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< h1 id = "函数" > < a class = "header" href = "#函数" > 函数< / a > < / h1 >
< p > Rust 的函数我们在之前已经见过不少，跟其他语言几乎没有什么区别。因此本章的学习之路将轻松和愉快，骚年们，请珍惜这种愉快，下一章你将体验到不一样的 Rust。< / p >
< p > 在函数界，有一个函数只闻其名不闻其声，可以止小孩啼！在程序界只有 < code > hello,world!< / code > 可以与之媲美，它就是 < code > add< / code > 函数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn add(i: i32, j: i32) -> i32 {
i + j
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 该函数如此简单，但是又是如此的五脏俱全，声明函数的关键字 < code > fn< / code > ，函数名 < code > add()< / code > ，参数 < code > i< / code > 和 < code > j< / code > ，参数类型和返回值类型都是 < code > i32< / code > ，总之一切那么的普通，但是又那么的自信，直到你看到了下面这张图：< / p >
< img alt = "" src = "https://pic2.zhimg.com/80/v2-54b3a6d435d2482243edc4be9ab98153_1440w.png" class = "center" / >
< p > 当你看懂了这张图，其实就等于差不多完成了函数章节的学习，但是这么短的章节显然对不起读者老爷们的厚爱，所以我们来展开下。< / p >
< h2 id = "函数要点" > < a class = "header" href = "#函数要点" > 函数要点< / a > < / h2 >
< ul >
< li > 函数名和变量名使用< a href = "https://course.rs/practice/naming.html" > 蛇形命名法(snake case)< / a > ，例如 < code > fn add_two() {}< / code > < / li >
< li > 函数的位置可以随便放， < / li >
< li > 每个函数参数都需要标注类型< / li >
< / ul >
< h2 id = "函数参数" > < a class = "header" href = "#函数参数" > 函数参数< / a > < / h2 >
< p > Rust 是静态类型语言，因此需要你为每一个函数参数都标识出它的具体类型，例如：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
another_function(5, 6.1);
}
fn another_function(x: i32, y: f32) {
println!("The value of x is: {}", x);
println!("The value of y is: {}", y);
}< / code > < / pre >
< p > < code > another_function< / code > 函数有两个参数，其中 < code > x< / code > 是 < code > i32< / code > 类型，< code > y< / code > 是 < code > f32< / code > 类型，然后在该函数内部，打印出这两个值。这里去掉 < code > x< / code > 或者 < code > y< / code > 的任何一个的类型，都会报错：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
another_function(5, 6.1);
}
fn another_function(x: i32, y) {
println!("The value of x is: {}", x);
println!("The value of y is: {}", y);
}< / code > < / pre >
< p > 错误如下:< / p >
< pre > < code class = "language-console" > error: expected one of `:`, `@`, or `|`, found `)`
--> src/main.rs:5:30
|
5 | fn another_function(x: i32, y) {
| ^ expected one of `:`, `@`, or `|` // 期待以下符号之一 `:`, `@`, or `|`
|
= note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
// 匿名参数在 Rust 2018 edition 中就已经移除
help: if this is a parameter name, give it a type // 如果y是一个参数名，
|
5 | fn another_function(x: i32, y: TypeName) {
| ~~~~~~~~~~~
help: if this is a type, explicitly ignore the parameter name // 如果y是一个类型，
|
5 | fn another_function(x: i32, _: y) {
| ~~~~
< / code > < / pre >
< h2 id = "函数返回" > < a class = "header" href = "#函数返回" > 函数返回< / a > < / h2 >
< p > 在上一章节语句和表达式中，我们有提到，在 Rust 中函数就是表达式，因此我们可以把函数的返回值直接赋给调用者。< / p >
< p > 函数的返回值就是函数体最后一条表达式的返回值，当然我们也可以使用 < code > return< / code > 提前返回，下面的函数使用最后一条表达式来返回一个值：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn plus_five(x:i32) -> i32 {
x + 5
}
fn main() {
let x = plus_five(5);
println!("The value of x is: {}", x);
}< / code > < / pre >
< p > < code > x + 5< / code > 是一条表达式，求值后，返回一个值，因为它是函数的最后一行，因此该表达式的值也是函数的返回值。< / p >
< p > 再来看两个重点：< / p >
< ol >
< li > < code > let x = plus_five(5)< / code > ，说明我们用一个函数的返回值来初始化 < code > x< / code > 变量，因此侧面说明了在 Rust 中函数也是表达式，这种写法等同于 < code > let x = 5 + 5;< / code > < / li >
