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<h1 id="基本数据布局-layout-"><a class="header" href="#基本数据布局-layout-">基本数据布局( Layout )</a></h1>
<p>发现一件尴尬的事情，之前介绍了这么多，但是竟然没有介绍链表是什么...亡羊补牢未为晚也，链表就是一系列存储在堆上的连续数据，大家是不是发现这个定义跟动态数据 <code>Vector</code> 非常相似，那么区别在于什么呢？</p>
<p>区别就在于链表中的每一个元素都指向下一个元素，最终形成 - 顾名思义的链表: <code>A1 -&gt; A2 -&gt; A3 -&gt; Null</code>。 而数组中的元素只是连续排列，并不存在前一个元素指向后一个元素的情况，而是每个元素通过下标索引来访问。</p>
<p>既然函数式语言的程序员最常使用链表，那么我们来看看他们给出的定义长什么样:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>List a = Empty | Elem a (List a)
<span class="boring">}</span></code></pre></pre>
<p>mu...看上去非常像一个数学定义，我们可以这样阅读它， 列表 a 要么是空，要么是一个元素后面再跟着一个列表。非常递归也不咋好懂的定义，果然，这很函数式语言。</p>
<p>下面我们再来使用 Rust 的方式对链表进行下定义，为了简单性，这先不使用泛型:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// in first.rs
pub enum List {
Empty,
Elem(i32, List),
}
<span class="boring">}</span></code></pre></pre>
<p>喔，看上去人模狗样，来，运行下看看：</p>
<pre><code class="language-shell">$ cargo run
error[E0072]: recursive type `List` has infinite size
--&gt; src/first.rs:1:1
|
1 | pub enum List {
| ^^^^^^^^^^^^^ recursive type has infinite size
2 | Empty,
3 | Elem(i32, List),
| ---- recursive without indirection
help: insert some indirection (e.g., a `Box`, `Rc`, or `&amp;`) to make `List` representable
|
3 | Elem(i32, Box&lt;List&gt;),
</code></pre>
<p>帅不过 3 秒的玩意儿～～ 好在这个问题，我们在<a href="https://course.rs/advance/smart-pointer/box.html#%E5%B0%86%E5%8A%A8%E6%80%81%E5%A4%A7%E5%B0%8F%E7%B1%BB%E5%9E%8B%E5%8F%98%E4%B8%BA-sized-%E5%9B%BA%E5%AE%9A%E5%A4%A7%E5%B0%8F%E7%B1%BB%E5%9E%8B">之前的章节</a>中就讲过了，简而言之，当前的类型是<a href="https://course.rs/advance/into-types/sized.html">不定长</a>的，对于 Rust 编译器而言，所有栈上的类型都必须在编译期有固定的长度，一个简单的解决方案就是使用 <code>Box</code> 将值封装到堆上，然后使用栈上的定长指针来指向堆上不定长的值。</p>
<p>实际上，如果大家有仔细看编译错误的话，它还给出了我们提示: <code>Elem(i32, Box&lt;List&gt;)</code>，和我们之前的结论一致，下面来试试：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub enum List {
Empty,
Elem(i32, Box&lt;List&gt;),
}
<span class="boring">}</span></code></pre></pre>
<pre><code class="language-shell">$ cargo build
Finished dev [unoptimized + debuginfo] target(s) in 0.22s
</code></pre>
<p>万能的编译器再一次拯救了我们，这次的代码成功完成了编译。但是这依然是不咋滴的 <code>List</code> 定义，有几个原因。</p>
<p>首先，考虑一个拥有两个元素的 <code>List</code>:</p>
<pre><code class="language-shell">[] = Stack
() = Heap
[Elem A, ptr] -&gt; (Elem B, ptr) -&gt; (Empty, *junk*)
</code></pre>
<p>这里有两个问题:</p>
<ul>
<li>最后一个节点分配在了堆上，但是它看上去根本不像一个 <code>Node</code></li>
<li>第一个 <code>Node</code> 是存储在栈上的，结果一家子不能整整齐齐的待在堆上了</li>
</ul>
<p>这两点看上去好像有点矛盾：你希望所有节点在堆上，但是又觉得最后一个节点不应该在堆上。那再来考虑另一种布局( Layout )方式：</p>
<pre><code class="language-shell">[ptr] -&gt; (Elem A, ptr) -&gt; (Elem B, *null*)
</code></pre>
<p>在这种布局下，我们无条件的在堆上创建所有节点，最大的区别就是这里不再有 <code>junk</code>。那么什么是 <code>junk</code>？为了理解这个概念，先来看看枚举类型的内存布局( Layout )长什么样:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>enum Foo {
D1(u8),
D2(u16),
D3(u32),
D4(u64)
}
<span class="boring">}</span></code></pre></pre>
<p>大家觉得 <code>Foo::D1(99)</code> 占用多少内存空间？是 <code>u8</code> 对应的 1 个字节吗？答案是 8 个字节( 为了好理解，这里不考虑 enum tag 所占用的额外空间 )，因为枚举有一个特点，枚举成员占用的内存空间大小跟最大的成员对齐，在这个例子中，所有的成员都会跟 <code>u64</code> 进行对齐。</p>
<p>在理解了这点后，再回到我们的 <code>List</code> 定义。这里最大的问题就是尽管 <code>List::Empty</code> 是空的节点，但是它依然得消耗和其它节点一样的内存空间。</p>
<p>与其让一个节点不进行内存分配，不如让它一直进行内存分配，无论是否有内容，因为后者会保证节点内存布局的一致性。这种一致性对于 <code>push</code> 和 <code>pop</code> 节点来说可能没有什么影响，但是对于链表的分割和合并而言，就有意义了。</p>
