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<h1 id="static-和-t-static"><a class="header" href="#static-和-t-static">&amp;'static 和 T: 'static</a></h1>
<p>Rust 的难点之一就在于它有不少容易混淆的概念，例如 <code>&amp;str</code> 、<code>str</code> 与 <code>String</code>， 再比如本文标题那两位。不过与字符串也有不同，这两位对于普通用户来说往往是无需进行区分的，但是当大家想要深入学习或使用 Rust 时，它们就会成为成功路上的拦路虎了。</p>
<p>与生命周期的其它章节不同，本文短小精悍，阅读过程可谓相当轻松愉快，话不多说，let's go。</p>
<p><code>'static</code> 在 Rust 中是相当常见的，例如字符串字面值就具有 <code>'static</code> 生命周期:</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let mark_twain: &amp;str = &quot;Samuel Clemens&quot;;
print_author(mark_twain);
}
fn print_author(author: &amp;'static str) {
println!(&quot;{}&quot;, author);
}</code></pre></pre>
<p>除此之外，特征对象的生命周期也是 <code>'static</code>，例如<a href="https://course.rs/compiler/fight-with-compiler/lifetime/closure-with-static.html#%E7%89%B9%E5%BE%81%E5%AF%B9%E8%B1%A1%E7%9A%84%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F">这里</a>所提到的。</p>
<p>除了 <code>&amp;'static</code> 的用法外，我们在另外一种场景中也可以见到 <code>'static</code> 的使用:</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::fmt::Display;
fn main() {
let mark_twain = &quot;Samuel Clemens&quot;;
print(&amp;mark_twain);
}
fn print&lt;T: Display + 'static&gt;(message: &amp;T) {
println!(&quot;{}&quot;, message);
}</code></pre></pre>
<p>在这里，很明显 <code>'static</code> 是作为生命周期约束来使用了。 <strong>那么问题来了， <code>&amp;'static</code> 和 <code>T: 'static</code> 的用法到底有何区别？</strong></p>
<h2 id="static"><a class="header" href="#static"><code>&amp;'static</code></a></h2>
<p><code>&amp;'static</code> 对于生命周期有着非常强的要求：一个引用必须要活得跟剩下的程序一样久，才能被标注为 <code>&amp;'static</code>。</p>
<p>对于字符串字面量来说，它直接被打包到二进制文件中，永远不会被 <code>drop</code>，因此它能跟程序活得一样久，自然它的生命周期是 <code>'static</code>。</p>
<p>但是，<strong><code>&amp;'static</code> 生命周期针对的仅仅是引用，而不是持有该引用的变量，对于变量来说，还是要遵循相应的作用域规则</strong> :</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::{slice::from_raw_parts, str::from_utf8_unchecked};
fn get_memory_location() -&gt; (usize, usize) {
// “Hello World” 是字符串字面量，因此它的生命周期是 `'static`.
// 但持有它的变量 `string` 的生命周期就不一样了，它完全取决于变量作用域，对于该例子来说，也就是当前的函数范围
let string = &quot;Hello World!&quot;;
let pointer = string.as_ptr() as usize;
let length = string.len();
(pointer, length)
// `string` 在这里被 drop 释放
// 虽然变量被释放，无法再被访问，但是数据依然还会继续存活
}
fn get_str_at_location(pointer: usize, length: usize) -&gt; &amp;'static str {
// 使用裸指针需要 `unsafe{}` 语句块
unsafe { from_utf8_unchecked(from_raw_parts(pointer as *const u8, length)) }
}
fn main() {
let (pointer, length) = get_memory_location();
let message = get_str_at_location(pointer, length);
println!(
&quot;The {} bytes at 0x{:X} stored: {}&quot;,
length, pointer, message
);
// 如果大家想知道为何处理裸指针需要 `unsafe`，可以试着反注释以下代码
// let message = get_str_at_location(1000, 10);
}</code></pre></pre>
<p>上面代码有两点值得注意：</p>
<ul>
<li><code>&amp;'static</code> 的引用确实可以和程序活得一样久，因为我们通过 <code>get_str_at_location</code> 函数直接取到了对应的字符串</li>
<li>持有 <code>&amp;'static</code> 引用的变量，它的生命周期受到作用域的限制，大家务必不要搞混了</li>
</ul>
<h2 id="t-static"><a class="header" href="#t-static"><code>T: 'static</code></a></h2>
<p>相比起来，这种形式的约束就有些复杂了。</p>
<p>首先，在以下两种情况下，<code>T: 'static</code> 与 <code>&amp;'static</code> 有相同的约束：<code>T</code> 必须活得和程序一样久。</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::fmt::Debug;
