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< h1 id = "方法-method" > < a class = "header" href = "#方法-method" > 方法 Method< / a > < / h1 >
< p > 从面向对象语言过来的同学对于方法肯定不陌生，< code > class< / code > 里面就充斥着方法的概念。在 Rust 中，方法的概念也大差不差，往往和对象成对出现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > object.method()
< span class = "boring" > }< / span > < / code > < / pre >
< p > 例如读取一个文件写入缓冲区，如果用函数的写法 < code > read(f, buffer)< / code > ，用方法的写法 < code > f.read(buffer)< / code > 。不过与其它语言 < code > class< / code > 跟方法的联动使用不同（ ） ， < / p >
< h2 id = "定义方法" > < a class = "header" href = "#定义方法" > 定义方法< / a > < / h2 >
< p > Rust 使用 < code > impl< / code > 来定义方法，例如以下代码：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct Circle {
x: f64,
y: f64,
radius: f64,
}
impl Circle {
// new是Circle的关联函数， ，
// 这种方法往往用于初始化当前结构体的实例
fn new(x: f64, y: f64, radius: f64) -> Circle {
Circle {
x: x,
y: y,
radius: radius,
}
}
// Circle的方法， & self表示借用当前的Circle结构体
fn area(& self) -> f64 {
std::f64::consts::PI * (self.radius * self.radius)
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 我们这里先不详细展开讲解，只是先建立对方法定义的大致印象。下面的图片将 Rust 方法定义与其它语言的方法定义做了对比：< / p >
< img alt = "" src = "https://pica.zhimg.com/80/v2-0d848e960f3279999eab4b1317f6538e_1440w.png" class = "center" / >
< p > 可以看出，其它语言中所有定义都在 < code > class< / code > 中，但是 Rust 的对象定义和方法定义是分离的，这种数据和使用分离的方式，会给予使用者极高的灵活度。< / p >
< p > 再来看一个例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
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 };
println!(
"The area of the rectangle is {} square pixels.",
rect1.area()
);
}< / code > < / pre >
< p > 该例子定义了一个 < code > Rectangle< / code > 结构体，并且在其上定义了一个 < code > area< / code > 方法，用于计算该矩形的面积。< / p >
< p > < code > impl Rectangle {}< / code > 表示为 < code > Rectangle< / code > 实现方法（< code > impl< / code > 是实现 < em > implementation< / em > 的缩写），这样的写法表明 < code > impl< / code > 语句块中的一切都是跟 < code > Rectangle< / code > 相关联的。< / p >
< h4 id = "selfself-和-mut-self" > < a class = "header" href = "#selfself-和-mut-self" > self、& self 和 & mut self< / a > < / h4 >
< p > 接下来的内容非常重要，请大家仔细看。在 < code > area< / code > 的签名中，我们使用 < code > & self< / code > 替代 < code > rectangle: & Rectangle< / code > ， < code > & self< / code > 其实是 < code > self: & Self< / code > 的简写（注意大小写）。在一个 < code > impl< / code > 块内，< code > Self< / code > 指代被实现方法的结构体类型，< code > self< / code > 指代此类型的实例，换句话说，< code > self< / code > 指代的是 < code > Rectangle< / code > 结构体实例，这样的写法会让我们的代码简洁很多，而且非常便于理解：我们为哪个结构体实现方法，那么 < code > self< / code > 就是指代哪个结构体的实例。< / p >
< p > 需要注意的是，< code > self< / code > 依然有所有权的概念：< / p >
< ul >
< li > < code > self< / code > 表示 < code > Rectangle< / code > 的所有权转移到该方法中，这种形式用的较少< / li >
< li > < code > & self< / code > 表示该方法对 < code > Rectangle< / code > 的不可变借用< / li >
< li > < code > & mut self< / code > 表示可变借用< / li >
< / ul >
< p > 总之，< code > self< / code > 的使用就跟函数参数一样，要严格遵守 Rust 的所有权规则。< / p >
< p > 回到上面的例子中，选择 < code > & self< / code > 的理由跟在函数中使用 < code > & Rectangle< / code > 是相同的：我们并不想获取所有权，也无需去改变它，只是希望能够读取结构体中的数据。如果想要在方法中去改变当前的结构体，需要将第一个参数改为 < code > & mut self< / code > 。仅仅通过使用 < code > self< / code > 作为第一个参数来使方法获取实例的所有权是很少见的，这种使用方式往往用于把当前的对象转成另外一个对象时使用，转换完后，就不再关注之前的对象，且可以防止对之前对象的误调用。< / p >
< p > 简单总结下，使用方法代替函数有以下好处：< / p >
< ul >
< li > 不用在函数签名中重复书写 < code > self< / code > 对应的类型< / li >
< li > 代码的组织性和内聚性更强，对于代码维护和阅读来说，好处巨大< / li >
< / ul >
< h4 id = "方法名跟结构体字段名相同" > < a class = "header" href = "#方法名跟结构体字段名相同" > 方法名跟结构体字段名相同< / a > < / h4 >
< p > 在 Rust 中，允许方法名跟结构体的字段名相同：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > impl Rectangle {
fn width(& self) -> bool {
self.width > 0
}
}
fn main() {
