rust
/
rust-course
mirror of https://github.com/sunface/rust-course
5
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity
You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
Branches Tags
${ item.name }
Create tag ${ searchTerm }
Create branch ${ searchTerm }
from '3ec8064ea8'
${ noResults }
rust-course/advance/into-types/enum-int.html

393 lines
18 KiB
Raw Normal View History Unescape

deploy
2 months ago
<!DOCTYPE HTML>
<html lang="zh-CN" class="light sidebar-visible" dir="ltr">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>枚举和整数 - Rust语言圣经(Rust Course)</title>
<!-- Custom HTML head -->
<meta name="description" content="">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff">
<link rel="icon" href="../../favicon.svg">
<link rel="shortcut icon" href="../../favicon.png">
<link rel="stylesheet" href="../../css/variables.css">
<link rel="stylesheet" href="../../css/general.css">
<link rel="stylesheet" href="../../css/chrome.css">
<link rel="stylesheet" href="../../css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="../../FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="../../fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" id="highlight-css" href="../../highlight.css">
<link rel="stylesheet" id="tomorrow-night-css" href="../../tomorrow-night.css">
<link rel="stylesheet" id="ayu-highlight-css" href="../../ayu-highlight.css">
<!-- Custom theme stylesheets -->
<link rel="stylesheet" href="../../theme/style.css">
<!-- Provide site root and default themes to javascript -->
<script>
const path_to_root = "../../";
const default_light_theme = "light";
const default_dark_theme = "navy";
</script>
<!-- Start loading toc.js asap -->
<script src="../../toc.js"></script>
</head>
<body>
<div id="body-container">
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script>
try {
let theme = localStorage.getItem('mdbook-theme');
let sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script>
const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme;
let theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
const html = document.documentElement;
html.classList.remove('light')
html.classList.add(theme);
html.classList.add("js");
</script>
<input type="checkbox" id="sidebar-toggle-anchor" class="hidden">
<!-- Hide / unhide sidebar before it is displayed -->
<script>
let sidebar = null;
const sidebar_toggle = document.getElementById("sidebar-toggle-anchor");
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
} else {
sidebar = 'hidden';
}
sidebar_toggle.checked = sidebar === 'visible';
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<!-- populated by js -->
<mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox>
<noscript>
<iframe class="sidebar-iframe-outer" src="../../toc.html"></iframe>
</noscript>
<div id="sidebar-resize-handle" class="sidebar-resize-handle">
<div class="sidebar-resize-indicator"></div>
</div>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky">
<div class="left-buttons">
<label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</label>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="default_theme">Auto</button></li>
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">Rust语言圣经(Rust Course)</h1>
<div class="right-buttons">
<a href="../../print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
<a href="https://github.com/sunface/rust-course" title="Git repository" aria-label="Git repository">
<i id="git-repository-button" class="fa fa-github"></i>
</a>
<a href="https://github.com/sunface/rust-course/edit/main/src/advance/into-types/enum-int.md" title="Suggest an edit" aria-label="Suggest an edit">
<i id="git-edit-button" class="fa fa-edit"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script>
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<h1 id="整数转换为枚举"><a class="header" href="#整数转换为枚举">整数转换为枚举</a></h1>
<p>在 Rust 中，从枚举到整数的转换很容易，但是反过来，就没那么容易，甚至部分实现还挺邪恶, 例如使用<code>transmute</code>。</p>
<h2 id="一个真实场景的需求"><a class="header" href="#一个真实场景的需求">一个真实场景的需求</a></h2>
<p>在实际场景中，从整数到枚举的转换有时还是非常需要的，例如你有一个枚举类型，然后需要从外面传入一个整数，用于控制后续的流程走向，此时就需要用整数去匹配相应的枚举(你也可以用整数匹配整数-, -，看看会不会被喷)。</p>
<p>既然有了需求，剩下的就是看看该如何实现，这篇文章的水远比你想象的要深，且看八仙过海各显神通。</p>
<h2 id="c-语言的实现"><a class="header" href="#c-语言的实现">C 语言的实现</a></h2>
<p>对于 C 语言来说，万物皆邪恶，因此我们不讨论安全，只看实现，不得不说很简洁：</p>
<pre><code class="language-C">#include &lt;stdio.h&gt;
enum atomic_number {
HYDROGEN = 1,
HELIUM = 2,
// ...
