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< h1 id = "泛型-generics" > < a class = "header" href = "#泛型-generics" > 泛型 Generics< / a > < / h1 >
< p > Go 语言在 2022 年，就要正式引入泛型，被视为在 1.0 版本后， ， ？ ？
本章将一一为你讲解。< / p >
< p > 我们在编程中，经常有这样的需求：用同一功能的函数处理不同类型的数据，例如两个数的加法，无论是整数还是浮点数，甚至是自定义类型，都能进行支持。在不支持泛型的编程语言中，通常需要为每一种类型编写一个函数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn add_i8(a:i8, b:i8) -> i8 {
a + b
}
fn add_i32(a:i32, b:i32) -> i32 {
a + b
}
fn add_f64(a:f64, b:f64) -> f64 {
a + b
}
fn main() {
println!("add i8: {}", add_i8(2i8, 3i8));
println!("add i32: {}", add_i32(20, 30));
println!("add f64: {}", add_f64(1.23, 1.23));
}< / code > < / pre >
< p > 上述代码可以正常运行，但是很啰嗦，如果你要支持更多的类型，那么会更繁琐。程序员或多或少都有强迫症，一个好程序员的公认特征就是 —— 懒，这么勤快的写一大堆代码，显然不是咱们的优良传统，是不？< / p >
< p > 在开始讲解 Rust 的泛型之前，先来看看什么是多态。< / p >
< p > 在编程的时候，我们经常利用多态。通俗的讲，多态就是好比坦克的炮管，既可以发射普通弹药，也可以发射制导炮弹（导弹），也可以发射贫铀穿甲弹，甚至发射子母弹，没有必要为每一种炮弹都在坦克上分别安装一个专用炮管，即使生产商愿意，炮手也不愿意，累死人啊。所以在编程开发中，我们也需要这样“通用的炮管”，这个“通用的炮管”就是多态。< / p >
< p > 实际上，泛型就是一种多态。泛型主要目的是为程序员提供编程的便利，减少代码的臃肿，同时可以极大地丰富语言本身的表达能力，为程序员提供了一个合适的炮管。想想，一个函数，可以代替几十个，甚至数百个函数，是一件多么让人兴奋的事情：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn add< T> (a:T, b:T) -> T {
a + b
}
fn main() {
println!("add i8: {}", add(2i8, 3i8));
println!("add i32: {}", add(20, 30));
println!("add f64: {}", add(1.23, 1.23));
}< / code > < / pre >
< p > 将之前的代码改成上面这样，就是 Rust 泛型的初印象，这段代码虽然很简洁，但是并不能编译通过，我们会在后面进行详细讲解，现在只要对泛型有个大概的印象即可。< / p >
< h2 id = "泛型详解" > < a class = "header" href = "#泛型详解" > 泛型详解< / a > < / h2 >
< p > 上面代码的 < code > T< / code > 就是< strong > 泛型参数< / strong > ，实际上在 Rust 中，泛型参数的名称你可以任意起，但是出于惯例，我们都用 < code > T< / code > （ < code > T< / code > 是 < code > type< / code > 的首字母）来作为首选，这个名称越短越好，除非需要表达含义，否则一个字母是最完美的。< / p >
< p > 使用泛型参数，有一个先决条件，必需在使用前对其进行声明：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn largest< T> (list: & [T]) -> T {
< span class = "boring" > }< / span > < / code > < / pre >
< p > 该泛型函数的作用是从列表中找出最大的值，其中列表中的元素类型为 T。首先 < code > largest< T> < / code > 对泛型参数 < code > T< / code > 进行了声明，然后才在函数参数中进行使用该泛型参数 < code > list: & [T]< / code > （还记得 < code > & [T]< / code > 类型吧？这是< a href = "https://course.rs/basic/compound-type/array.html#数组切片" > 数组切片< / a > ）。< / p >
< p > 总之，我们可以这样理解这个函数定义：函数 < code > largest< / code > 有泛型类型 < code > T< / code > ，它有个参数 < code > list< / code > ，其元素是类型为 < code > T< / code > 的数组切片，最后，该函数返回值的类型也是 < code > T< / code > 。< / p >
< p > 下面是一个错误的泛型函数的实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn largest< T> (list: & [T]) -> T {
let mut largest = list[0];
for & item in list.iter() {
if item > largest {
largest = item;
}
}
largest
}
fn main() {
let number_list = vec![34, 50, 25, 100, 65];
