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Learning Rust By Practice, narrowing the gap between beginner and skilled-dev with challenging examples, exercises and projects.
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Replies: 38 comments 20 replies
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请教一下,第5题里面,为什么如下写法也可以编译通过并执行成功呢?
fn main() { let light = TrafficLight::new(); assert_eq!(light.get_state(), "red"); }
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😕 3
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为什么不行呢😂
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因为assert_eq!调用的是类型的PartialEq特征,而String有实现对str和&str类型的PartialEq特征。
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👍 3
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回复 @religou:我猜没加解引用符的self是被"自动引用和解引用"了(如果我说的不对还请大佬们指正)
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👍 3
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@tn233 正确的,rust具有自动引用和解引用的功能
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这里的自动引用和解引用有条件限制,比如该事例加上*****就不会编译通过:
#![allow(unused)]
enum Message {
Quit,
Move{x:i32, y: i32},
Write(String),
ChangeColor(i32, i32, i32),
}
impl Message {
fn call(&self) {
match *self {
Message::Write(message) => print!("{}", message),
_ =>()
}
}
}
fn main() {
let m = Message::Write(String::from("test"));
m.call();
}
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done
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最后一题应该返回字面值吧
#[derive(Debug)] enum TrafficLightColor { Red, Yellow, Green, } // 为 TrafficLightColor 实现所需的方法 impl TrafficLightColor { fn color(&self) -> &str { match *self { TrafficLightColor::Red => "red", TrafficLightColor::Yellow => "yellow", TrafficLightColor::Green => "Green", } } } fn main() { let c = TrafficLightColor::Yellow; assert_eq!(c.color(), "yellow"); println!("{:?}",c); }
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这样也能通过
impl TrafficLightColor {
pub fn color(&self) -> &str {
match *self {
Self::Red => "red",
Self::Green => "green",
Self::Yellow => "yellow",
}
}
}Beta Was this translation helpful? Give feedback.
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👍 2 -
🎉 1
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fn color(&self)-> &str {
match &self {
TrafficLightColor::Red=>"red",
TrafficLightColor::Yellow=>"yellow",
TrafficLightColor::Green=>"green",
}
}
我这样也行, 自动解引用真的让我习惯c++之后感到有点混乱。。总是不自主去思考指针。。
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没有生命周期的话,会导致悬垂指针
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fn color(&self) ->&str{
match self {
Yellow => "yellow",
_ => "other"
}
}
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self前面加不加*有什么区别?
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均通过了测试
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rust的自动引用和解引用机制
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👍 1
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向match传入一个引用,匹配一个非引用模式(模式前不包含"&"),Rust会尝试copy传入的指针,如果指针不支持copy(例如可变引用就没有实现copy),那么就会move指针,传入的指针被销毁,创建一个相同指向的新指针取代。由此,match获得了指针的"所有权"(指针其实不因该谈所有权,但是借用这个说法好理解)。之后新指针会被解引用,拿到所指向数据的值。
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👍 2
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impl TrafficLightColor {
fn color(&self) -> &str {
match *self {
TrafficLightColor::Red => "red",
TrafficLightColor::Yellow => "yellow",
TrafficLightColor::Green => "Green",
}
}
}
这个能改成 if let 匹配吗?
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但题目的目的是针对不同的颜色返回不同的字符串
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done
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done
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Done
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DONE
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done
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为什么用 self 也能正常运行?
