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    <span class="token punctuation">{</span>
        <span class="token keyword">let</span> r<span class="token punctuation">;</span>
        <span class="token punctuation">{</span>
            <span class="token keyword">let</span> x <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">;</span>
            r <span class="token operator">=</span> <span class="token operator">&amp;</span>x<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;{}&quot;</span><span class="token punctuation">,</span> r<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>上面这段代码会在 <code>r = &amp;x;</code> 处报错，因为当打印 <code>r</code> 的值的时候，<code>x</code> 已经离开了他的作用域，这时 <code>r</code> 指向的 <code>x</code> 的内存已经被释放，因此会报错。</p> <p>Rust 实际上是通过<strong>借用检查器</strong>来检查一些变量的生命周期。</p> <h3 id="借用检查器"><a href="#借用检查器" class="header-anchor">#</a> 借用检查器</h3> <p>Rust 编译器的借用检查器（borrow checker），用来比较作用域来判断所有的借用是否合法</p> <p>在上例中，借用检查器检测到 <code>r</code> 的生命周期大于 <code>x</code>，即被引用者的生命周期小于引用者的生命周期，因此编译会报错。</p> <h3 id="函数中的泛型生命周期"><a href="#函数中的泛型生命周期" class="header-anchor">#</a> 函数中的泛型生命周期</h3> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test02</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> string1 <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Congratulations&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">let</span> string2 <span class="token operator">=</span> <span class="token string">&quot;fantastic&quot;</span><span class="token punctuation">;</span>
    <span class="token keyword">let</span> result <span class="token operator">=</span> <span class="token function">longest</span><span class="token punctuation">(</span>string1<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> string2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The longest string is {}&quot;</span><span class="token punctuation">,</span> result<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token keyword">str</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> x<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> y<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        x
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        y
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>编译报错：</p> <div class="language-rust extra-class"><pre class="language-rust"><code>➜  ~<span class="token operator">/</span><span class="token class-name">Code</span><span class="token operator">/</span>rust<span class="token operator">/</span>life_cycle git<span class="token punctuation">:</span><span class="token punctuation">(</span>master<span class="token punctuation">)</span> ✗ cargo run           
   <span class="token class-name">Compiling</span> life_cycle v0<span class="token number">.1</span><span class="token punctuation">.</span><span class="token number">0</span> <span class="token punctuation">(</span><span class="token operator">/</span>home<span class="token operator">/</span>cherry<span class="token operator">/</span><span class="token class-name">Code</span><span class="token operator">/</span>rust<span class="token operator">/</span>life_cycle<span class="token punctuation">)</span>
error<span class="token punctuation">[</span><span class="token constant">E0106</span><span class="token punctuation">]</span><span class="token punctuation">:</span> missing lifetime specifier
  <span class="token operator">-</span><span class="token punctuation">-&gt;</span> src<span class="token operator">/</span>main<span class="token punctuation">.</span>rs<span class="token punctuation">:</span><span class="token number">24</span><span class="token punctuation">:</span><span class="token number">33</span>
   <span class="token operator">|</span>
<span class="token number">24</span> <span class="token operator">|</span> <span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token keyword">str</span> <span class="token punctuation">{</span>
   <span class="token operator">|</span>               <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span>     <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span>     <span class="token operator">^</span> expected named lifetime parameter
   <span class="token operator">|</span>
