Rust by Example

Associated types

The use of "Associated types" improves the overall readability of code by moving inner types locally into a trait as output types. Syntax for the trait definition is as follows:

#![allow(unused)]

fn main() {

// `A` and `B` are defined in the trait via the `type` keyword.

// (Note: `type` in this context is different from `type` when used for

// aliases).

trait Contains {

type A;

type B;

// Updated syntax to refer to these new types generically.

fn contains(&self, &Self::A, &Self::B) -> bool;

}

}

Note that functions that use the trait Contains are no longer required to express A or B at all:

// Without using associated types

fn difference(container: &C) -> i32 where

C: Contains { ... }

// Using associated types

fn difference(container: &C) -> i32 { ... }

Let's rewrite the example from the previous section using associated types:

struct Container(i32, i32);

// A trait which checks if 2 items are stored inside of container.

// Also retrieves first or last value.

trait Contains {

// Define generic types here which methods will be able to utilize.

type A;

type B;

fn contains(&self, _: &Self::A, _: &Self::B) -> bool;

fn first(&self) -> i32;

fn last(&self) -> i32;

}

impl Contains for Container {

// Specify what types `A` and `B` are. If the `input` type

// is `Container(i32, i32)`, the `output` types are determined

// as `i32` and `i32`.

type A = i32;

type B = i32;

// `&Self::A` and `&Self::B` are also valid here.

fn contains(&self, number_1: &i32, number_2: &i32) -> bool {

(&self.0 == number_1) && (&self.1 == number_2)

}

// Grab the first number.

fn first(&self) -> i32 { self.0 }

// Grab the last number.

fn last(&self) -> i32 { self.1 }

}

fn difference(container: &C) -> i32 {

container.last() - container.first()

}

fn main() {

let number_1 = 3;

let number_2 = 10;

let container = Container(number_1, number_2);

println!("Does container contain {} and {}: {}",

&number_1, &number_2,

container.contains(&number_1, &number_2));

println!("First number: {}", container.first());

println!("Last number: {}", container.last());

println!("The difference is: {}", difference(&container));

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX