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<main>
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<h2 id="迭代器"><a class="header" href="#迭代器">迭代器</a></h2>
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<p>集合类型可以通过 <code>Iterator</code> 特征进行迭代，该特征看起来比 <code>Drop</code> 要复杂点：</p>
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<pre><pre class="playground"><code class="language-rust edition2021">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>pub trait Iterator {
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type Item;
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fn next(&mut self) -> Option<Self::Item>;
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}
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<span class="boring">}
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</span></code></pre></pre>
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<p>这里的 <code>Item</code> 是<a href="https://course.rs/basic/trait/advance-trait.html#%E5%85%B3%E8%81%94%E7%B1%BB%E5%9E%8B">关联类型</a>，用来指代迭代器中具体的元素类型，<code>next</code> 方法返回的也是该类型。</p>
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<p>其实上面的说法有点不够准确，原因是 <code>next</code> 方法返回的是 <code>Option<Self::Item></code>，使用 <code>Option<T></code> 枚举的原因是为了方便用户，不然用户需要 <code>has_next</code> 和 <code>get_next</code> 才能满足使用需求。有值时返回 <code>Some(T)</code>，无值时返回 <code>None</code>，这种 API 设计工程性更好，也更加安全，完美！</p>
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<p>有点悲剧的是, Rust 截至目前还没有 <code>yield</code> 语句，因此我们需要自己来实现相关的逻辑。还有点需要注意，每个集合类型应该实现 3 种迭代器类型：</p>
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<ul>
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<li><code>IntoIter</code> - <code>T</code></li>
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<li><code>IterMut</code> - <code>&mut T</code></li>
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<li><code>Iter</code> - <code>&T</code></li>
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</ul>
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<p>也许大家不认识它们，但是其实很好理解，<code>IntoIter</code> 类型迭代器的 <code>next</code> 方法会拿走被迭代值的所有权，<code>IterMut</code> 是可变借用， <code>Iter</code> 是不可变借用。事实上，类似的<a href="https://course.rs/practice/naming.html#%E4%B8%80%E4%B8%AA%E9%9B%86%E5%90%88%E4%B8%8A%E7%9A%84%E6%96%B9%E6%B3%95%E5%A6%82%E6%9E%9C%E8%BF%94%E5%9B%9E%E8%BF%AD%E4%BB%A3%E5%99%A8%E9%9C%80%E9%81%B5%E5%BE%AA%E5%91%BD%E5%90%8D%E8%A7%84%E5%88%99iteriter_mutinto_iter-c-iter">命名规则</a>在 Rust 中随处可见，当熟悉后，以后见到类似的命名大家就可以迅速的理解其对值的运用方式。</p>
|
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|
<h2 id="intoiter"><a class="header" href="#intoiter">IntoIter</a></h2>
|
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<p>先来看看 <code>IntoIter</code> 该怎么实现:</p>
|
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<pre><pre class="playground"><code class="language-rust edition2021">
|
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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|
|
</span>pub struct IntoIter<T>(List<T>);
|
|
|
|
|
|
|
|
|
|
|
|
impl<T> List<T> {
|
|
|
|
|
|
pub fn into_iter(self) -> IntoIter<T> {
|
|
|
|
|
|
IntoIter(self)
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<T> Iterator for IntoIter<T> {
|
|
|
|
|
|
type Item = T;
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
|
|
|
|
// access fields of a tuple struct numerically
|
|
|
|
|
|
self.0.pop()
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>这里我们通过<a href="https://course.rs/basic/compound-type/struct.html#%E5%85%83%E7%BB%84%E7%BB%93%E6%9E%84%E4%BD%93tuple-struct">元组结构体</a>的方式定义了 <code>IntoIter</code>，下面来测试下:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>#[test]