   <span class="token operator">=</span> help<span class="token punctuation">:</span> this function<span class="token lifetime-annotation symbol">'s</span> <span class="token keyword">return</span> <span class="token keyword">type</span> <span class="token type-definition class-name">contains</span> a borrowed value<span class="token punctuation">,</span> but the signature does not say whether it is borrowed from `x` or `y`
help<span class="token punctuation">:</span> consider introducing a named lifetime parameter
   <span class="token operator">|</span>
<span class="token number">24</span> <span class="token operator">|</span> <span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span> <span class="token punctuation">{</span>
   <span class="token operator">|</span>           <span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span><span class="token operator">+</span>     <span class="token operator">+</span><span class="token operator">+</span>          <span class="token operator">+</span><span class="token operator">+</span>          <span class="token operator">+</span><span class="token operator">+</span>

<span class="token class-name">For</span> more information about this error<span class="token punctuation">,</span> <span class="token keyword">try</span> `rustc <span class="token operator">-</span><span class="token operator">-</span>explain <span class="token constant">E0106</span>`<span class="token punctuation">.</span>
</code></pre></div><p>我们发现编译器会提示<strong>缺少一个命名的生命周期参数</strong>，这个函数返回一个借用的值，但是没有声明这个借用的值是来自 <code>x</code> 还是来自 <code>y</code>。值得说明的是，这个返回值的借用跟函数体的逻辑没有关系，要从函数签名就要看出返回值借用的值来自哪一个参数。</p> <p>根据编译器提示，我们声明一个泛型生命周期 <code>'a</code>，代码修改如下：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> x<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> y<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        x
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        y
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h3 id="生命周期标注语法"><a href="#生命周期标注语法" class="header-anchor">#</a> 生命周期标注语法</h3> <ul><li>生命周期的标注不会改变引用的生命周期长度</li> <li>当指定了泛型生命周期参数，函数可以接收带有任何生命周期的引用</li> <li>生命周期的标注：描述了多个引用的生命周期间的关系，但不影响生命周期</li></ul> <p>生命周期参数名语法如下：</p> <ul><li>以 <code>'</code> 开头</li> <li>通常全小写且非常短</li> <li>习惯以 <code>'a</code> 表示</li></ul> <p>生命周期标注的位置：</p> <ul><li>在引用符号 <code>&amp;</code> 后面标注</li> <li>使用空格将标注和引用类型区分开</li></ul> <p>生命周期标注的例子：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token operator">&amp;</span><span class="token keyword">i32</span>        <span class="token comment">// 一个引用</span>
<span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">i32</span>     <span class="token comment">// 带有显式生命周期的引用</span>
<span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">mut</span> <span class="token keyword">i32</span> <span class="token comment">// 带有显式生命周期的可变引用</span>
</code></pre></div><p>值得注意的是，单个生命周期标注本身没有意义，我们再看上面的 <code>longest</code> 函数：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> x<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> y<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        x
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        y
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>泛型的生命周期参数声明在函数名和参数列表之间的 <code>&lt;&gt;</code> 中。</p> <p>我们仔细分析这个函数：</p> <p><code>longest</code> 函数的两个参数都声明了生命周期，就要求这两个引用必须和泛型的生命周期存活相同的时间，而且函数所返回的字符串切片的存活时长也不能小于 <code>'a</code> 这个生命周期。为引用指明生命周期，是要确保当引用失去了所有权后而被移出内存。当在函数参数中指明生命周期时，我们并没有改变参数和返回值的生命周期，只是向调用检查器指出了一些可用于检查非法调用的约束。而 <code>longest</code> 函数本身并不需要知道参数 <code>x</code> 和 <code>y</code> 具体的存活时长，只需要某个可以代替 <code>'a</code> 的作用域，同时满足函数的签名约束。实际上，若函数引用其外部的代码或者被外部代码引用，只靠 rust 本身确定参数和返回值的生命周期时不可能的，这样的话，函数所使用的生命周期在每次调用中都会发生变化，正因为如此，我们才需要手动对生命周期进行标注。</p> <p>当我们将两个引用传入函数时，<code>x</code> 和 <code>y</code> 作用域重叠的部分将用来代替 <code>'a</code> 这个生命周期的作用域，换句话说，这个泛型生命周期得到的具体的生命周期就是 <code>x</code> 和 <code>y</code> 两者生命周期较短的那个，因为返回值也标注了相同的生命周期，因此返回值的引用在两者比较短的生命周期内都是有效的。</p> <p>那么生命周期标注是如何对 <code>longest</code> 函数进行限制的？我么修改一下代码：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test02</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> string1 <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Congratulations&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">let</span> string2 <span class="token operator">=</span> <span class="token string">&quot;fantastic&quot;</span><span class="token punctuation">;</span>
