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<main>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<pre><code class="language-C">#include <stdio.h>
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enum atomic_number {
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HYDROGEN = 1,
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HELIUM = 2,
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// ...
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IRON = 26,
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};
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int main(void)
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{
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enum atomic_number element = 26;
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if (element == IRON) {
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printf("Beware of Rust!\n");
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}
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return 0;
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}
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</code></pre>
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<p>但是在 Rust 中，以下代码：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">enum MyEnum {
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A = 1,
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B,
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C,
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}
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fn main() {
|
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// 将枚举转换成整数，顺利通过
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let x = MyEnum::C as i32;
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// 将整数转换为枚举，失败
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match x {
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MyEnum::A => {}
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MyEnum::B => {}
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MyEnum::C => {}
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_ => {}
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}
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}</code></pre></pre>
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<p>就会报错: <code>MyEnum::A => {} mismatched types, expected i32, found enum MyEnum</code>。</p>
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<h2 id="使用三方库"><a class="header" href="#使用三方库">使用三方库</a></h2>
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<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>
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<p>在<code>Cargo.toml</code>中引入：</p>
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<pre><code class="language-toml">[dependencies]
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num-traits = "0.2.14"
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num-derive = "0.3.3"
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</code></pre>
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<p>代码如下:</p>
|
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|
<pre><pre class="playground"><code class="language-rust edition2021">use num_derive::FromPrimitive;
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use num_traits::FromPrimitive;
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#[derive(FromPrimitive)]
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enum MyEnum {
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A = 1,
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B,
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C,
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}
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fn main() {
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let x = 2;
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match FromPrimitive::from_i32(x) {
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Some(MyEnum::A) => println!("Got A"),
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Some(MyEnum::B) => println!("Got B"),
|
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Some(MyEnum::C) => println!("Got C"),
|
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None => println!("Couldn't convert {}", x),
|
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}
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|
}</code></pre></pre>
|
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|
<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>
|
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|
<h2 id="tryfrom--宏"><a class="header" href="#tryfrom--宏">TryFrom + 宏</a></h2>
|
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|
<p>在 Rust 1.34 后，可以实现<code>TryFrom</code>特征来做转换:</p>
|
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|
|
|
<pre><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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|
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|
</span>use std::convert::TryFrom;
|
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|
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|
|
impl TryFrom<i32> for MyEnum {
|
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|
type Error = ();
|
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|
|
|
|
|
|
|
|
|
fn try_from(v: i32) -> Result<Self, Self::Error> {
|
|
|
|
|
|
match v {
|
|
|
|
|
|
x if x == MyEnum::A as i32 => Ok(MyEnum::A),
|
|
|
|
|
|
x if x == MyEnum::B as i32 => Ok(MyEnum::B),
|
|
|
|
|
|
x if x == MyEnum::C as i32 => Ok(MyEnum::C),
|
|
|
|
|
|
_ => 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) => println!("a"),
|
|
|
|
|
|
Ok(MyEnum::B) => println!("b"),
|
|
|
|
|
|
Ok(MyEnum::C) => println!("c"),
|
|
|
|
|
|
Err(_) => 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)?,)*
|
|
|
|
|
|
}) => {
|
|
|
|
|
|
$(#[$meta])*
|
|
|
|
|
|
$vis enum $name {
|
|
|
|
|
|
$($(#[$vmeta])* $vname $(= $val)?,)*
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl std::convert::TryFrom<i32> for $name {
|
|
|
|
|
|
type Error = ();
|
|
|
|
|
|
|
|
|
|
|
|
fn try_from(v: i32) -> Result<Self, Self::Error> {
|
|
|
|
|
|
match v {
|
|
|
|
|
|
$(x if x == $name::$vname as i32 => Ok($name::$vname),)*
|
|
|
|
|
|
_ => 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 => { println!("Found A"); }
|
|
|
|
|
|
MyEnum::B => { println!("Found B"); }
|
|
|
|
|
|
MyEnum::C => { 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>
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