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<main>
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<h1 id="当闭包碰到特征对象-1"><a class="header" href="#当闭包碰到特征对象-1">当闭包碰到特征对象 1</a></h1>
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<p>特征对象是一个好东西，闭包也是一个好东西，但是如果两者你都想要时，可能就会火星撞地球，boom! 至于这两者为何会勾搭到一起？考虑一个常用场景：使用闭包作为回调函数.</p>
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<h2 id="学习目标"><a class="header" href="#学习目标">学习目标</a></h2>
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<p>如何使用闭包作为特征对象，并解决以下错误：<code>the parameter type </code> `impl Fn(&str) -> Res` <code> may not live long enough</code></p>
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<h2 id="报错的代码"><a class="header" href="#报错的代码">报错的代码</a></h2>
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<p>在下面代码中，我们通过闭包实现了一个简单的回调函数(错误代码已经标注)：</p>
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<pre class="playground"><code class="language-rust edition2021">pub struct Res<'a> {
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value: &'a str,
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}
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impl<'a> Res<'a> {
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pub fn new(value: &str) -> Res {
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Res { value }
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}
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}
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pub struct Container<'a> {
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name: &'a str,
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callback: Option<Box<dyn Fn(&str) -> Res>>,
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}
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impl<'a> Container<'a> {
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pub fn new(name: &str) -> Container {
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Container {
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name,
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callback: None,
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}
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}
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pub fn set(&mut self, cb: impl Fn(&str) -> Res) {
|
|
|
self.callback = Some(Box::new(cb));
|
|
|
}
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let mut inl = Container::new("Inline");
|
|
|
|
|
|
inl.set(|val| {
|
|
|
println!("Inline: {}", val);
|
|
|
Res::new("inline")
|
|
|
});
|
|
|
|
|
|
if let Some(cb) = inl.callback {
|
|
|
cb("hello, world");
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<pre><code>error[E0310]: the parameter type `impl Fn(&str) -> Res` may not live long enough
|
|
|
--> src/main.rs:25:30
|
|
|
|
|
|
|
24 | pub fn set(&mut self, cb: impl Fn(&str) -> Res) {
|
|
|
| -------------------- help: consider adding an explicit lifetime bound...: `impl Fn(&str) -> Res + 'static`
|
|
|
25 | self.callback = Some(Box::new(cb));
|
|
|
| ^^^^^^^^^^^^ ...so that the type `impl Fn(&str) -> Res` will meet its required lifetime bounds
|
|
|
</code></pre>
|
|
|
<p>从第一感觉来说，报错属实不应该，因为我们连引用都没有用，生命周期都不涉及，怎么就报错了？在继续深入之前，先来观察下该闭包是如何被使用的：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>callback: Option<Box<dyn Fn(&str) -> Res>>,
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>众所周知，闭包跟哈姆雷特一样，每一个都有<a href="https://course.rs/advance/functional-programing/closure.html#闭包作为函数返回值">自己的类型</a>，因此我们无法通过类型标注的方式来声明一个闭包，那么只有一个办法，就是使用特征对象，因此上面代码中，通过<code>Box<dyn Trait></code>的方式把闭包特征封装成一个特征对象。</p>
|
|
|
<h2 id="深入挖掘报错原因"><a class="header" href="#深入挖掘报错原因">深入挖掘报错原因</a></h2>
|
|
|
<p>事出诡异必有妖，那接下来我们一起去会会这只妖。</p>
|
|
|
<h4 id="特征对象的生命周期"><a class="header" href="#特征对象的生命周期">特征对象的生命周期</a></h4>
|
|
|
<p>首先编译器报错提示我们闭包活得不够久，那可以大胆推测，正因为使用了闭包作为特征对象，所以才活得不够久。因此首先需要调查下特征对象的生命周期。</p>
|
|
|
<p>首先给出结论：<strong>特征对象隐式的具有<code>'static</code>生命周期</strong>。</p>
|
|
|
<p>其实在 Rust 中，<code>'static</code>生命周期很常见，例如一个没有引用字段的结构体它其实也是<code>'static</code>。当<code>'static</code>用于一个类型时，该类型不能包含任何非<code>'static</code>引用字段，例如以下结构体：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>struct Foo<'a> {
|
|
|
x : &'a [u8]
|
|
|
};
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>除非<code>x</code>字段借用了<code>'static</code>的引用，否则<code>'a</code>肯定比<code>'static</code>要小，那么该结构体实例的生命周期肯定不是<code>'static</code>: <code>'a: 'static</code>的限制不会被满足(<a href="https://course.rs/advance/lifetime/advance.html#生命周期约束HRTB">HRTB</a>)。</p>
|
|
|
<p>对于特征对象来说，它没有包含非<code>'static</code>的引用，因此它隐式的具有<code>'static</code>生命周期, <code>Box<dyn Trait></code>就跟<code>Box<dyn Trait + 'static></code>是等价的。</p>
|
|
|
<h4 id="static-闭包的限制"><a class="header" href="#static-闭包的限制">’static 闭包的限制</a></h4>
|
|
|
<p>其实以上代码的错误很好解决，甚至编译器也提示了我们：</p>
|
|
|
<pre><code class="language-console">help: consider adding an explicit lifetime bound...: `impl Fn(&str) -> Res + 'static`
|
|
|
</code></pre>
|
|
|
<p>但是解决问题不是本文的目标，我们还是要继续深挖一下，如果闭包使用了<code>'static</code>会造成什么问题。</p>
|
|
|
<h5 id="1-无本地变量被捕获"><a class="header" href="#1-无本地变量被捕获">1. 无本地变量被捕获</a></h5>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>inl.set(|val| {
|
|
|
println!("Inline: {}", val);
|
|
|
Res::new("inline")
|
|
|
});
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>以上代码只使用了闭包中传入的参数，并没有本地变量被捕获，因此<code>'static</code>闭包一切 OK。</p>
|
|
|
<h5 id="2-有本地变量被捕获"><a class="header" href="#2-有本地变量被捕获">2. 有本地变量被捕获</a></h5>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let local = "hello".to_string();
|
|
|
|
|
|
// 编译错误： 闭包不是'static!