|
|
|
|
|
|
fn into_iter() {
|
|
|
|
|
|
let mut list = List::new();
|
|
|
|
|
|
list.push(1); list.push(2); list.push(3);
|
|
|
|
|
|
|
|
|
|
|
|
let mut iter = list.into_iter();
|
|
|
|
|
|
assert_eq!(iter.next(), Some(3));
|
|
|
|
|
|
assert_eq!(iter.next(), Some(2));
|
|
|
|
|
|
assert_eq!(iter.next(), Some(1));
|
|
|
|
|
|
assert_eq!(iter.next(), None);
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
|
|
|
|
|
|
Running target/debug/lists-5c71138492ad4b4a
|
|
|
|
|
|
|
|
|
|
|
|
running 4 tests
|
|
|
|
|
|
test first::test::basics ... ok
|
|
|
|
|
|
test second::test::basics ... ok
|
|
|
|
|
|
test second::test::into_iter ... ok
|
|
|
|
|
|
test second::test::peek ... ok
|
|
|
|
|
|
|
|
|
|
|
|
test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<h2 id="iter"><a class="header" href="#iter">Iter</a></h2>
|
|
|
|
|
|
<p>相对来说，<code>IntoIter</code> 是最好实现的，因为它只是简单的拿走值，不涉及到引用，也不涉及到生命周期，而 <code>Iter</code> 就有所不同了。</p>
|
|
|
|
|
|
<p>这里的基本逻辑是我们持有一个当前节点的指针，当生成一个值后，该指针将指向下一个节点。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub struct Iter<T> {
|
|
|
|
|
|
next: Option<&Node<T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<T> List<T> {
|
|
|
|
|
|
pub fn iter(&self) -> Iter<T> {
|
|
|
|
|
|
Iter { next: self.head.map(|node| &node) }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<T> Iterator for Iter<T> {
|
|
|
|
|
|
type Item = &T;
|
|
|
|
|
|
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
|
|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.next.map(|node| &node);
|
|
|
|
|
|
&node.elem
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
|
|
|
|
|
|
error[E0106]: missing lifetime specifier
|
|
|
|
|
|
--> src/second.rs:72:18
|
|
|
|
|
|
|
|
|
|
|
|
|
72 | next: Option<&Node<T>>,
|
|
|
|
|
|
| ^ expected lifetime parameter
|
|
|
|
|
|
|
|
|
|
|
|
error[E0106]: missing lifetime specifier
|
|
|
|
|
|
--> src/second.rs:82:17
|
|
|
|
|
|
|
|
|
|
|
|
|
82 | type Item = &T;
|
|
|
|
|
|
| ^ expected lifetime parameter
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>许久不见的错误又冒了出来，而且这次直指 Rust 中最难的点之一：生命周期。关于生命周期的讲解，这里就不再展开，如果大家还不熟悉，强烈建议看看<a href="https://course.rs/advance/lifetime/intro.html">此章节</a>，然后再继续。</p>
|
|
|
|
|
|
<p>首先，先加一个生命周期试试：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub struct Iter<'a, T> {
|
|
|
|
|
|
next: Option<&'a Node<T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
|
|
|
|
|
|
error[E0106]: missing lifetime specifier
|
|
|
|
|
|
--> src/second.rs:83:22
|
|
|
|
|
|
|
|
|
|
|
|
|
83 | impl<T> Iterator for Iter<T> {
|
|
|
|
|
|
| ^^^^^^^ expected lifetime parameter
|
|
|
|
|
|
|
|
|
|
|
|
error[E0106]: missing lifetime specifier
|
|
|
|
|
|
--> src/second.rs:84:17
|
|
|
|
|
|
|
|
|
|
|
|
|
84 | type Item = &T;
|
|
|
|
|
|
| ^ expected lifetime parameter
|
|
|
|
|
|
|
|
|
|
|
|
error: aborting due to 2 previous errors
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>好的，现在有了更多的提示，来按照提示修改下代码:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub struct Iter<'a, T> {
|
|
|
|
|
|
next: Option<&'a Node<T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<'a, T> List<T> {
|
|
|
|
|
|
pub fn iter(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: self.head.map(|node| &'a node) }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
|
|
|
|
type Item = &'a T;
|
|
|
|
|
|
fn next(&'a mut self) -> Option<Self::Item> {
|
|
|
|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.next.map(|node| &'a node);
|
|
|
|
|
|
&'a node.elem
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
|
|
|
|
|
|
error: expected `:`, found `node`
|
|
|
|
|
|