let rect1 = Rectangle {
width: 30,
height: 50,
};
if rect1.width() {
println!("The rectangle has a nonzero width; it is {}", rect1.width);
}
}< / code > < / pre >
< p > 当我们使用 < code > rect1.width()< / code > 时， < code > rect1.width< / code > ，则是访问它的字段。< / p >
< p > 一般来说，方法跟字段同名，往往适用于实现 < code > getter< / code > 访问器，例如:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > mod my {
pub struct Rectangle {
width: u32,
pub height: u32,
}
impl Rectangle {
pub fn new(width: u32, height: u32) -> Self {
Rectangle { width, height }
}
pub fn width(& self) -> u32 {
return self.width;
}
pub fn height(& self) -> u32 {
return self.height;
}
}
}
fn main() {
let rect1 = my::Rectangle::new(30, 50);
println!("{}", rect1.width()); // OK
println!("{}", rect1.height()); // OK
// println!("{}", rect1.width); // Error - the visibility of field defaults to private
println!("{}", rect1.height); // OK
}< / code > < / pre >
< p > 当从模块外部访问结构体时，结构体的字段默认是私有的，其目的是隐藏信息（封装）。我们如果想要从模块外部获取 < code > Rectangle< / code > 的字段，只需把它的 < code > new< / code > ， < code > width< / code > 和 < code > height< / code > 方法设置为公开可见，那么用户就可以创建一个矩形，同时通过访问器 < code > rect1.width()< / code > 和 < code > rect1.height()< / code > 方法来获取矩形的宽度和高度。< / p >
< p > 因为 < code > width< / code > 字段是私有的，当用户访问 < code > rect1.width< / code > 字段时，就会报错。注意在此例中，< code > Self< / code > 指代的就是被实现方法的结构体 < code > Rectangle< / code > 。< / p >
< p > 特别的是，这种默认的可见性（私有的）可以通过 < code > pub< / code > 进行覆盖，这样对于模块外部来说，就可以直接访问使用 < code > pub< / code > 修饰的字段而无需通过访问器。这种可见性仅当从定义结构的模块外部访问时才重要，并且具有隐藏信息（封装）的目的。< / p >
< blockquote >
< h3 id = "--运算符到哪去了" > < a class = "header" href = "#--运算符到哪去了" > < code > -> < / code > 运算符到哪去了？< / a > < / h3 >
< p > 在 C/C++ 语言中，有两个不同的运算符来调用方法：< code > .< / code > 直接在对象上调用方法，而 < code > -> < / code > 在一个对象的指针上调用方法，这时需要先解引用指针。换句话说，如果 < code > object< / code > 是一个指针，那么 < code > object-> something()< / code > 和 < code > (*object).something()< / code > 是一样的。< / p >
< p > Rust 并没有一个与 < code > -> < / code > 等效的运算符； ， < strong > 自动引用和解引用< / strong > 的功能。方法调用是 Rust 中少数几个拥有这种行为的地方。< / p >
< p > 他是这样工作的：当使用 < code > object.something()< / code > 调用方法时， < code > object< / code > 添加 < code > & < / code > （视可见性添加< code > & mut< / code > )、 < code > *< / code > 以便使 < code > object< / code > 与方法签名匹配。也就是说，这些代码是等价的：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < span class = "boring" > #[derive(Debug,Copy,Clone)]
< / span > < span class = "boring" > struct Point {
< / span > < span class = "boring" > x: f64,
< / span > < span class = "boring" > y: f64,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > < span class = "boring" > impl Point {
< / span > < span class = "boring" > fn distance(& self, other: & Point) -> f64 {
< / span > < span class = "boring" > let x_squared = f64::powi(other.x - self.x, 2);
< / span > < span class = "boring" > let y_squared = f64::powi(other.y - self.y, 2);
< / span > < span class = "boring" >
< / span > < span class = "boring" > f64::sqrt(x_squared + y_squared)
< / span > < span class = "boring" > }
< / span > < span class = "boring" > }
< / span > < span class = "boring" > let p1 = Point { x: 0.0, y: 0.0 };
< / span > < span class = "boring" > let p2 = Point { x: 5.0, y: 6.5 };
< / span > p1.distance(& p2);
(& p1).distance(& p2);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 第一行看起来简洁的多。这种自动引用的行为之所以有效，是因为方法有一个明确的接收者———— < code > self< / code > 的类型。在给出接收者和方法名的前提下， < code > & self< / code > ），做出修改（< code > & mut self< / code > ）或者是获取所有权（< code > self< / code > ） ， < / p >
< / blockquote >
< h2 id = "带有多个参数的方法" > < a class = "header" href = "#带有多个参数的方法" > 带有多个参数的方法< / a > < / h2 >