<p>下面，我们对之前两种不同布局的 List 进行下分割:</p>
<pre><code class="language-shell">layout 1:
[Elem A, ptr] -&gt; (Elem B, ptr) -&gt; (Elem C, ptr) -&gt; (Empty *junk*)
split off C:
[Elem A, ptr] -&gt; (Elem B, ptr) -&gt; (Empty *junk*)
[Elem C, ptr] -&gt; (Empty *junk*)
</code></pre>
<pre><code class="language-shell">layout 2:
[ptr] -&gt; (Elem A, ptr) -&gt; (Elem B, ptr) -&gt; (Elem C, *null*)
split off C:
[ptr] -&gt; (Elem A, ptr) -&gt; (Elem B, *null*)
[ptr] -&gt; (Elem C, *null*)
</code></pre>
<p>可以看出，在布局 1 中，需要将 <code>C</code> 节点从堆上拷贝到栈中，而布局 2 则无需此过程。而且从分割后的布局清晰度而言，2 也要优于 1。</p>
<p>现在，我们应该都相信布局 1 更糟糕了，而且不幸的是，我们之前的实现就是布局 1， 那么该如何实现新的布局呢 ？也许，我们可以实现类似如下的 <code>List</code> :</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub enum List {
Empty,
ElemThenEmpty(i32),
ElemThenNotEmpty(i32, Box&lt;List&gt;),
}
<span class="boring">}</span></code></pre></pre>
<p>但是，你们有没有觉得更糟糕了...有些不忍直视的感觉。这让我们的代码复杂度大幅提升，例如你现在得实现一个完全不合法的状态: <code>ElemThenNotEmpty(0, Box(Empty))</code>，而且这种实现依然有之前的不一致性的问题。</p>
<p>之前我们提到过枚举成员的内存空间占用和 enum tag 问题，实际上我们可以创建一个特例：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>enum Foo {
A,
B(ContainsANonNullPtr),
}
<span class="boring">}</span></code></pre></pre>
<p>在这里 <code>null</code> 指针的优化就开始介入了，它会消除枚举成员 <code>A</code> 占用的额外空间，原因在于编译器可以直接将 <code>A</code> 优化成 <code>0</code>，而 <code>B</code> 则不行，因为它包含了非 <code>null</code> 指针。这样一来，编译器就无需给 <code>A</code> 打 tag 进行识别了，而是直接通过 <code>0</code> 就能识别出这是 <code>A</code> 成员，非 <code>0</code> 的自然就是 <code>B</code> 成员。</p>
<p>事实上，编译器还会对枚举做一些其他优化，但是 <code>null</code> 指针优化是其中最重要的一条。</p>
<p>所以我们应该怎么避免多余的 <code>junk</code>，保持内存分配的一致性，还能保持 <code>null</code> 指针优化呢？枚举可以让我们声明一个类型用于表达多个不同的值，而结构体可以声明一个类型同时包含多个值，只要将这两个类型结合在一起，就能实现之前的目标: 枚举类型用于表示 <code>List</code>，结构体类型用于表示 <code>Node</code>.</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>struct Node {
elem: i32,
next: List,
}
pub enum List {
Empty,
More(Box&lt;Node&gt;),
}
<span class="boring">}</span></code></pre></pre>
<p>让我们看看新的定义是否符合之前的目标：</p>
<ul>
<li><code>List</code> 的尾部不会再分配多余的 junk 值，通过!</li>
<li><code>List</code> 枚举的形式可以享受 <code>null</code> 指针优化，完美！</li>
<li>所有的元素都拥有统一的内存分配，Good!</li>
</ul>
<p>很好，我们准确构建了之前想要的内存布局，并且证明了最初的内存布局问题多多，编译下试试：</p>
<pre><code class="language-shell">error[E0446]: private type `Node` in public interface
--&gt; src/first.rs:8:10
|
1 | struct Node {
| ----------- `Node` declared as private
...
8 | More(Box&lt;Node&gt;),
| ^^^^^^^^^ can't leak private type
</code></pre>
<p>在英文书中，这里是一个 warning ，但是在笔者使用的最新版中(Rust 1.59)，该 warning 已经变成一个错误。主要原因在于 <code>pub enum</code> 会要求它的所有成员必须是 <code>pub</code>，但是由于 <code>Node</code> 没有声明为 <code>pub</code>，因此产生了冲突。</p>
<p>这里最简单的解决方法就是将 <code>Node</code> 结构体和它的所有字段都标记为 <code>pub</code> :</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct Node {
pub elem: i32,
pub next: List,
}
<span class="boring">}</span></code></pre></pre>
<p>但是从编程的角度而言，我们还是希望让实现细节只保留在内部，而不是对外公开，因此以下代码相对会更加适合：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct List {
head: Link,
}
enum Link {
Empty,
More(Box&lt;Node&gt;),
}
struct Node {
elem: i32,
next: Link,
}
<span class="boring">}</span></code></pre></pre>
<p>从代码层面看，貌似多了一层封装，但是实际上 <code>List</code> 只有一个字段，因此结构体的大小跟字段大小是相等的，没错，传说中的零开销抽象！</p>
<p>至此，一个令人满意的数据布局就已经设计完成，下面一起来看看该如何使用这些数据。</p>
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