        <span class="token keyword">let</span> result <span class="token operator">=</span> <span class="token function">longest</span><span class="token punctuation">(</span>string1<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> string2<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The longest string is {}&quot;</span><span class="token punctuation">,</span> result<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token keyword">str</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> x<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> y<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        x
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        y
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>将 string1 下面三行代码放到一个单独的作用域里，string2 是一个字符串字面值（字符串切片），他的生命周期相当于是一个静态的生命周期，在整个程序运行期间都存活，而 result 引用也会在 Line 7 大括号结束之前保持有效，因此代码不会报错。</p> <p>【注】：<code>&amp;str</code> 是直接在可执行文件中加载的，即这块内存直接放到可执行文件里面的，所以整个程序运行期间，这块内存比较特殊，不会由于所有权而消失，所以指这块内存的引用，一定会一直指向一个合法内存，所以其引用的生命周期是 <code>'static</code>，也就是全局静态，也不可能出现什么悬垂引用。</p> <p>再改一下代码，将 result 声明放到外面，然后将 print 也放到外面，将 string2 改成 String 类型：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test02</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> string1 <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Congratulations&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">let</span> result<span class="token punctuation">;</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">let</span> string2 <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;fantastic&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        result <span class="token operator">=</span> <span class="token function">longest</span><span class="token punctuation">(</span>string1<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> string2<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The longest string is {}&quot;</span><span class="token punctuation">,</span> result<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>我们发现 Line 6 报错了。string1 的生命周期为 Line 2~9，string2 的生命周期为 Line 5~7，所以 <code>'a</code> 所表示的生命周期为 Line 5~7，而 result 的生命周期为 Line 3~9，不在 <code>'a</code> 的范围内，因此编译报错，我们来看一下编译具体的错误：</p> <div class="language-rust extra-class"><pre class="language-rust"><code>➜  ~<span class="token operator">/</span><span class="token class-name">Code</span><span class="token operator">/</span>rust<span class="token operator">/</span>life_cycle git<span class="token punctuation">:</span><span class="token punctuation">(</span>master<span class="token punctuation">)</span> ✗ cargo run
   <span class="token class-name">Compiling</span> life_cycle v0<span class="token number">.1</span><span class="token punctuation">.</span><span class="token number">0</span> <span class="token punctuation">(</span><span class="token operator">/</span>home<span class="token operator">/</span>cherry<span class="token operator">/</span><span class="token class-name">Code</span><span class="token operator">/</span>rust<span class="token operator">/</span>life_cycle<span class="token punctuation">)</span>
error<span class="token punctuation">[</span><span class="token constant">E0597</span><span class="token punctuation">]</span><span class="token punctuation">:</span> `string2` does not live long enough
  <span class="token operator">-</span><span class="token punctuation">-&gt;</span> src<span class="token operator">/</span>main<span class="token punctuation">.</span>rs<span class="token punctuation">:</span><span class="token number">30</span><span class="token punctuation">:</span><span class="token number">44</span>
   <span class="token operator">|</span>
<span class="token number">30</span> <span class="token operator">|</span>         result <span class="token operator">=</span> <span class="token function">longest</span><span class="token punctuation">(</span>string1<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> string2<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token operator">|</span>                                            <span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span><span class="token operator">^</span> borrowed value does not live long enough
<span class="token number">31</span> <span class="token operator">|</span>     <span class="token punctuation">}</span>
