Imported Upstream version 1.1.0+dfsg1

[rustc.git] / src / doc / trpl / structs.md

% Structs

Structs are a way of creating more complex data types. For example, if we were
doing calculations involving coordinates in 2D space, we would need both an `x`
and a `y` value:

```rust
let origin_x = 0;
let origin_y = 0;
```

A struct lets us combine these two into a single, unified datatype:

```rust
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let origin = Point { x: 0, y: 0 }; // origin: Point

    println!("The origin is at ({}, {})", origin.x, origin.y);
}
```

There’s a lot going on here, so let’s break it down. We declare a `struct` with
the `struct` keyword, and then with a name. By convention, `struct`s begin with
a capital letter and are camel cased: `PointInSpace`, not `Point_In_Space`.

We can create an instance of our struct via `let`, as usual, but we use a `key:
value` style syntax to set each field. The order doesn’t need to be the same as
in the original declaration.

Finally, because fields have names, we can access the field through dot
notation: `origin.x`.

The values in structs are immutable by default, like other bindings in Rust.
Use `mut` to make them mutable:

```rust
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let mut point = Point { x: 0, y: 0 };

    point.x = 5;

    println!("The point is at ({}, {})", point.x, point.y);
}
```

This will print `The point is at (5, 0)`.

Rust does not support field mutability at the language level, so you cannot
write something like this:

```rust,ignore
struct Point {
    mut x: i32,
    y: i32,
}
```

Mutability is a property of the binding, not of the structure itself. If you’re
used to field-level mutability, this may seem strange at first, but it
significantly simplifies things. It even lets you make things mutable for a short
time only:


```rust,ignore
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let mut point = Point { x: 0, y: 0 };

    point.x = 5;

    let point = point; // this new binding can’t change now

    point.y = 6; // this causes an error
}
```

# Update syntax

A `struct` can include `..` to indicate that you want to use a copy of some
other struct for some of the values. For example:

```rust
struct Point3d {
    x: i32,
    y: i32,
    z: i32,
}

let mut point = Point3d { x: 0, y: 0, z: 0 };
point = Point3d { y: 1, .. point };
```

This gives `point` a new `y`, but keeps the old `x` and `z` values. It doesn’t
have to be the same `struct` either, you can use this syntax when making new
ones, and it will copy the values you don’t specify:

```rust
# struct Point3d {
#     x: i32,
#     y: i32,
#     z: i32,
# }
let origin = Point3d { x: 0, y: 0, z: 0 };
let point = Point3d { z: 1, x: 2, .. origin };
```

# Tuple structs

Rust has another data type that’s like a hybrid between a [tuple][tuple] and a
struct, called a ‘tuple struct’. Tuple structs have a name, but
their fields don’t:

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

[tuple]: primitive-types.html#tuples

These two will not be equal, even if they have the same values:

```rust
# struct Color(i32, i32, i32);
# struct Point(i32, i32, i32);
let black = Color(0, 0, 0);
let origin = Point(0, 0, 0);
```

It is almost always better to use a struct than a tuple struct. We would write
`Color` and `Point` like this instead:

```rust
struct Color {
    red: i32,
    blue: i32,
    green: i32,
}

struct Point {
    x: i32,
    y: i32,
    z: i32,
}
```

Now, we have actual names, rather than positions. Good names are important,
and with a struct, we have actual names.

There _is_ one case when a tuple struct is very useful, though, and that’s a
tuple struct with only one element. We call this the ‘newtype’ pattern, because
it allows you to create a new type, distinct from that of its contained value
and expressing its own semantic meaning:

```rust
struct Inches(i32);

let length = Inches(10);

let Inches(integer_length) = length;
println!("length is {} inches", integer_length);
```

As you can see here, you can extract the inner integer type through a
destructuring `let`, just as with regular tuples. In this case, the
`let Inches(integer_length)` assigns `10` to `integer_length`.

# Unit-like structs

You can define a struct with no members at all:

```rust
struct Electron;
```

Such a struct is called ‘unit-like’ because it resembles the empty
tuple, `()`, sometimes called ‘unit’. Like a tuple struct, it defines a
new type.

This is rarely useful on its own (although sometimes it can serve as a
marker type), but in combination with other features, it can become
useful. For instance, a library may ask you to create a structure that
implements a certain [trait][trait] to handle events. If you don’t have
any data you need to store in the structure, you can just create a
unit-like struct.

[trait]: traits.html