src/custom_types/structs.md

# Structures

There are three types of structures ("structs") that can be created using the
`struct` keyword:

* Tuple structs, which are, basically, named tuples.
* The classic [C structs][c_struct]
* Unit structs, which are field-less, are useful for generics.

```rust,editable
#[derive(Debug)]
struct Person<'a> {
    // The 'a defines a lifetime
    name: &'a str,
    age: u8,
}

// A unit struct
struct Nil;

// A tuple struct
struct Pair(i32, f32);

// A struct with two fields
struct Point {
    x: f32,
    y: f32,
}

// Structs can be reused as fields of another struct
#[allow(dead_code)]
struct Rectangle {
    // A rectangle can be specified by where the top left and bottom right
    // corners are in space.
    top_left: Point,
    bottom_right: Point,
}

fn main() {
    // Create struct with field init shorthand
    let name = "Peter";
    let age = 27;
    let peter = Person { name, age };

    // Print debug struct
    println!("{:?}", peter);


    // Instantiate a `Point`
    let point: Point = Point { x: 10.3, y: 0.4 };

    // Access the fields of the point
    println!("point coordinates: ({}, {})", point.x, point.y);

    // Make a new point by using struct update syntax to use the fields of our
    // other one
    let bottom_right = Point { x: 5.2, ..point };

    // `bottom_right.y` will be the same as `point.y` because we used that field
    // from `point`
    println!("second point: ({}, {})", bottom_right.x, bottom_right.y);

    // Destructure the point using a `let` binding
    let Point { x: top_edge, y: left_edge } = point;

    let _rectangle = Rectangle {
        // struct instantiation is an expression too
        top_left: Point { x: left_edge, y: top_edge },
        bottom_right: bottom_right,
    };

    // Instantiate a unit struct
    let _nil = Nil;

    // Instantiate a tuple struct
    let pair = Pair(1, 0.1);

    // Access the fields of a tuple struct
    println!("pair contains {:?} and {:?}", pair.0, pair.1);

    // Destructure a tuple struct
    let Pair(integer, decimal) = pair;

    println!("pair contains {:?} and {:?}", integer, decimal);
}
```

### Activity

1. Add a function `rect_area` which calculates the area of a rectangle (try
   using nested destructuring).
2. Add a function `square` which takes a `Point` and a `f32` as arguments, and returns a `Rectangle` with its lower left corner on the point, and a width and height corresponding to the `f32`.

### See also:

[`attributes`][attributes], [lifetime][lifetime] and [destructuring][destructuring]

[attributes]: ../attribute.md
[c_struct]: https://en.wikipedia.org/wiki/Struct_(C_programming_language)
[destructuring]: ../flow_control/match/destructuring.md
[lifetime]: ../scope/lifetime.md