   <span class="token operator">|</span>     <span class="token operator">-</span> `string2` dropped here <span class="token keyword">while</span> still borrowed
<span class="token number">32</span> <span class="token operator">|</span>     <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The longest string is {}&quot;</span><span class="token punctuation">,</span> result<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token operator">|</span>                                          <span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span><span class="token operator">-</span> borrow later used here

<span class="token class-name">For</span> more information about this error<span class="token punctuation">,</span> <span class="token keyword">try</span> `rustc <span class="token operator">-</span><span class="token operator">-</span>explain <span class="token constant">E0597</span>`<span class="token punctuation">.</span>
error<span class="token punctuation">:</span> could not compile `life_cycle` due to previous error
</code></pre></div><p>这个报错的含义是，为了让 result 这个变量在打印时是有效的，那么 string2 必须在外部作用域结束之前一直保持有效，因为在函数声明中参数和返回值都使用了相同的生命周期。</p> <p>在上例中，尽管 string1 的长度大于 string2 的长度，函数返回的是 string1 的引用，但是编译器并不知道这一点，编译器只知道 <code>longest</code> 函数返回引用的生命周期是 <code>x</code> 和 <code>y</code> 生命周期比较短的那个。</p> <h2 id="深入理解生命周期"><a href="#深入理解生命周期" class="header-anchor">#</a> 深入理解生命周期</h2> <ul><li>指定生命周期参数的方式依赖于函数所做的事情，在上面的例子中，若 <code>longest</code> 函数改为：</li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token keyword">str</span> <span class="token punctuation">{</span>
    x
<span class="token punctuation">}</span>
</code></pre></div><p>这个时候，函数只返回变量 <code>x</code>，而与 <code>y</code> 无关，因此无需为 <code>y</code> 指定生命周期。</p> <ul><li>从函数返回引用时，返回类型的生命周期参数需要与其中一个参数的生命周期匹配</li> <li>如果返回的引用没有指向任何参数，那么他只能引用函数内创建的值
<ul><li>这就是<strong>悬垂引用</strong>，该值在函数结束时就走出了作用域，见下面的例子</li></ul></li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">test02</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> string1 <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Congratulations&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">let</span> string2 <span class="token operator">=</span> <span class="token string">&quot;fantastic&quot;</span><span class="token punctuation">;</span>
    <span class="token keyword">let</span> result <span class="token operator">=</span> <span class="token function">longest</span><span class="token punctuation">(</span>string1<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> string2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;The longest string is {}&quot;</span><span class="token punctuation">,</span> result<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> res <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;abc&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    res<span class="token punctuation">.</span><span class="token function">as_str</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre></div><p>上面的代码中，<code>longest</code> 函数中返回了局部变量 <code>res</code>，当函数执行完毕时，局部变量 <code>res</code> 所指向的内存已经被释放掉，因此 <code>test02</code> 中的 <code>result</code> 变量指向的 <code>res</code> 内存已经被清理，这就造成了<strong>悬垂引用</strong>，非常类似于 <code>C/C++</code> 的野指针。</p> <p>那么我就是想返回函数中的局部变量，应该怎么办呢？解决办法也很简单，就是直接返回这个值而不是返回引用，这样就将变量的所有权移交出去了，如下所示：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">longest</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token class-name">String</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> res <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;abc&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    res
<span class="token punctuation">}</span>
</code></pre></div><p><strong>因此从根本上讲，生命周期这种语法规则，是用来关联函数的不同参数及返回值之间的生命周期，一旦他们取得了某种联系，rust 就会获得足够的信息来支持保证内存安全的操作，并且阻止那些可能会导致悬垂指针或者其他违反内存安全的行为。</strong></p> <h2 id="struct-定义中的生命周期标注"><a href="#struct-定义中的生命周期标注" class="header-anchor">#</a> Struct 定义中的生命周期标注</h2> <p>struct 里可以包括：</p> <ul><li>自持有类型（类似于 i32 等）</li> <li>引用：需要在每个引用上添加生命周期标注</li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">ImportantExcerpt</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    part<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span>

