练习题来自：https://practice-zh.course.rs/generics-traits/advanced-traits.html
1
struct Container(i32, i32);// 使用关联类型实现重新实现以下特征
// trait Contains {
// type A;
// type B;trait Contains<A, B> {fn contains(&self, _: &A, _: &B) -> bool;fn first(&self) -> i32;fn last(&self) -> i32;
}impl Contains<i32, i32> for Container {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<A, B, C: Contains<A, B>>(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));
}
所谓关联类型，个人觉得更像是一种语法糖。它将本来需要用泛型表示的类型写在特征里面。实现特征，不仅需要实现特征中的方法，还要实现特征中的类型。
将A和B作为关联类型写入特征：
trait Contains{type A;type B;fn contains(&self, _: &Self::A, _: &Self::B) -> bool;fn first(&self) -> i32;fn last(&self) -> i32;
}
实现特征时，显式指定A和B的类型：
impl Contains for Container {type A = i32;type B = i32;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 }
}
无需在上层指定类型，使用者暗含了类型：
fn difference<C: Contains>(container: &C) -> i32 {container.last() - container.first()
}
2
use std::ops::Sub;#[derive(Debug, PartialEq)]
struct Point<T> {x: T,y: T,
}// 用三种方法填空: 其中两种使用默认的泛型参数，另外一种不使用
impl __ {type Output = Self;fn sub(self, other: Self) -> Self::Output {Point {x: self.x - other.x,y: self.y - other.y,}}
}fn main() {assert_eq!(Point { x: 2, y: 3 } - Point { x: 1, y: 0 },Point { x: 1, y: 3 });println!("Success!")
}
这题目是第一个我没做出来，然后不得不求助答案的题目… …这三种语法是逐渐变简单的，第一种是最复杂的，核心是impl T，但是对T进行Sub的特征限定，而且输出必须也是T（否则后面x和y无法相减）；其次Sub的类型需要是Point，最后Point也需要泛型参数。
impl<T: Sub<Output = T>> Sub<Point<T>> for Point<T> {type Output = Self;fn sub(self, other: Self) -> Self::Output {Point {x: self.x - other.x,y: self.y - other.y,}}
}
由于Point就是Self，所以这就是第二种语法：
impl<T: Sub<Output = T>> Sub<Self> for Point<T> {type Output = Self;fn sub(self, other: Self) -> Self::Output {Point {x: self.x - other.x,y: self.y - other.y,}}
}
而Sub本身的泛型参数是存在默认值的，就是Self，因此这里可以继续省略：
impl<T: Sub<Output = T>> Sub for Point<T> {type Output = Self;fn sub(self, other: Self) -> Self::Output {Point {x: self.x - other.x,y: self.y - other.y,}}
}
3
trait Pilot {fn fly(&self) -> String;
}trait Wizard {fn fly(&self) -> String;
}struct Human;impl Pilot for Human {fn fly(&self) -> String {String::from("This is your captain speaking.")}
}impl Wizard for Human {fn fly(&self) -> String {String::from("Up!")}
}impl Human {fn fly(&self) -> String {String::from("*waving arms furiously*")}
}fn main() {let person = Human;assert_eq!(__, "This is your captain speaking.");assert_eq!(__, "Up!");assert_eq!(__, "*waving arms furiously*");println!("Success!")
}
这里涉及到一个多继承的问题，即一个类同时实现了两个特征，其中各有一个签名相同的方法，类本身也有一个签名相同的方法，需要加限定符号区分。
fn main() {let person = Human;assert_eq!(Pilot::fly(&person), "This is your captain speaking.");assert_eq!(Wizard::fly(&person), "Up!");assert_eq!(Human::fly(&person), "*waving arms furiously*");println!("Success!")
}
当然，自己调用自己的方法还有更简单，也是更常见的写法：
assert_eq!(person.fly(), "*waving arms furiously*");
C++里也存在多继承的问题，虽然就我的工作经验来看很少出现，比如下面的代码：
struct P1
{virtual void get(){cout << "P1 get" << endl;}
};struct P2
{virtual void get(){cout << "P2 get" << endl;}
};struct Child: P1, P2
{// virtual void get()// {// cout << "Child get" << endl;// }
};int main()
{Child child;child.get();
}这里的get就会出现歧义的问题。解决办法也是加上限定符：
int main()
{Child child;child.P1::get();child.P2::get();
}
直接像Rust那么写肯定是不行的，这种直接用双冒号开头的写法，在C++中是静态方法（static）的特权。
4
trait Person {fn name(&self) -> String;
}// Person 是 Student 的 supertrait .
// 实现 Student 需要同时实现 Person.
trait Student: Person {fn university(&self) -> String;
}trait Programmer {fn fav_language(&self) -> String;
}// CompSciStudent (computer science student) 是 Programmer
// 和 Student 的 subtrait. 实现 CompSciStudent 需要先实现这两个 supertraits.
trait CompSciStudent: Programmer + Student {fn git_username(&self) -> String;
}fn comp_sci_student_greeting(student: &dyn CompSciStudent) -> String {format!("My name is {} and I attend {}. My favorite language is {}. My Git username is {}",student.name(),student.university(),student.fav_language(),student.git_username())
}struct CSStudent {name: String,university: String,fav_language: String,git_username: String
}// 为 CSStudent 实现所需的特征
impl ...fn main() {let student = CSStudent {name: "Sunfei".to_string(),university: "XXX".to_string(),fav_language: "Rust".to_string(),git_username: "sunface".to_string()};// 填空println!("{}", comp_sci_student_greeting(__));
}
逐个实现特征即可，不过Rust居然不能一次实现所有的特征（感觉这些特征方法也可以写在同一个里）
impl Person for CSStudent {fn name(&self) -> String {self.name.clone()}
}impl Student for CSStudent{fn university(&self) -> String{self.university.clone()}
}impl Programmer for CSStudent {fn fav_language(&self) -> String {self.fav_language.clone()}
}impl CompSciStudent for CSStudent{fn git_username(&self) -> String{self.git_username.clone()}
}
5
use std::fmt;// 定义一个 newtype `Pretty`impl fmt::Display for Pretty {fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {write!(f, "\"{}\"", self.0.clone() + ", world")}
}fn main() {let w = Pretty("hello".to_string());println!("w = {}", w);
}
这玩意让我觉得孤儿规则没什么意义。。。
use std::fmt;// 定义一个 newtype `Pretty`
struct Pretty(String);impl fmt::Display for Pretty {fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {write!(f, "\"{}\"", self.0.clone() + ", world")}
}fn main() {let w = Pretty("hello".to_string());println!("w = {}", w);
}