关联类型

通过将内部类型作为*输出*类型局部移动到特征中，“关联类型”的使用提高了代码的整体可读性。`trait` 定义的语法如下

#![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;
}
}

请注意，使用 `Contains` 特征的函数不再需要表达 `A` 或 `B`

// 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 { ... }

让我们使用关联类型重写上一节中的示例

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));
}