< li > < code > x + 5< / code > 没有分号，因为它是一条表达式，这个在上一节中我们也有详细介绍< / li >
< / ol >
< p > 再来看一段代码，同时使用 < code > return< / code > 和表达式作为返回值：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn plus_or_minus(x:i32) -> i32 {
if x > 5 {
return x - 5
}
x + 5
}
fn main() {
let x = plus_or_minus(5);
println!("The value of x is: {}", x);
}< / code > < / pre >
< p > < code > plus_or_minus< / code > 函数根据传入 < code > x< / code > 的大小来决定是做加法还是减法，若 < code > x > 5< / code > 则通过 < code > return< / code > 提前返回 < code > x - 5< / code > 的值,否则返回 < code > x + 5< / code > 的值。< / p >
< h4 id = "rust-中的特殊返回类型" > < a class = "header" href = "#rust-中的特殊返回类型" > Rust 中的特殊返回类型< / a > < / h4 >
< h5 id = "无返回值" > < a class = "header" href = "#无返回值" > 无返回值< code > ()< / code > < / a > < / h5 >
< p > 对于 Rust 新手来说，有些返回类型很难理解，而且如果你想通过百度或者谷歌去搜索，都不好查询，因为这些符号太常见了，根本难以精确搜索到。< / p >
< p > 例如单元类型 < code > ()< / code > ，是一个零长度的元组。它没啥作用，但是可以用来表达一个函数没有返回值：< / p >
< ul >
< li > 函数没有返回值，那么返回一个 < code > ()< / code > < / li >
< li > 通过 < code > ;< / code > 结尾的语句返回一个 < code > ()< / code > < / li >
< / ul >
< p > 例如下面的 < code > report< / code > 函数会隐式返回一个 < code > ()< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::fmt::Debug;
fn report< T: Debug> (item: T) {
println!("{:?}", item);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 与上面的函数返回值相同，但是下面的函数显式的返回了 < code > ()< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn clear(text: & mut String) -> () {
*text = String::from("");
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 在实际编程中，你会经常在错误提示中看到该 < code > ()< / code > 的身影出没，假如你的函数需要返回一个 < code > u32< / code > 值，但是如果你不幸的以 < code > 表达式;< / code > 的语句形式作为函数的最后一行代码，就会报错：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn add(x:u32,y:u32) -> u32 {
x + y;
}
< span class = "boring" > }< / span > < / 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:6:24
|
6 | fn add(x:u32,y:u32) -> u32 {
| --- ^^^ expected `u32`, found `()` // 期望返回u32,却返回()
| |
| implicitly returns `()` as its body has no tail or `return` expression
7 | x + y;
| - help: consider removing this semicolon
< / code > < / pre >
< p > 还记得我们在< a href = "https://course.rs/basic/base-type/statement-expression.html" > 语句与表达式< / a > 中讲过的吗？只有表达式能返回值，而 < code > ;< / code > 结尾的是语句，在 Rust 中，一定要严格区分< strong > 表达式< / strong > 和< strong > 语句< / strong > 的区别，这个在其它语言中往往是被忽视的点。< / p >
< h5 id = "永不返回的发散函数-" > < a class = "header" href = "#永不返回的发散函数-" > 永不返回的发散函数 < code > !< / code > < / a > < / h5 >
< p > 当用 < code > !< / code > 作函数返回类型的时候，表示该函数永不返回( diverging functions )，特别的，这种语法往往用做会导致程序崩溃的函数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn dead_end() -> ! {
panic!("你已经到了穷途末路，崩溃吧！");
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 下面的函数创建了一个无限循环，该循环永不跳出，因此函数也永不返回：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn forever() -> ! {
loop {
//...
};
}
< span class = "boring" > }< / span > < / code > < / pre >
< 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/basic-types/functions.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/basic-types/functions.md" > 习题解答< / a > 。< / p >
< / blockquote >
< / main >
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