fn foo(self: &Point)
fn foo(&self)

Just like any other parameter, you can take self in three forms. Here’s the list, with the most common form first:

fn foo(&self) // take self by reference
fn foo(&mut self) // take self by mutable reference
fn foo(self) // take self by ownership

In this case, we only need a reference. We don’t need to mutate either Point to get the distance between them, so we won't take a mutable reference to the Point that we call the method on. Methods that take ownership of self are rarely used. An example of a time to do that would be if we wanted to have a method that would transform self into something else and prevent other code from using the value of self after the transformation happens.

Methods and automatic referencing

We’ve left out an important detail. It’s in this line of the example:

assert_eq!(8.200609733428363, p1.distance(&p2));

When we defined distance(), we took both self and the other argument by reference. Yet, we needed a & for p2 but not p1. What gives?

This feature is called automatic referencing, and calling methods is one of the few places in Rust that has behavior like this. Here’s how it works: when you call a method with self.(, Rust will automatically add in &s or &muts to match the signature. In other words, these are the same:

p1.distance(&p2);
(&p1).distance(&p2);

The first one looks much, much cleaner. Here’s another example:

let mut s = String::from("Hello,");

s.push_str(" world!");

// The above is the same as:
// (&mut s).push_str(" world!");

assert_eq!("Hello, world!", s);

Because push_str() has the following signature:

fn push_str(&mut self, string: &str) {

This automatic referencing behavior works because methods have a clear receiver — the type of self — and in most cases it’s clear given the receiver and name of a method whether the method is just reading (so needs &self), mutating (so &mut self), or consuming (so self). The fact that Rust makes borrowing implicit for method receivers is a big part of making ownership ergonomic in practice.