let result = largest(& number_list);
println!("The largest number is {}", result);
let char_list = vec!['y', 'm', 'a', 'q'];
let result = largest(& char_list);
println!("The largest char is {}", result);
}< / code > < / pre >
< p > 运行后报错：< / p >
< pre > < code class = "language-console" > error[E0369]: binary operation `> ` cannot be applied to type `T` // `> `操作符不能用于类型`T`
--> src/main.rs:5:17
|
5 | if item > largest {
| ---- ^ ------- T
| |
| T
|
help: consider restricting type parameter `T` // 考虑对T进行类型上的限制 :
|
1 | fn largest< T: std::cmp::PartialOrd> (list: & [T]) -> T {
| ++++++++++++++++++++++
< / code > < / pre >
< p > 因为 < code > T< / code > 可以是任何类型，但不是所有的类型都能进行比较，因此上面的错误中，编译器建议我们给 < code > T< / code > 添加一个类型限制：使用 < code > std::cmp::PartialOrd< / code > 特征（ ） < code > T< / code > 进行限制，特征在下一节会详细介绍，现在你只要理解，该特征的目的就是让< strong > 类型实现可比较的功能< / strong > 。< / p >
< p > 还记得我们一开始的 < code > add< / code > 泛型函数吗？如果你运行它，会得到以下的报错：< / p >
< pre > < code class = "language-console" > error[E0369]: cannot add `T` to `T` // 无法将 `T` 类型跟 `T` 类型进行相加
--> src/main.rs:2:7
|
2 | a + b
| - ^ - T
| |
| T
|
help: consider restricting type parameter `T`
|
1 | fn add< T: std::ops::Add< Output = T> > (a:T, b:T) -> T {
| +++++++++++++++++++++++++++
< / code > < / pre >
< p > 同样的，不是所有 < code > T< / code > 类型都能进行相加操作，因此我们需要用 < code > std::ops::Add< Output = T> < / code > 对 < code > T< / code > 进行限制：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn add< T: std::ops::Add< Output = T> > (a:T, b:T) -> T {
a + b
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 进行如上修改后，就可以正常运行。< / p >
< h3 id = "显式地指定泛型的类型参数" > < a class = "header" href = "#显式地指定泛型的类型参数" > 显式地指定泛型的类型参数< / a > < / h3 >
< p > 有时候，编译器无法推断你想要的泛型参数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::fmt::Display;
fn create_and_print< T> () where T: From< i32> + Display {
let a: T = 100.into(); // 创建了类型为 T 的变量 a，
println!("a is: {}", a);
}
fn main() {
create_and_print();
}< / code > < / pre >
< p > 如果运行以上代码，会得到报错：< / p >
< pre > < code class = "language-console" > error[E0283]: type annotations needed // 需要标明类型
--> src/main.rs:9:5
|
9 | create_and_print();
| ^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `create_and_print` // 无法推断函数 `create_and_print` 的类型参数 `T` 的类型
|
= note: multiple `impl`s satisfying `_: From< i32> ` found in the `core` crate:
- impl From< i32> for AtomicI32;
- impl From< i32> for f64;
- impl From< i32> for i128;
- impl From< i32> for i64;
note: required by a bound in `create_and_print`
--> src/main.rs:3:35
|
3 | fn create_and_print< T> () where T: From< i32> + Display {
| ^^^^^^^^^ required by this bound in `create_and_print`
help: consider specifying the generic argument // 尝试指定泛型参数
|
9 | create_and_print::< T> ();
| +++++
< / code > < / pre >
< p > 报错里说得很清楚，编译器不知道 < code > T< / code > 到底应该是什么类型。不过好心的编译器已经帮我们列出了满足条件的类型，然后告诉我们解决方法：显式指定类型：< code > create_and_print::< T> ()< / code > 。< / p >
< p > 于是，我们修改代码：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::fmt::Display;
fn create_and_print< T> () where T: From< i32> + Display {
let a: T = 100.into(); // 创建了类型为 T 的变量 a，
println!("a is: {}", a);
}
fn main() {
create_and_print::< i64> ();
}< / code > < / pre >
< p > 即可成功运行。< / p >
< h2 id = "结构体中使用泛型" > < a class = "header" href = "#结构体中使用泛型" > 结构体中使用泛型< / a > < / h2 >
< p > 结构体中的字段类型也可以用泛型来定义，下面代码定义了一个坐标点 < code > Point< / code > ，它可以存放任何类型的坐标值：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > struct Point< T> {
x: T,
y: T,
}
fn main() {
let integer = Point { x: 5, y: 10 };
let float = Point { x: 1.0, y: 4.0 };
}< / code > < / pre >
< p > 这里有两点需要特别的注意：< / p >
< ul >
< li > < strong > 提前声明< / strong > ，跟泛型函数定义类似，首先我们在使用泛型参数之前必需要进行声明 < code > Point< T> < / code > ，接着就可以在结构体的字段类型中使用 < code > T< / code > 来替代具体的类型< / li >
< li > < strong > x 和 y 是相同的类型< / strong > < / li >
< / ul >
< p > 第二点非常重要，如果使用不同的类型，那么它会导致下面代码的报错：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > struct Point< T> {
x: T,
y: T,
}
fn main() {
let p = Point{x: 1, y :1.1};
}< / code > < / pre >
< p > 错误如下：< / p >
< pre > < code class = "language-console" > error[E0308]: mismatched types //类型不匹配
--> src/main.rs:7:28
|
7 | let p = Point{x: 1, y :1.1};
| ^^^ expected integer, found floating-point number //期望y是整数，
< / code > < / pre >
< p > 当把 < code > 1< / code > 赋值给 < code > x< / code > 时，变量 < code > p< / code > 的 < code > T< / code > 类型就被确定为整数类型，因此 < code > y< / code > 也必须是整数类型，但是我们却给它赋予了浮点数，因此导致报错。< / p >
< p > 如果想让 < code > x< / code > 和 < code > y< / code > 既能类型相同，又能类型不同，就需要使用不同的泛型参数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > struct Point< T,U> {
x: T,
y: U,
}
fn main() {
let p = Point{x: 1, y :1.1};
}< / code > < / pre >
< p > 切记，所有的泛型参数都要提前声明：< code > Point< T,U> < / code > ! 但是如果你的结构体变成这鬼样：< code > struct Woo< T,U,V,W,X> < / code > ，那么你需要考虑拆分这个结构体，减少泛型参数的个数和代码复杂度。< / p >
< h2 id = "枚举中使用泛型" > < a class = "header" href = "#枚举中使用泛型" > 枚举中使用泛型< / a > < / h2 >
< p > 提到枚举类型，< code > Option< / code > 永远是第一个应该被想起来的，在之前的章节中，它也多次出现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > enum Option< T> {
Some(T),
None,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > Option< T> < / code > 是一个拥有泛型 < code > T< / code > 的枚举类型，它第一个成员是 < code > Some(T)< / code > ，存放了一个类型为 < code > T< / code > 的值。得益于泛型的引入，我们可以在任何一个需要返回值的函数中，去使用 < code > Option< T> < / code > 枚举类型来做为返回值，用于返回一个任意类型的值 < code > Some(T)< / code > ，或者没有值 < code > None< / code > 。< / p >