<span class="token keyword">fn</span> <span class="token function-definition function">test04</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> novel <span class="token operator">=</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">&quot;Today is Tuesday. And I will take part in a meeting.&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">let</span> first_sentence <span class="token operator">=</span> novel<span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">&quot;.&quot;</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">expect</span><span class="token punctuation">(</span><span class="token string">&quot;Can't find a '.'&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token class-name">ImportantExcerpt</span> <span class="token punctuation">{</span>
        part<span class="token punctuation">:</span> first_sentence<span class="token punctuation">,</span>
    <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="生命周期的省略"><a href="#生命周期的省略" class="header-anchor">#</a> 生命周期的省略</h2> <p>每个引用都有生命周期，需要为使用生命周期的函数或 struct 指定生命周期参数</p> <p>但是下面这个例子，没有任何生命周期的标注，仍然可以通过编译：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">fn</span> <span class="token function-definition function">first_word</span><span class="token punctuation">(</span>s<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token keyword">str</span> <span class="token punctuation">{</span>
    <span class="token keyword">let</span> byte <span class="token operator">=</span> s<span class="token punctuation">.</span><span class="token function">as_bytes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span>i<span class="token punctuation">,</span> <span class="token operator">&amp;</span>item<span class="token punctuation">)</span> <span class="token keyword">in</span> byte<span class="token punctuation">.</span><span class="token function">iter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">enumerate</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> item <span class="token operator">==</span> <span class="token char">b' '</span> <span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token operator">&amp;</span>s<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">..</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token operator">&amp;</span>s<span class="token punctuation">[</span><span class="token punctuation">..</span><span class="token punctuation">]</span>
<span class="token punctuation">}</span>
</code></pre></div><p>按照原来的 rust 规范，函数声明、参数和返回类型前都是要加上生命周期标注的，但是 rust 团队发现程序员总是一遍又一遍地标注同样的生命周期，而且这些场景是可以预测的，有着明确的模式，因此 rust 团队就将这些模式写入了编译器，使得借用检查器可以自动对这些模式进行推导而无需显式标注。</p> <p><strong>生命周期省略规则</strong></p> <ul><li>在 Rust 引用分析中所编入的模式称为<strong>生命周期省略规则</strong> <ul><li>这些规则无需开发者来遵守</li> <li>它们是一些特殊情况，由编译器来考虑</li> <li>如果你的代码符合这些情况，那么就无需显式标注生命周期</li></ul></li> <li>生命周期省略规则不会提供完整的推断：
<ul><li>如果应用规则后，引用的生命周期仍然模糊不清→编译错误</li> <li>解决办法：添加生命周期标注，表明引用间的相互关系</li></ul></li></ul> <h3 id="输入、输出生命周期"><a href="#输入、输出生命周期" class="header-anchor">#</a> 输入、输出生命周期</h3> <p>生命周期在：</p> <ul><li>函数/方法的参数中，叫做输入生命周期</li> <li>函数/方法的返回值中，叫输出生命周期</li></ul> <h3 id="生命周期省略的三个规则"><a href="#生命周期省略的三个规则" class="header-anchor">#</a> 生命周期省略的三个规则</h3> <p>编译器使用三个规则在没有显式标注生命周期的情况下，来确定引用的生命周期</p> <ul><li>规则 1 应用于输入生命周期</li> <li>规则 2、3 应用于输出生命周期</li> <li>如果编译器应用完三个规则后，仍然无法确定有效的生命周期，则报错</li> <li>这些规则适用于 fn 和 impl 块</li></ul> <p><strong>规则 1：</strong> 每个引用类型都有自己的生命周期
<strong>规则 2：</strong> 如果只有 1 个输入生命周期参数，那么该生命周期被赋给所有输出生命周期参数
<strong>规则 3：</strong> 如果有多个输入生命周期参数，但其中一个是 <code>&amp;self</code> 或 <code>&amp;mut self</code>，那么 <code>self</code> 的生命周期会被赋给所有的输出生命周期参数</p> <p><strong>生命周期省略的三个规则-例子</strong></p> <p>假设我们是编译器:</p> <p><code>fn first_word(s: &amp;str) -&gt; &amp;str {}</code> <code>fn first_word&lt;'a&gt;(s: &amp;'a str) -&gt; &amp;str {}</code> <code>fn first_word&lt;'a&gt;(s: &amp;'a str) -&gt; &amp;'a str {}</code></p> <p><code>fn longest(x: &amp;str, y: &amp;str) -&gt; &amp;str {}</code> <code>fn longest&lt;'a, 'b&gt;(x: &amp;'a str, y: &amp;'b str) -&gt; &amp;str {}</code></p> <h3 id="方法定义中的生命周期标注"><a href="#方法定义中的生命周期标注" class="header-anchor">#</a> 方法定义中的生命周期标注</h3> <ul><li>在 struct 上使用生命周期实现方法，语法和泛型参数的语法一样</li> <li>在哪声明和使用生命周期参数，依赖于：
<ul><li>生命周期参数是否和字段、方法的参数或返回值有关</li></ul></li> <li>struct 字段的生命周期名：
<ul><li>在 impl 后声明</li> <li>在 struct 名后使用</li> <li>这些生命周期是 struct 类型的一部分</li></ul></li> <li>impl 块内的方法签名中
