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<main>
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<h1 id="函数"><a class="header" href="#函数">函数</a></h1>
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<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>
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<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>
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>fn add(i: i32, j: i32) -> i32 {
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i + j
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}
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<span class="boring">}</span></code></pre>
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<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>
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<img alt="" src="https://pic2.zhimg.com/80/v2-54b3a6d435d2482243edc4be9ab98153_1440w.png" class="center" />
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<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>
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<h2 id="函数要点"><a class="header" href="#函数要点">函数要点</a></h2>
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<ul>
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<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>
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<li>函数的位置可以随便放，Rust 不关心我们在哪里定义了函数，只要有定义即可</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，请给予它一个类型
|
|
|
|
|
|
|
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，请使用_忽略参数名
|
|
|
|
|
|
|
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>
|
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</blockquote>
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