fn color(&self) -> &str{
match self {
Yellow => "yellow",
Red => "red",
Green => "red",
_ => "unknow"
}
}
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这样写会有警告吧。
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自动解引用。作者不是说这样写更好吗?让使用者知道这里使用的是引用
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请问这里一定得进行解引用这一步吗(不管是显式还是隐式),代码里
let c = TrafficLightColor::Yellow;
c本身就是TrafficLightColor的一个值,那color函数里的self是值的引用,不可以直接进行模式匹配吗
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lala
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done
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1、方法的声明
struct Rectangle { width: u32, height: u32, } impl Rectangle { // 完成 area 方法,返回矩形 Rectangle 的面积 fn area(&self)->u32{ self.width*self.height } } fn main() { let rect1 = Rectangle { width: 30, height: 50 }; assert_eq!(rect1.area(), 1500); }
2、方法中&self和self的区别
// 只填空,不要删除任何代码行! #[derive(Debug)] struct TrafficLight { color: String, } impl TrafficLight { pub fn show_state(&self) {//不可以用self,会导致所有权转移 println!("the current state is {}", self.color); } } fn main() { let light = TrafficLight{ color: "red".to_owned(), }; // 不要拿走 `light` 的所有权 light.show_state(); // 否则下面代码会报错 println!("{:?}", light); }
3、方法中&self和&mut self的用法
struct TrafficLight { color: String, } impl TrafficLight { // 使用 `Self` 填空 pub fn show_state(&self) { println!("the current state is {}", self.color); } // 填空,不要使用 `Self` 或其变体 pub fn change_state(&mut self) { self.color = "green".to_string() } } fn main() {}
4、Self代替对类型的说明
#[derive(Debug)] struct TrafficLight { color: String, } impl TrafficLight { // 1. 实现下面的关联函数 `new`, // 2. 该函数返回一个 TrafficLight 实例,包含 `color` "red" // 3. 该函数必须使用 `Self` 作为类型,不能在签名或者函数体中使用 `TrafficLight` pub fn new()->Self{ Self{ color:String::from("red"), } } pub fn get_state(&self) -> &str { &self.color } } fn main() { let light = TrafficLight::new(); //println!("{:?}",light); assert_eq!(light.get_state(), "red"); }
5、每个结构体可以对应多个impl模块
struct Rectangle { width: u32, height: u32, } // 使用多个 `impl` 语句块重写下面的代码 impl Rectangle { fn area(&self) -> u32 { self.width * self.height } } impl Rectangle{ fn can_hold(&self, other: &Rectangle) -> bool { self.width > other.width && self.height > other.height } } fn main() {}
6、
//自己的答案,可以度过,但是很明显逻辑看上去很奇怪 #[derive(Debug)] enum TrafficLightColor { Red, Yellow, Green, } // 为 TrafficLightColor 实现所需的方法 impl TrafficLightColor { pub fn color(&self)->&str{ "yellow" } } fn main() { let c = TrafficLightColor::Yellow; assert_eq!(c.color(), "yellow"); println!("{:?}",c); }
官方答案
#[derive(Debug)] enum TrafficLightColor { Red, Yellow, Green, } // implement TrafficLightColor with a method impl TrafficLightColor { fn color(&self) -> String { //模式匹配赛高 match *self { TrafficLightColor::Red => "red".to_string(), TrafficLightColor::Yellow => "yellow".to_string(), TrafficLightColor::Green => "green".to_string(), } } } fn main() { let c = TrafficLightColor::Yellow; assert_eq!(c.color(), "yellow"); println!("{:?}", c); }
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done
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assert_eq!()函数可以同时比较String与&str这两个类型具体的内容是否相同。
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done~~~
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mark finished
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impl TrafficLightColor {
fn color(&self) -> &str {
match self {
Self::Red => "red",
Self::Yellow => "yellow",
Self::Green => "green"
}
}
}
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impl TrafficLightColor {
fn color(&self) -> String {
let f = format!("{:?}", self);
f.to_lowercase()
}
}
最后一题这样也行
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day4 方法
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done
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Done
// 练习
// 语法糖:一种语法;使代码更已读或者更简洁。但不增加语法功能
// 字段初始化简写
// struct Point {
// x: i32,
// y: i32,
// }
// fn main() {
// let x = 5;
// let y = 10;
// let p = Point { x, y }; // 等同于 Point { x: x, y: y }
// }
// 模式匹配中的if let和while let
// let some_option = Some(5);
// // if let
// if let Some(x) = some_option {
// println!("Matched: {}", x);
// }
// // while let
// let mut stack = vec![1, 2, 3];
// while let Some(top) = stack.pop() {
// println!("{}", top);
// }
// // 如果不用语法糖,都需要用match进行匹配,上面的代码会变成这样:
// let some_option = Some(5);
// // 使用 match 代替 if let
// match some_option {
// Some(x) => println!("Matched: {}", x),
// None => {},
// }
// // 使用 match 代替 while let
// let mut stack = vec![1, 2, 3];
// loop {
// match stack.pop() {
// Some(top) => println!("{}", top),
// None => break,
// }
// }
// 方法调用的自动引用和解引用
// struct Circle {
// radius: f64,
// }
// impl Circle {
// fn area(&self) -> f64 {
// std::f64::consts::PI * self.radius * self.radius
// }
// }
// fn main() {
// let circle = Circle { radius: 5.0 };
// println!("Area: {}", circle.area()); // 自动引用
// // println!("Area: {}", Circle::area(&circle)); // 显式传递引用
// }
// 闭包的简写
// let add = |a, b| a + b; // 等同于 |a: i32, b: i32| -> i32 { a + b }
// println!("Sum: {}", add(2, 3));
// 1.