fn print_it&lt;T: Debug + 'static&gt;( input: T) {
println!( &quot;'static value passed in is: {:?}&quot;, input );
}
fn print_it1( input: impl Debug + 'static ) {
println!( &quot;'static value passed in is: {:?}&quot;, input );
}
fn main() {
let i = 5;
print_it(&amp;i);
print_it1(&amp;i);
}</code></pre></pre>
<p>以上代码会报错，原因很简单: <code>&amp;i</code> 的生命周期无法满足 <code>'static</code> 的约束，如果大家将 <code>i</code> 修改为常量，那自然一切 OK。</p>
<p>见证奇迹的时候，请不要眨眼，现在我们来稍微修改下 <code>print_it</code> 函数:</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::fmt::Debug;
fn print_it&lt;T: Debug + 'static&gt;( input: &amp;T) {
println!( &quot;'static value passed in is: {:?}&quot;, input );
}
fn main() {
let i = 5;
print_it(&amp;i);
}</code></pre></pre>
<p>这段代码竟然不报错了！原因在于我们约束的是 <code>T</code>，但是使用的却是它的引用 <code>&amp;T</code>，换而言之，我们根本没有直接使用 <code>T</code>，因此编译器就没有去检查 <code>T</code> 的生命周期约束！它只要确保 <code>&amp;T</code> 的生命周期符合规则即可，在上面代码中，它自然是符合的。</p>
<p>再来看一个例子:</p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::fmt::Display;
fn main() {
let r1;
let r2;
{
static STATIC_EXAMPLE: i32 = 42;
r1 = &amp;STATIC_EXAMPLE;
let x = &quot;&amp;'static str&quot;;
r2 = x;
// r1 和 r2 持有的数据都是 'static 的，因此在花括号结束后，并不会被释放
}
println!(&quot;&amp;'static i32: {}&quot;, r1); // -&gt; 42
println!(&quot;&amp;'static str: {}&quot;, r2); // -&gt; &amp;'static str
let r3: &amp;str;
{
let s1 = &quot;String&quot;.to_string();
// s1 虽然没有 'static 生命周期，但是它依然可以满足 T: 'static 的约束
// 充分说明这个约束是多么的弱。。
static_bound(&amp;s1);
// s1 是 String 类型，没有 'static 的生命周期，因此下面代码会报错
r3 = &amp;s1;
// s1 在这里被 drop
}
println!(&quot;{}&quot;, r3);
}
fn static_bound&lt;T: Display + 'static&gt;(t: &amp;T) {
println!(&quot;{}&quot;, t);
}</code></pre></pre>
<h2 id="static-到底针对谁"><a class="header" href="#static-到底针对谁">static 到底针对谁？</a></h2>
<p>大家有没有想过，到底是 <code>&amp;'static</code> 这个引用还是该引用指向的数据活得跟程序一样久呢？</p>
<p><strong>答案是引用指向的数据</strong>，而引用本身是要遵循其作用域范围的，我们来简单验证下：</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
{
let static_string = &quot;I'm in read-only memory&quot;;
println!(&quot;static_string: {}&quot;, static_string);
// 当 `static_string` 超出作用域时，该引用不能再被使用，但是数据依然会存在于 binary 所占用的内存中
}
println!(&quot;static_string reference remains alive: {}&quot;, static_string);
}</code></pre></pre>
<p>以上代码不出所料会报错，原因在于虽然字符串字面量 &quot;I'm in read-only memory&quot; 的生命周期是 <code>'static</code>，但是持有它的引用并不是，它的作用域在内部花括号 <code>}</code> 处就结束了。</p>
<h2 id="课后练习"><a class="header" href="#课后练习">课后练习</a></h2>
<blockquote>
<p><a href="https://practice-zh.course.rs/lifetime/static.html">Rust By Practice</a>，支持代码在线编辑和运行，并提供详细的习题解答。（本节暂无习题解答）</p>
</blockquote>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>总之， <code>&amp;'static</code> 和 <code>T: 'static</code> 大体上相似，相比起来，后者的使用形式会更加复杂一些。</p>
<p>至此，相信大家对于 <code>'static</code> 和 <code>T: 'static</code> 也有了清晰的理解，那么我们应该如何使用它们呢？</p>
<p>作为经验之谈，可以这么来:</p>
<ul>
<li>如果你需要添加 <code>&amp;'static</code> 来让代码工作，那很可能是设计上出问题了</li>
<li>如果你希望满足和取悦编译器，那就使用 <code>T: 'static</code>，很多时候它都能解决问题</li>
</ul>
<blockquote>
<p>一个小知识，在 Rust 标准库中，有 48 处用到了 &amp;'static ，112 处用到了 <code>T: 'static</code> ，看来取悦编译器不仅仅是菜鸟需要的，高手也经常用到 :)</p>
</blockquote>
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