Associated types - Rust By Example
Description: A description

# 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<A, B, C>(container: &C) -> i32 where
        C: Contains<A, B> { ... }

    // Using associated types
    fn difference<C: Contains>(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<C: Contains>(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));
    }