< p > 对于枚举而言，卧龙凤雏永远是绕不过去的存在：如果是 < code > Option< / code > 是卧龙，那么 < code > Result< / code > 就一定是凤雏，得两者可得天下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > enum Result< T, E> {
Ok(T),
Err(E),
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这个枚举和 < code > Option< / code > 一样，主要用于函数返回值，与 < code > Option< / code > 用于值的存在与否不同，< code > Result < / code > 关注的主要是值的正确性。< / p >
< p > 如果函数正常运行，则最后返回一个 < code > Ok(T)< / code > ， < code > T< / code > 是函数具体的返回值类型，如果函数异常运行，则返回一个 < code > Err(E)< / code > ， < code > E< / code > 是错误类型。例如打开一个文件：如果成功打开文件，则返回 < code > Ok(std::fs::File)< / code > ，因此 < code > T< / code > 对应的是 < code > std::fs::File< / code > 类型；而当打开文件时出现问题时，返回 < code > Err(std::io::Error)< / code > ， < code > E< / code > 对应的就是 < code > std::io::Error< / code > 类型。< / p >
< h2 id = "方法中使用泛型" > < a class = "header" href = "#方法中使用泛型" > 方法中使用泛型< / a > < / h2 >
< p > 上一章中，我们讲到什么是方法以及如何在结构体和枚举上定义方法。方法上也可以使用泛型：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > struct Point< T> {
x: T,
y: T,
}
impl< T> Point< T> {
fn x(& self) -> & T {
& self.x
}
}
fn main() {
let p = Point { x: 5, y: 10 };
println!("p.x = {}", p.x());
}< / code > < / pre >
< p > 使用泛型参数前，依然需要提前声明：< code > impl< T> < / code > ，只有提前声明了，我们才能在< code > Point< T> < / code > 中使用它，这样 Rust 就知道 < code > Point< / code > 的尖括号中的类型是泛型而不是具体类型。需要注意的是，这里的 < code > Point< T> < / code > 不再是泛型声明，而是一个完整的结构体类型，因为我们定义的结构体就是 < code > Point< T> < / code > 而不再是 < code > Point< / code > 。< / p >
< p > 除了结构体中的泛型参数，我们还能在该结构体的方法中定义额外的泛型参数，就跟泛型函数一样：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > struct Point< T, U> {
x: T,
y: U,
}
impl< T, U> Point< T, U> {
fn mixup< V, W> (self, other: Point< V, W> ) -> Point< T, W> {
Point {
x: self.x,
y: other.y,
}
}
}
fn main() {
let p1 = Point { x: 5, y: 10.4 };
let p2 = Point { x: "Hello", y: 'c'};
let p3 = p1.mixup(p2);
println!("p3.x = {}, p3.y = {}", p3.x, p3.y);
}< / code > < / pre >
< p > 这个例子中，< code > T,U< / code > 是定义在结构体 < code > Point< / code > 上的泛型参数，< code > V,W< / code > 是单独定义在方法 < code > mixup< / code > 上的泛型参数，它们并不冲突，说白了，你可以理解为，一个是结构体泛型，一个是函数泛型。< / p >
< h4 id = "为具体的泛型类型实现方法" > < a class = "header" href = "#为具体的泛型类型实现方法" > 为具体的泛型类型实现方法< / a > < / h4 >
< p > 对于 < code > Point< T> < / code > 类型，你不仅能定义基于 < code > T< / code > 的方法，还能针对特定的具体类型，进行方法定义：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl Point< f32> {
fn distance_from_origin(& self) -> f32 {
(self.x.powi(2) + self.y.powi(2)).sqrt()