IRON = 26,
};
int main(void)
{
enum atomic_number element = 26;
if (element == IRON) {
printf("Beware of Rust!\n");
}
return 0;
}
</code></pre>
<p>但是在 Rust 中，以下代码：</p>
<pre><pre class="playground"><code class="language-rust edition2021">enum MyEnum {
A = 1,
B,
C,
}
fn main() {
// 将枚举转换成整数，顺利通过
let x = MyEnum::C as i32;
// 将整数转换为枚举，失败
match x {
MyEnum::A =&gt; {}
MyEnum::B =&gt; {}
MyEnum::C =&gt; {}
_ =&gt; {}
}
}</code></pre></pre>
<p>就会报错: <code>MyEnum::A =&gt; {} mismatched types, expected i32, found enum MyEnum</code>。</p>
<h2 id="使用三方库"><a class="header" href="#使用三方库">使用三方库</a></h2>
<p>首先可以想到的肯定是三方库，毕竟 Rust 的生态目前已经发展的很不错，类似的需求总是有的，这里我们先使用<code>num-traits</code>和<code>num-derive</code>来试试。</p>
<p>在<code>Cargo.toml</code>中引入：</p>
<pre><code class="language-toml">[dependencies]
num-traits = "0.2.14"
num-derive = "0.3.3"
</code></pre>
<p>代码如下:</p>
<pre><pre class="playground"><code class="language-rust edition2021">use num_derive::FromPrimitive;
use num_traits::FromPrimitive;
#[derive(FromPrimitive)]
enum MyEnum {
A = 1,
B,
C,
}
fn main() {
let x = 2;
match FromPrimitive::from_i32(x) {
Some(MyEnum::A) =&gt; println!("Got A"),
Some(MyEnum::B) =&gt; println!("Got B"),
Some(MyEnum::C) =&gt; println!("Got C"),
None =&gt; println!("Couldn't convert {}", x),
}
}</code></pre></pre>
<p>除了上面的库，还可以使用一个较新的库: <a href="https://github.com/illicitonion/num_enum"><code>num_enums</code></a>。</p>
<h2 id="tryfrom--宏"><a class="header" href="#tryfrom--宏">TryFrom + 宏</a></h2>
<p>在 Rust 1.34 后，可以实现<code>TryFrom</code>特征来做转换:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::convert::TryFrom;
impl TryFrom&lt;i32&gt; for MyEnum {
type Error = ();
fn try_from(v: i32) -&gt; Result&lt;Self, Self::Error&gt; {
match v {
x if x == MyEnum::A as i32 =&gt; Ok(MyEnum::A),
x if x == MyEnum::B as i32 =&gt; Ok(MyEnum::B),
x if x == MyEnum::C as i32 =&gt; Ok(MyEnum::C),
_ =&gt; Err(()),
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>以上代码定义了从<code>i32</code>到<code>MyEnum</code>的转换，接着就可以使用<code>TryInto</code>来实现转换：</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::convert::TryInto;
fn main() {
let x = MyEnum::C as i32;
match x.try_into() {
Ok(MyEnum::A) =&gt; println!("a"),
Ok(MyEnum::B) =&gt; println!("b"),
Ok(MyEnum::C) =&gt; println!("c"),
Err(_) =&gt; eprintln!("unknown number"),
}
}</code></pre></pre>
<p>但是上面的代码有个问题，你需要为每个枚举成员都实现一个转换分支，非常麻烦。好在可以使用宏来简化，自动根据枚举的定义来实现<code>TryFrom</code>特征:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[macro_export]
macro_rules! back_to_enum {
($(#[$meta:meta])* $vis:vis enum $name:ident {
$($(#[$vmeta:meta])* $vname:ident $(= $val:expr)?,)*
}) =&gt; {
$(#[$meta])*
$vis enum $name {
$($(#[$vmeta])* $vname $(= $val)?,)*
}
impl std::convert::TryFrom&lt;i32&gt; for $name {
type Error = ();
fn try_from(v: i32) -&gt; Result&lt;Self, Self::Error&gt; {
match v {
$(x if x == $name::$vname as i32 =&gt; Ok($name::$vname),)*
_ =&gt; Err(()),
}
}
}
}
}
back_to_enum! {
enum MyEnum {
A = 1,
B,
C,
}
}
<span class="boring">}</span></code></pre></pre>
<h2 id="邪恶之王-stdmemtransmute"><a class="header" href="#邪恶之王-stdmemtransmute">邪恶之王 std::mem::transmute</a></h2>
<p><strong>这个方法原则上并不推荐，但是有其存在的意义，如果要使用，你需要清晰的知道自己为什么使用</strong>。</p>
<p>在之前的类型转换章节，我们提到过非常邪恶的<a href="https://course.rs/advance/into-types/converse.html#%E5%8F%98%E5%BD%A2%E8%AE%B0transmutes"><code>transmute</code>转换</a>，其实，当你知道数值一定不会超过枚举的范围时(例如枚举成员对应 1，2，3，传入的整数也在这个范围内)，就可以使用这个方法完成变形。</p>
<blockquote>
<p>最好使用#[repr(..)]来控制底层类型的大小，免得本来需要 i32，结果传入 i64，最终内存无法对齐，产生奇怪的结果</p>
</blockquote>
<pre><pre class="playground"><code class="language-rust edition2021">#[repr(i32)]
enum MyEnum {
A = 1, B, C
}
fn main() {
let x = MyEnum::C;
let y = x as i32;
let z: MyEnum = unsafe { std::mem::transmute(y) };
// match the enum that came from an int
match z {
MyEnum::A =&gt; { println!("Found A"); }
MyEnum::B =&gt; { println!("Found B"); }
MyEnum::C =&gt; { println!("Found C"); }
}
}</code></pre></pre>
<p>既然是邪恶之王，当然得有真本事，无需标准库、也无需 unstable 的 Rust 版本，我们就完成了转换！awesome!??</p>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>本文列举了常用(其实差不多也是全部了，还有一个 unstable 特性没提到)的从整数转换为枚举的方式，推荐度按照出现的先后顺序递减。</p>
<p>但是推荐度最低，不代表它就没有出场的机会，只要使用边界清晰，一样可以大放光彩，例如最后的<code>transmute</code>函数.</p>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="../../advance/into-types/sized.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="../../advance/smart-pointer/intro.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="../../advance/into-types/sized.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="../../advance/smart-pointer/intro.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script>
window.playground_copyable = true;
</script>
<script src="../../ace.js"></script>
<script src="../../editor.js"></script>
<script src="../../mode-rust.js"></script>
<script src="../../theme-dawn.js"></script>
<script src="../../theme-tomorrow_night.js"></script>
<script src="../../elasticlunr.min.js"></script>
<script src="../../mark.min.js"></script>
<script src="../../searcher.js"></script>
<script src="../../clipboard.min.js"></script>
<script src="../../highlight.js"></script>
<script src="../../book.js"></script>
<!-- Custom JS scripts -->
<script src="../../assets/custom2.js"></script>
<script src="../../assets/bigPicture.js"></script>
</div>
</body>
</html>