--> src/second.rs:77:47
|
|
|
|
|
|
|
|
|
|
|
|
|
77 | Iter { next: self.head.map(|node| &'a node) }
|
|
|
|
|
|
| ---- while parsing this struct ^^^^ expected `:`
|
|
|
|
|
|
|
|
|
|
|
|
error: expected `:`, found `node`
|
|
|
|
|
|
--> src/second.rs:85:50
|
|
|
|
|
|
|
|
|
|
|
|
|
85 | self.next = node.next.map(|node| &'a node);
|
|
|
|
|
|
| ^^^^ expected `:`
|
|
|
|
|
|
|
|
|
|
|
|
error[E0063]: missing field `next` in initializer of `second::Iter<'_, _>`
|
|
|
|
|
|
--> src/second.rs:77:9
|
|
|
|
|
|
|
|
|
|
|
|
|
77 | Iter { next: self.head.map(|node| &'a node) }
|
|
|
|
|
|
| ^^^^ missing `next`
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>怎么回事。。感觉错误犹如雨后春笋般冒了出来，Rust 是不是被我们搞坏了 :(</p>
|
|
|
|
|
|
<p>现在看来，我们的生命周期是用错了，聪明的同学可能已经看出了端倪，那么再修改下试试；</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub struct Iter<'a, T> {
|
|
|
|
|
|
next: Option<&'a Node<T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
// 这里无需生命周期，因为 List 没有使用生命周期的关联项
|
|
|
|
|
|
impl<T> List<T> {
|
|
|
|
|
|
// 这里我们为 `iter` 声明一个生命周期 'a , 此时 `&self` 需要至少和 `Iter` 活得一样久
|
|
|
|
|
|
pub fn iter<'a>(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: self.head.map(|node| &node) }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
// 这里声明生命周期是因为下面的关联类型 Item 需要
|
|
|
|
|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
|
|
|
|
type Item = &'a T;
|
|
|
|
|
|
|
|
|
|
|
|
// 这里无需更改，因为上面已经处理了.
|
|
|
|
|
|
// Self 依然是这么棒
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
|
|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.next.map(|node| &node);
|
|
|
|
|
|
&node.elem
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>现在，我们也许可以自信的编译下试试了：</p>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
|
|
|
|
|
|
error[E0308]: mismatched types
|
|
|
|
|
|
--> src/second.rs:77:22
|
|
|
|
|
|
|
|
|
|
|
|
|
77 | Iter { next: self.head.map(|node| &node) }
|
|
|
|
|
|
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `second::Node`, found struct `std::boxed::Box`
|
|
|
|
|
|
|
|
|
|
|
|
|
= note: expected type `std::option::Option<&second::Node<T>>`
|
|
|
|
|
|
found type `std::option::Option<&std::boxed::Box<second::Node<T>>>`
|
|
|
|
|
|
|
|
|
|
|
|
error[E0308]: mismatched types
|
|
|
|
|
|
--> src/second.rs:85:25
|
|
|
|
|
|
|
|
|
|
|
|
|
85 | self.next = node.next.map(|node| &node);
|
|
|
|
|
|
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `second::Node`, found struct `std::boxed::Box`
|
|
|
|
|
|
|
|
|
|
|
|
|
= note: expected type `std::option::Option<&'a second::Node<T>>`
|
|
|
|
|
|
found type `std::option::Option<&std::boxed::Box<second::Node<T>>>`
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>(╯°□°)╯︵ ┻━┻</p>
|
|
|
|
|
|
<p>这么看，生命周期的问题解决了，但是又引入了新的错误。原因在于，我们希望存储 <code>&Node</code> 但是获取的却是 <code>&Box<Node></code>。嗯，小问题，解引用搞定：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>impl<T> List<T> {
|
|
|
|
|
|
pub fn iter<'a>(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: self.head.map(|node| &*node) }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
|
|
|
|
type Item = &'a T;
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
|
|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.next.map(|node| &*node);
|
|
|
|
|
|
&node.elem
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
|
|
|
|
|
|
Compiling lists v0.1.0 (/Users/ABeingessner/dev/temp/lists)
|
|
|
|
|
|
error[E0515]: cannot return reference to local data `*node`
|
|
|
|
|
|
--> src/second.rs:77:43
|
|
|
|
|
|
|
|
|
|
|
|
|
77 | Iter { next: self.head.map(|node| &*node) }
|
|
|
|
|
|
| ^^^^^^ returns a reference to data owned by the current function
|
|
|
|
|
|
|
|
|
|
|
|