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码意味着 < code > Point< f32> < / code > 类型会有一个方法 < code > distance_from_origin< / code > ，而其他 < code > T< / code > 不是 < code > f32< / code > 类型的 < code > Point< T> < / code > 实例则没有定义此方法。这个方法计算点实例与坐标< code > (0.0, 0.0)< / code > 之间的距离，并使用了只能用于浮点型的数学运算符。< / p >
< p > 这样我们就能针对特定的泛型类型实现某个特定的方法，对于其它泛型类型则没有定义该方法。< / p >
< h2 id = "const-泛型rust-151-版本引入的重要特性" > < a class = "header" href = "#const-泛型rust-151-版本引入的重要特性" > const 泛型（ < / a > < / h2 >
< p > 在之前的泛型中，可以抽象为一句话：针对类型实现的泛型，所有的泛型都是为了抽象不同的类型，那有没有针对值的泛型？可能很多同学感觉很难理解，值怎么使用泛型？不急，我们先从数组讲起。< / p >
< p > 在< a href = "https://course.rs/basic/compound-type/array.html" > 数组< / a > 那节，有提到过很重要的一点：< code > [i32; 2]< / code > 和 < code > [i32; 3]< / code > 是不同的数组类型，比如下面的代码：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn display_array(arr: [i32; 3]) {
println!("{:?}", arr);
}
fn main() {
let arr: [i32; 3] = [1, 2, 3];
display_array(arr);
let arr: [i32; 2] = [1, 2];
display_array(arr);
}< / code > < / pre >
< p > 运行后报错：< / p >
< pre > < code class = "language-console" > error[E0308]: mismatched types // 类型不匹配
--> src/main.rs:10:19
|
10 | display_array(arr);
| ^^^ expected an array with a fixed size of 3 elements, found one with 2 elements
// 期望一个长度为3的数组，
< / code > < / pre >
< p > 结合代码和报错，可以很清楚的看出，< code > [i32; 3]< / code > 和 < code > [i32; 2]< / code > 确实是两个完全不同的类型，因此无法用同一个函数调用。< / p >
< p > 首先，让我们修改代码，让 < code > display_array< / code > 能打印任意长度的 < code > i32< / code > 数组：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn display_array(arr: & [i32]) {
println!("{:?}", arr);
}
fn main() {
let arr: [i32; 3] = [1, 2, 3];
display_array(& arr);
let arr: [i32; 2] = [1, 2];
display_array(& arr);
}< / code > < / pre >
< p > 很简单，只要使用数组切片，然后传入 < code > arr< / code > 的不可变引用即可。< / p >
< p > 接着，将 < code > i32< / code > 改成所有类型的数组：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn display_array< T: std::fmt::Debug> (arr: & [T]) {
println!("{:?}", arr);
}
fn main() {
let arr: [i32; 3] = [1, 2, 3];
display_array(& arr);
let arr: [i32; 2] = [1, 2];
display_array(& arr);
}< / code > < / pre >
< p > 也不难，唯一要注意的是需要对 < code > T< / code > 加一个限制 < code > std::fmt::Debug< / code > ，该限制表明 < code > T< / code > 可以用在 < code > println!("{:?}", arr)< / code > 中，因为 < code > {:?}< / code > 形式的格式化输出需要 < code > arr< / code > 实现该特征。< / p >
< p > 通过引用，我们可以很轻松的解决处理任何类型数组的问题，但是如果在某些场景下引用不适宜用或者干脆不能用呢？你们知道为什么以前 Rust 的一些数组库，在使用的时候都限定长度不超过 32 吗？因为它们会为每个长度都单独实现一个函数，简直。。。毫无人性。难道没有什么办法可以解决这个问题吗？< / p >
< p > 好在，现在咱们有了 const 泛型，也就是针对值的泛型，正好可以用于处理数组长度的问题：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn display_array< T: std::fmt::Debug, const N: usize> (arr: [T; N]) {
println!("{:?}", arr);
}
fn main() {
let arr: [i32; 3] = [1, 2, 3];
display_array(arr);
let arr: [i32; 2] = [1, 2];
display_array(arr);