<ul><li>引用必须绑定于 struct 字段引用的生命周期，或者引用是独立的也可以</li> <li>生命周期省略规则经常使得方法中的生命周期标注不是必须的</li></ul></li></ul> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">ImportantExcerpt</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    part<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span>

<span class="token keyword">impl</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token class-name">ImportantExcerpt</span><span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
    <span class="token keyword">fn</span> <span class="token function-definition function">level</span><span class="token punctuation">(</span><span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token keyword">u32</span> <span class="token punctuation">{</span>
        <span class="token number">3</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">fn</span> <span class="token function-definition function">printSome</span><span class="token punctuation">(</span><span class="token operator">&amp;</span><span class="token keyword">self</span><span class="token punctuation">,</span> words<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token keyword">str</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token operator">&amp;</span><span class="token keyword">str</span> <span class="token punctuation">{</span>
        <span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">&quot;There are some words: {}&quot;</span><span class="token punctuation">,</span> <span class="token keyword">self</span><span class="token punctuation">.</span>part<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">self</span><span class="token punctuation">.</span>part
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="静态生命周期"><a href="#静态生命周期" class="header-anchor">#</a> 静态生命周期</h2> <ul><li><code>'static</code> 是一个特殊的生命周期：整个程序的持续时间
<ul><li>例如：所有的字符串字面值都拥有 <code>'static</code> 生命周期
<ul><li><code>let s: &amp;'static str = &quot;I have a static lifetime.&quot;;</code></li> <li>字符串字面值是存在二进制程序中，总是可用</li></ul></li></ul></li> <li>为引用指定 <code>'static</code> 之前要三思
<ul><li>是否需要引用在整个生命周期内都存活</li></ul></li></ul> <h3 id="一个泛型参数类型-trait-bound-和生命周期的综合例子"><a href="#一个泛型参数类型-trait-bound-和生命周期的综合例子" class="header-anchor">#</a> 一个泛型参数类型，Trait Bound 和生命周期的综合例子</h3> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">use</span> <span class="token namespace">std<span class="token punctuation">::</span>fmt<span class="token punctuation">::</span></span><span class="token class-name">Display</span><span class="token punctuation">;</span>

<span class="token keyword">fn</span> <span class="token function-definition function">longest_with_an</span> announcement<span class="token operator">&lt;</span><span class="token lifetime-annotation symbol">'a</span><span class="token punctuation">,</span><span class="token class-name">T</span><span class="token operator">&gt;</span>
    <span class="token punctuation">(</span>x<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span> y<span class="token punctuation">:</span> <span class="token operator">&amp;</span><span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span><span class="token punctuation">,</span> ann<span class="token punctuation">:</span> <span class="token class-name">T</span><span class="token punctuation">)</span> <span class="token punctuation">-&gt;</span> <span class="token lifetime-annotation symbol">'a</span> <span class="token keyword">str</span>
<span class="token keyword">where</span>
    <span class="token class-name">T</span><span class="token punctuation">:</span> <span class="token class-name">Display</span><span class="token punctuation">,</span>
<span class="token punctuation">{</span>
    <span class="token macro property">println!</span> <span class="token punctuation">(</span><span class="token string">&quot;Announcement! {}&quot;</span><span class="token punctuation">,</span> ann<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> x<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&gt;</span> y<span class="token punctuation">.</span><span class="token function">len</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        x
    <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
        y
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p><strong>要注意的是，生命周期也是泛型的一种。</strong></p></div> <footer class="page-edit"><!----> <!----></footer> <!----> </main></div><div class="global-ui"></div></div>
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