error[E0507]: cannot move out of borrowed content
|
|
|
|
|
|
--> src/second.rs:77:22
|
|
|
|
|
|
|
|
|
|
|
|
|
77 | Iter { next: self.head.map(|node| &*node) }
|
|
|
|
|
|
| ^^^^^^^^^ cannot move out of borrowed content
|
|
|
|
|
|
|
|
|
|
|
|
error[E0515]: cannot return reference to local data `*node`
|
|
|
|
|
|
--> src/second.rs:85:46
|
|
|
|
|
|
|
|
|
|
|
|
|
85 | self.next = node.next.map(|node| &*node);
|
|
|
|
|
|
| ^^^^^^ returns a reference to data owned by the current function
|
|
|
|
|
|
|
|
|
|
|
|
error[E0507]: cannot move out of borrowed content
|
|
|
|
|
|
--> src/second.rs:85:25
|
|
|
|
|
|
|
|
|
|
|
|
|
85 | self.next = node.next.map(|node| &*node);
|
|
|
|
|
|
| ^^^^^^^^^ cannot move out of borrowed content
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>又怎么了! (ﾉಥ益ಥ）ﾉ ┻━┻</p>
|
|
|
|
|
|
<p>大家还记得之前章节的内容吗？原因是这里我们忘记了 <code>as_ref</code> ，然后值的所有权被转移到了 <code>map</code> 中，结果我们在内部引用了一个局部值，造成一个悬垂引用：</p>
|
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<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
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|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub struct Iter<'a, T> {
|
|
|
|
|
|
next: Option<&'a Node<T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<T> List<T> {
|
|
|
|
|
|
pub fn iter<'a>(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: self.head.as_ref().map(|node| &*node) }
|
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|
|
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|
}
|
|
|
|
|
|
}
|
|
|
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|
|
|
|
|
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|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
|
|
|
|
type Item = &'a T;
|
|
|
|
|
|
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
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|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.next.as_ref().map(|node| &*node);
|
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|
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|
&node.elem
|
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|
|
|
|
})
|
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|
|
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}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
|
|
|
|
|
|
Compiling lists v0.1.0 (/Users/ABeingessner/dev/temp/lists)
|
|
|
|
|
|
error[E0308]: mismatched types
|
|
|
|
|
|
--> src/second.rs:77:22
|
|
|
|
|
|
|
|
|
|
|
|
|
77 | Iter { next: self.head.as_ref().map(|node| &*node) }
|
|
|
|
|
|
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `second::Node`, found struct `std::boxed::Box`
|
|
|
|
|
|
|
|
|
|
|
|
|
= note: expected type `std::option::Option<&second::Node<T>>`
|
|
|
|
|
|
found type `std::option::Option<&std::boxed::Box<second::Node<T>>>`
|
|
|
|
|
|
|
|
|
|
|
|
error[E0308]: mismatched types
|
|
|
|
|
|
--> src/second.rs:85:25
|
|
|
|
|
|
|
|
|
|
|
|
|
85 | self.next = node.next.as_ref().map(|node| &*node);
|
|
|
|
|
|
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `second::Node`, found struct `std::boxed::Box`
|
|
|
|
|
|
|
|
|
|
|
|
|
= note: expected type `std::option::Option<&'a second::Node<T>>`
|
|
|
|
|
|
found type `std::option::Option<&std::boxed::Box<second::Node<T>>>`
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>😭</p>
|
|
|
|
|
|
<p>错误的原因是，<code>as_ref</code> 增加了一层间接引用，需要被移除，这里使用另外一种方式来实现:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub struct Iter<'a, T> {
|
|
|
|
|
|
next: Option<&'a Node<T>>,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<T> List<T> {
|
|
|
|
|
|