}< / code > < / pre >
< p > 如上所示，我们定义了一个类型为 < code > [T; N]< / code > 的数组，其中 < code > T< / code > 是一个基于类型的泛型参数，这个和之前讲的泛型没有区别，而重点在于 < code > N< / code > 这个泛型参数，它是一个基于值的泛型参数！因为它用来替代的是数组的长度。< / p >
< p > < code > N< / code > 就是 const 泛型，定义的语法是 < code > const N: usize< / code > ，表示 const 泛型 < code > N< / code > ，它基于的值类型是 < code > usize< / code > 。< / p >
< p > 在泛型参数之前， < / p >
< h4 id = "const-泛型表达式" > < a class = "header" href = "#const-泛型表达式" > const 泛型表达式< / a > < / h4 >
< p > 假设我们某段代码需要在内存很小的平台上工作，因此需要限制函数参数占用的内存大小，此时就可以使用 const 泛型表达式来实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > // 目前只能在nightly版本下使用
#![allow(incomplete_features)]
#![feature(generic_const_exprs)]
fn something< T> (val: T)
where
Assert< { core::mem::size_of::< T> () < 768 }> : IsTrue,
// ^-----------------------------^ 这里是一个 const 表达式，换成其它的 const 表达式也可以
{
//
}
fn main() {
something([0u8; 0]); // ok
something([0u8; 512]); // ok
something([0u8; 1024]); // 编译错误， ，
}
// ---
pub enum Assert< const CHECK: bool> {
//
}
pub trait IsTrue {
//
}
impl IsTrue for Assert< true> {
//
}< / code > < / pre >
< h4 id = "const-fn" > < a class = "header" href = "#const-fn" > const fn< / a > < / h4 >
< p > 在讨论完 < code > const< / code > 泛型后，不得不提及另一个与之密切相关且强大的特性：< code > const fn< / code > ，即常量函数。< code > const fn< / code > 允许我们在编译期对函数进行求值，从而实现更高效、更灵活的代码设计。< / p >
< h5 id = "为什么需要-const-fn" > < a class = "header" href = "#为什么需要-const-fn" > 为什么需要 const fn< / a > < / h5 >
< p > 通常情况下，函数是在运行时被调用和执行的。然而，在某些场景下，我们希望在编译期就计算出一些值，以提高运行时的性能或满足某些编译期的约束条件。例如，定义数组的长度、计算常量值等。< / p >
< p > 有了 < code > const fn< / code > ，我们可以在编译期执行这些函数，从而将计算结果直接嵌入到生成的代码中。这不仅提高了运行时的性能，还使代码更加简洁和安全。< / p >
< h5 id = "const-fn-的基本用法" > < a class = "header" href = "#const-fn-的基本用法" > const fn 的基本用法< / a > < / h5 >
< p > 要定义一个常量函数，只需要在函数声明前加上 < code > const< / code > 关键字。例如：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > const fn add(a: usize, b: usize) -> usize {
a + b
}
const RESULT: usize = add(5, 10);
fn main() {
println!("The result is: {}", RESULT);
}< / code > < / pre >
< h5 id = "const-fn-的限制" > < a class = "header" href = "#const-fn-的限制" > const fn 的限制< / a > < / h5 >
< p > 虽然 < code > const fn< / code > 提供了很多便利，但是由于其在编译期执行，以确保函数能在编译期被安全地求值，因此有一些限制，例如，不可将随机数生成器写成 < code > const fn< / code > 。< / p >
< p > 无论在编译时还是运行时调用 < code > const fn< / code > ，它们的结果总是相同，即使多次调用也是如此。唯一的例外是，如果你在极端情况下进行复杂的浮点操作，你可能会得到（非常轻微的）不同结果。因此，不建议使 < code > 数组长度 (arr.len())< / code > 和 < code > Enum判别式< / code > 依赖于浮点计算。< / p >
< h5 id = "结合-const-fn-与-const-泛型" > < a class = "header" href = "#结合-const-fn-与-const-泛型" > 结合 const fn 与 const 泛型< / a > < / h5 >