< p > 方法和函数一样，可以使用多个参数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > impl Rectangle {
fn area(& self) -> u32 {
self.width * self.height
}
fn can_hold(& self, other: & Rectangle) -> bool {
self.width > other.width & & self.height > other.height
}
}
fn main() {
let rect1 = Rectangle { width: 30, height: 50 };
let rect2 = Rectangle { width: 10, height: 40 };
let rect3 = Rectangle { width: 60, height: 45 };
println!("Can rect1 hold rect2? {}", rect1.can_hold(& rect2));
println!("Can rect1 hold rect3? {}", rect1.can_hold(& rect3));
}< / code > < / pre >
< h2 id = "关联函数" > < a class = "header" href = "#关联函数" > 关联函数< / a > < / h2 >
< p > 现在大家可以思考一个问题，如何为一个结构体定义一个构造器方法？也就是接受几个参数，然后构造并返回该结构体的实例。其实答案在开头的代码片段中就给出了，很简单，参数中不包含 < code > self< / code > 即可。< / p >
< p > 这种定义在 < code > impl< / code > 中且没有 < code > self< / code > 的函数被称之为< strong > 关联函数< / strong > ： < code > self< / code > ，不能用 < code > f.read()< / code > 的形式调用，因此它是一个函数而不是方法，它又在 < code > impl< / code > 中，与结构体紧密关联，因此称为关联函数。< / p >
< p > 在之前的代码中，我们已经多次使用过关联函数，例如 < code > String::from< / code > ，用于创建一个动态字符串。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < span class = "boring" > #[derive(Debug)]
< / span > < span class = "boring" > struct Rectangle {
< / span > < span class = "boring" > width: u32,
< / span > < span class = "boring" > height: u32,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > impl Rectangle {
fn new(w: u32, h: u32) -> Rectangle {
Rectangle { width: w, height: h }
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< blockquote >
< p > Rust 中有一个约定俗成的规则，使用 < code > new< / code > 来作为构造器的名称， ， < code > new< / code > 作为关键字。< / p >
< / blockquote >
< p > 因为是函数，所以不能用 < code > .< / code > 的方式来调用，我们需要用 < code > ::< / code > 来调用，例如 < code > let sq = Rectangle::new(3, 3);< / code > 。这个方法位于结构体的命名空间中：< code > ::< / code > 语法用于关联函数和模块创建的命名空间。< / p >
< h2 id = "多个-impl-定义" > < a class = "header" href = "#多个-impl-定义" > 多个 impl 定义< / a > < / h2 >
< p > Rust 允许我们为一个结构体定义多个 < code > impl< / code > 块，目的是提供更多的灵活性和代码组织性，例如当方法多了后，可以把相关的方法组织在同一个 < code > impl< / code > 块中，那么就可以形成多个 < code > impl< / code > 块，各自完成一块儿目标：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > < span class = "boring" > #[derive(Debug)]
< / span > < span class = "boring" > struct Rectangle {
< / span > < span class = "boring" > width: u32,
< / span > < span class = "boring" > height: u32,
< / span > < span class = "boring" > }
< / span > < span class = "boring" >
< / span > 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
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 当然，就这个例子而言，我们没必要使用两个 < code > impl< / code > 块，这里只是为了演示方便。< / p >
< h2 id = "为枚举实现方法" > < a class = "header" href = "#为枚举实现方法" > 为枚举实现方法< / a > < / h2 >
< p > 枚举类型之所以强大，不仅仅在于它好用、可以< a href = "https://course.rs/basic/compound-type/enum.html#同一化类型" > 同一化类型< / a > ，还在于，我们可以像结构体一样，为枚举实现方法：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #![allow(unused)]
enum Message {
Quit,
Move { x: i32, y: i32 },
Write(String),
ChangeColor(i32, i32, i32),
}
impl Message {
fn call(& self) {
// 在这里定义方法体
}
}
fn main() {
let m = Message::Write(String::from("hello"));
m.call();
}< / code > < / pre >
< p > 除了结构体和枚举，我们还能为特征(trait)实现方法，这将在下一章进行讲解，在此之前，先来看看泛型。< / p >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > < a href = "https://practice-zh.course.rs/method.html" > Rust By Practice< / a > ，支持代码在线编辑和运行，并提供详细的< a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/method.md" > 习题解答< / a > 。< / p >
< / blockquote >
< / main >
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< script src = "../assets/bigPicture-be03f9c8.js" > < / script >
< / div >
< / body >
< / html >