pub fn iter<'a>(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: self.head.as_deref() }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
impl<'a, T> Iterator for Iter<'a, T> {
|
|
|
|
|
|
type Item = &'a T;
|
|
|
|
|
|
|
|
|
|
|
|
fn next(&mut self) -> Option<Self::Item> {
|
|
|
|
|
|
self.next.map(|node| {
|
|
|
|
|
|
self.next = node.next.as_deref();
|
|
|
|
|
|
&node.elem
|
|
|
|
|
|
})
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>🎉 🎉 🎉</p>
|
|
|
|
|
|
<p><code>as_deref</code> 和 <code>as_deref_mut</code> 函数在 Rust 1.40 版本中正式稳定下来。在那之前，你只能在 <code>stable</code> 版本中使用 <code>map(|node| &**node)</code> 和 <code>map(|node| &mut**node)</code> 的方式来替代。</p>
|
|
|
|
|
|
<p>大家可能会觉得 <code>&**</code> 的形式看上去有些烂，没错，确实如此。但是就像一瓶好酒一样，Rust 也随着时间的推进变得越来越好，因此现在我们已经无需再这么做了。事实上，Rust 很擅长隐式地做类似的转换，或者可以称之为 <a href="https://course.rs/advance/smart-pointer/deref.html"><code>Deref</code></a>。</p>
|
|
|
|
|
|
<p>但是 <code>Deref</code> 在这里并不能很好的完成自己的任务，原因是在闭包中使用 <code>Option<&T></code> 而不是 <code>&T</code> 对于它来说有些过于复杂了，因此我们需要显式地去帮助它完成任务。好在根据我的经验来看，这种情况还是相当少见的。</p>
|
|
|
|
|
|
<p>事实上，还可以使用另一种方式来实现：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>self.next = node.next.as_ref().map::<&Node<T>, _>(|node| &node);
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>这种类型暗示的方式可以使用的原因在于 <code>map</code> 是一个泛型函数:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>pub fn map<U, F>(self, f: F) -> Option<U>
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>turbofish 形式的符号 <code>::<></code> 可以告诉编译器我们希望用哪个具体的类型来替代泛型类型，在这种情况里，<code>::<&Node<T>, _></code> 意味着: 它应该返回一个 <code>&Node<T></code>。这种方式可以让编译器知道它需要对 <code>&node</code> 应用 <code>deref</code>，这样我们就不用手动的添加 <code>**</code> 来进行解引用。</p>
|
|
|
|
|
|
<p>好了，既然编译通过，那就写个测试来看看运行结果:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>#[test]
|
|
|
|
|
|
fn iter() {
|
|
|
|
|
|
let mut list = List::new();
|
|
|
|
|
|
list.push(1); list.push(2); list.push(3);
|
|
|
|
|
|
|
|
|
|
|
|
let mut iter = list.iter();
|
|
|
|
|
|
assert_eq!(iter.next(), Some(&3));
|
|
|
|
|
|
assert_eq!(iter.next(), Some(&2));
|
|
|
|
|
|
assert_eq!(iter.next(), Some(&1));
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
|
|
|
|
|
|
Running target/debug/lists-5c71138492ad4b4a
|
|
|
|
|
|
|
|
|
|
|
|
running 5 tests
|
|
|
|
|
|
test first::test::basics ... ok
|
|
|
|
|
|
test second::test::basics ... ok
|
|
|
|
|
|
test second::test::into_iter ... ok
|
|
|
|
|
|
test second::test::iter ... ok
|
|
|
|
|
|
test second::test::peek ... ok
|
|
|
|
|
|
|
|
|
|
|
|
test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>最后，还有一点值得注意，之前的代码事实上可以应用<a href="https://course.rs/basic/lifetime.html#%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F%E6%B6%88%E9%99%A4">生命周期消除原则</a>:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>impl<T> List<T> {
|
|
|
|
|
|
pub fn iter<'a>(&'a self) -> Iter<'a, T> {
|
|
|
|
|
|
Iter { next: self.head.as_deref() }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>这段代码跟以下代码是等价的:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>impl<T> List<T> {
|
|
|
|
|
|
pub fn iter(&self) -> Iter<T> {
|
|
|
|
|
|
Iter { next: self.head.as_deref() }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
|
|
|
|
|
|
<p>当然，如果你就喜欢生命周期那种自由、飘逸的 feeling，还可以使用 Rust 2018 引入的“显式生命周期消除"语法 <code>'_</code>：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">
|
|
|
|
|
|
<span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>impl<T> List<T> {
|
|
|
|
|
|
pub fn iter(&self) -> Iter<'_, T> {
|
|
|
|
|
|
Iter { next: self.head.as_deref() }
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}
|
|
|
|
|
|
</span></code></pre></pre>
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