< p > 将 < code > const fn< / code > 与 < code > const 泛型< / code > 结合，可以实现更加灵活和高效的代码设计。例如，创建一个固定大小的缓冲区结构，其中缓冲区大小由编译期计算确定：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > struct Buffer< const N: usize> {
data: [u8; N],
}
const fn compute_buffer_size(factor: usize) -> usize {
factor * 1024
}
fn main() {
const SIZE: usize = compute_buffer_size(4);
let buffer = Buffer::< SIZE> {
data: [0; SIZE],
};
println!("Buffer size: {} bytes", buffer.data.len());
}< / code > < / pre >
< p > 在这个例子中，< code > compute_buffer_size< / code > 是一个常量函数，它根据传入的 < code > factor< / code > 计算缓冲区的大小。在 < code > main< / code > 函数中，我们使用 < code > compute_buffer_size(4)< / code > 来计算缓冲区大小为 4096 字节，并将其作为泛型参数传递给 < code > Buffer< / code > 结构体。这样，缓冲区的大小在编译期就被确定下来，避免了运行时的计算开销。< / p >
< h2 id = "泛型的性能" > < a class = "header" href = "#泛型的性能" > 泛型的性能< / a > < / h2 >
< p > 在 Rust 中泛型是零成本的抽象，意味着你在使用泛型时，完全不用担心性能上的问题。< / p >
< p > 但是任何选择都是权衡得失的， ， ？ < / p >
< p > 具体来说：< / p >
< p > Rust 通过在编译时进行泛型代码的 < strong > 单态化< / strong > (< em > monomorphization< / em > )来保证效率。单态化是一个通过填充编译时使用的具体类型，将通用代码转换为特定代码的过程。< / p >
< p > 编译器所做的工作正好与我们创建泛型函数的步骤相反，编译器寻找所有泛型代码被调用的位置并针对具体类型生成代码。< / p >
< p > 让我们看看一个使用标准库中 < code > Option< / code > 枚举的例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let integer = Some(5);
let float = Some(5.0);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 当 Rust 编译这些代码的时候，它会进行单态化。编译器会读取传递给 < code > Option< T> < / code > 的值并发现有两种 < code > Option< T> < / code > ：一种对应 < code > i32< / code > 另一种对应 < code > f64< / code > 。为此，它会将泛型定义 < code > Option< T> < / code > 展开为 < code > Option_i32< / code > 和 < code > Option_f64< / code > ，接着将泛型定义替换为这两个具体的定义。< / p >
< p > 编译器生成的单态化版本的代码看起来像这样：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > enum Option_i32 {
Some(i32),
None,
}
enum Option_f64 {
Some(f64),
None,
}
fn main() {
let integer = Option_i32::Some(5);
let float = Option_f64::Some(5.0);
}< / code > < / pre >
< p > 我们可以使用泛型来编写不重复的代码，而 Rust 将会为每一个实例编译其特定类型的代码。这意味着在使用泛型时没有运行时开销；当代码运行，它的执行效率就跟好像手写每个具体定义的重复代码一样。这个单态化过程正是 Rust 泛型在运行时极其高效的原因。< / p >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > Rust By Practice， ， < / p >
< ul >
< li > < a href = "https://practice-zh.course.rs/generics-traits/generics.html" > 泛型< / a >
< ul >
< li > < a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/generics-traits/generics.md" > 习题解答< / a > < / li >
< / ul >
< / li >
< li > < a href = "https://practice-zh.course.rs/generics-traits/const-generics.html" > const 泛型< / a >
< ul >
< li > < a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/generics-traits/const-generics.md" > 习题解答< / a > < / li >
< / ul >
< / li >
< / ul >
< / blockquote >
< / main >
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