// struct Rectangle {
// width: u32,
// height: u32,
// }
// impl Rectangle {
// fn area(&self) -> u32 {
// self.width * self.height
// }
// }
// fn main() {
// let rect1 = Rectangle { width: 30, height: 50 };
// assert_eq!(rect1.area(), 1500);
// }
// 2.
// #[derive(Debug)]
// struct TrafficLight {
// color: String,
// }
// impl TrafficLight {
// pub fn show_state(&self) {
// println!("the current state is: {}", &self.color);
// }
// }
// fn main() {
// let light = TrafficLight {
// color: "red".to_owned(),
// };
// light.show_state();
// println!("{:?}", light);
// }
// 3.
// struct TrafficLight {
// color: String,
// }
// impl TrafficLight {
// pub fn show_state(&self) {
// println!("the current state is: {}", self.color);
// }
// pub fn change_state(& mut self) {
// self.color = "green".to_string();
// }
// }
// fn main() {}
// 4.
// #![]和#[]区别
// #![]crate级别属性
// #[]项级别属性
// #[derive(Debug)]
// struct TrafficLight {
// color: String,
// }
// impl TrafficLight {
// pub fn new() -> TrafficLight {
// TrafficLight {
// color: "red".to_string(),
// }
// }
// pub fn get_status(&self) -> &str {
// &self.color
// }
// }
// fn main() {
// let light = TrafficLight::new();
// assert_eq!(light.get_status(), "red");
// }
// 5.
// struct Rectangle {
// width: u32,
// height: u32,
// }
// impl Rectangle {
// fn area(&self) -> u32 {
// self.width * self.height
// }
// }
// impl Rectangle {
// fn can_hold(&self, other: &Rectangle) -> bool {
// self.width > other.width && self.height > other.height
// }
// }
// fn main() {}
// 6.
#[derive(Debug)]
enum TrafficLightColor {
Red,
Yellow,
Green,
}
impl TrafficLightColor {
fn color(&self) -> &str {
match self {
TrafficLightColor::Red => "red",
TrafficLightColor::Yellow => "yellow",
TrafficLightColor::Green => "green",
}
}
}
fn main() {
let c = TrafficLightColor::Yellow;
assert_eq!(c.color(), "yellow");
println!("{:?}", c);
}
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请教一下,为什么第二题用的是self.color?
// println!("the current state is: {}", &self.color);
println!("the current state is: {}", self.color);
两种方式我都试过了,都可以正常编译,使用答案中提示的self.color有什么好处?
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done ✅
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done
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第 6 题
#[derive(Debug)] enum TrafficLightColor { Red, Yellow, Green, } // 为 TrafficLightColor 实现所需的方法 impl TrafficLightColor { fn color(&self) -> &str { if let TrafficLightColor::Yellow = self {"yellow"} else { "not yellow" } } } fn main() { let c = TrafficLightColor::Yellow; assert_eq!(c.color(), "yellow"); println!("{:?}",c); }
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done
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第六题也可以这样实现:
impl TrafficLightColor {
fn color(&self) -> String {
format!("{:?}", &self).to_lowercase()
}
}
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// 为 TrafficLightColor 实现所需的方法
impl TrafficLightColor { fn color(&self) -> String { format!("{:?}", &self).to_lowercase() } }
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