|
|
|
inl.set(|val| {
|
|
|
println!("Inline: {}", val);
|
|
|
println!("{}", local);
|
|
|
Res::new("inline")
|
|
|
});
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这里我们在闭包中捕获了本地环境变量<code>local</code>，因为<code>local</code>不是<code>'static</code>，那么闭包也不再是<code>'static</code>。</p>
|
|
|
<h5 id="3-将本地变量-move-进闭包"><a class="header" href="#3-将本地变量-move-进闭包">3. 将本地变量 move 进闭包</a></h5>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let local = "hello".to_string();
|
|
|
|
|
|
inl.set(move |val| {
|
|
|
println!("Inline: {}", val);
|
|
|
println!("{}", local);
|
|
|
Res::new("inline")
|
|
|
});
|
|
|
|
|
|
// 编译错误: local已经被移动到闭包中，这里无法再被借用
|
|
|
// println!("{}", local);
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>如上所示，你也可以选择将本地变量的所有权<code>move</code>进闭包中，此时闭包再次具有<code>'static</code>生命周期</p>
|
|
|
<h5 id="4-非要捕获本地变量的引用"><a class="header" href="#4-非要捕获本地变量的引用">4. 非要捕获本地变量的引用？</a></h5>
|
|
|
<p>对于第 2 种情况，如果非要这么干，那<code>'static</code>肯定是没办法了，我们只能给予闭包一个新的生命周期:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub struct Container<'a, 'b> {
|
|
|
name: &'a str,
|
|
|
callback: Option<Box<dyn Fn(&str) -> Res + 'b>>,
|
|
|
}
|
|
|
|
|
|
impl<'a, 'b> Container<'a, 'b> {
|
|
|
pub fn new(name: &str) -> Container {
|
|
|
Container {
|
|
|
name,
|
|
|
callback: None,
|
|
|
}
|
|
|
}
|
|
|
|
|
|
pub fn set(&mut self, cb: impl Fn(&str) -> Res + 'b) {
|
|
|
self.callback = Some(Box::new(cb));
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>肉眼可见，代码复杂度哐哐哐提升，不得不说<code>'static</code>真香！</p>
|
|
|
<p>友情提示：由此修改引发的一系列错误，需要你自行修复: ) (再次友情小提示，可以考虑把<code>main</code>中的<code>local</code>变量声明位置挪到<code>inl</code>声明位置之前)</p>
|
|
|
<h2 id="姗姗来迟的正确代码"><a class="header" href="#姗姗来迟的正确代码">姗姗来迟的正确代码</a></h2>
|
|
|
<p>其实，大家应该都知道该如何修改了，不过出于严谨，我们还是继续给出完整的正确代码:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub fn set(&mut self, cb: impl Fn(&str) -> Res + 'static) {
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>可能大家觉得我重新定义了<code>完整</code>两个字，其实是我不想水篇幅:)</p>
|
|
|
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
|
|
<p>闭包和特征对象的相爱相杀主要原因就在于特征对象默认具备<code>'static</code>的生命周期，同时我们还对什么样的类型具备<code>'static</code>进行了简单的分析。</p>
|
|
|
<p>同时，如果一个闭包拥有<code>'static</code>生命周期，那闭包无法通过引用的方式来捕获本地环境中的变量。如果你想要非要捕获，只能使用非<code>'static</code>。</p>
|
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