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<main>
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<h1 id="peek-函数"><a class="header" href="#peek-函数">Peek 函数</a></h1>
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<p>在之前章节中，我们定义了 <code>push</code>、<code>pop</code> 等基础操作，下面一起添加几个进阶操作，让我们的链表有用起来。</p>
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<p>首先实现的就是 <code>peek</code> 函数，它会返回链表的表头元素的引用:</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>pub fn peek(&self) -> Option<&T> {
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self.head.map(|node| {
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&node.elem
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})
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}
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<span class="boring">}</span></code></pre>
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<pre><code class="language-shell">$ cargo build
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error[E0515]: cannot return reference to local data `node.elem`
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--> src/second.rs:37:13
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37 | &node.elem
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| ^^^^^^^^^^ returns a reference to data owned by the current function
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error[E0507]: cannot move out of borrowed content
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--> src/second.rs:36:9
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36 | self.head.map(|node| {
|
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| ^^^^^^^^^ cannot move out of borrowed content
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</code></pre>
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<p>哎，Rust 大爷，您又哪里不满意了。不过问题倒是也很明显: <code>map</code> 方法是通过 <code>self</code> 获取的值，我们相当于把内部值的引用返回给函数外面的调用者。</p>
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<p>一个比较好的解决办法就是让 <code>map</code> 作用在引用上，而不是直接作用在 <code>self.head</code> 上，为此我们可以使用 <code>Option</code> 的 <code>as_ref</code> 方法：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>impl<T> Option<T> {
|
|
|
pub fn as_ref(&self) -> Option<&T>;
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>该方法将一个 <code>Option<T></code> 变成了 <code>Option<&T></code>，然后再调用 <code>map</code> 就会对引用进行处理了：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub fn peek(&self) -> Option<&T> {
|
|
|
self.head.as_ref().map(|node| {
|
|
|
&node.elem
|
|
|
})
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<pre><code class="language-shell">$ cargo build
|
|
|
|
|
|
Finished dev [unoptimized + debuginfo] target(s) in 0.32s
|
|
|
</code></pre>
|
|
|
<p>当然，我们还可以通过类似的方式获取一个可变引用:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub fn peek_mut(&mut self) -> Option<&mut T> {
|
|
|
self.head.as_mut().map(|node| {
|
|
|
&mut node.elem
|
|
|
})
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>至此 <code>peek</code> 已经完成，为了测试它的功能，我们还需要编写一个测试用例:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>#[test]
|
|
|
fn peek() {
|
|
|
let mut list = List::new();
|
|
|
assert_eq!(list.peek(), None);
|
|
|
assert_eq!(list.peek_mut(), None);
|
|
|
list.push(1); list.push(2); list.push(3);
|
|
|
|
|
|
assert_eq!(list.peek(), Some(&3));
|
|
|
assert_eq!(list.peek_mut(), Some(&mut 3));
|
|
|
list.peek_mut().map(|&mut value| {
|
|
|
value = 42
|
|
|
});
|
|
|
|
|
|
assert_eq!(list.peek(), Some(&42));
|
|
|
assert_eq!(list.pop(), Some(42));
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
error[E0384]: cannot assign twice to immutable variable `value`
|
|
|
--> src/second.rs:100:13
|
|
|
|
|
|
|
99 | list.peek_mut().map(|&mut value| {
|
|
|
| -----
|
|
|
| |
|
|
|
| first assignment to `value`
|
|
|
| help: make this binding mutable: `mut value`
|
|
|
100 | value = 42
|
|
|
| ^^^^^^^^^^ cannot assign twice to immutable variable ^~~~~
|
|
|
</code></pre>
|
|
|
<p>天呐，错误源源不断，这次编译器抱怨说 <code>value</code> 是不可变的，但是我们明明使用 <code>&mut value</code>，发生了什么？难道说在闭包中通过这种方式申明的可变性实际上没有效果？</p>
|
|
|
<p>实际上 <code>&mut value</code> 是一个模式匹配，它用 <code>&mut value</code> 模式去匹配一个可变的引用，此时匹配出来的 <code>value</code> 显然是一个值，而不是可变引用，因为只有完整的形式才是可变引用！</p>
|
|
|
<p>因此我们没必要画蛇添足，这里直接使用 <code>|value|</code> 来匹配可变引用即可，那么此时匹配出来的 <code>value</code> 就是一个可变引用。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>#[test]
|
|
|
fn peek() {
|
|
|
let mut list = List::new();
|
|
|
assert_eq!(list.peek(), None);
|
|
|
assert_eq!(list.peek_mut(), None);
|
|
|
list.push(1); list.push(2); list.push(3);
|
|
|
|
|
|
assert_eq!(list.peek(), Some(&3));
|
|
|
assert_eq!(list.peek_mut(), Some(&mut 3));
|
|
|
|
|
|
list.peek_mut().map(|value| {
|
|
|
*value = 42
|
|
|
});
|
|
|
|
|
|
assert_eq!(list.peek(), Some(&42));
|
|
|
assert_eq!(list.pop(), Some(42));
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这次我们直接匹配出来可变引用 <code>value</code>，然后对其修改即可。</p>
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
Running target/debug/lists-5c71138492ad4b4a
|
|
|
|
|
|
running 3 tests
|
|
|
test first::test::basics ... ok
|
|
|
test second::test::basics ... ok
|
|
|
test second::test::peek ... ok
|
|
|
|
|
|
test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured
|
|
|
</code></pre>
|
|
|
|
|
|
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