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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example 中文版: Defer</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'panic';
}
if (e.key == "ArrowRight") {
window.location.href = 'recover';
}
}
</script>
<body>
<div class="example" id="defer">
<h2><a href="./">Go by Example 中文版</a>: Defer</h2>
<table>
<tr>
<td class="docs">
<p><em>Defer</em> 用于确保程序在执行完成后，会调用某个函数，一般是执行清理工作。
Defer 的用途跟其他语言的 <code>ensure</code> 或 <code>finally</code> 类似。</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="https://play.studygolang.com/p/Q-zIr8aXjMM"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
<span class="s">"fmt"</span>
<span class="s">"os"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>假设我们想要创建一个文件、然后写入数据、最后在程序结束时关闭该文件。
这里展示了如何通过 <code>defer</code> 来做到这一切。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>在 <code>createFile</code> 后立即得到一个文件对象，
我们使用 defer 通过 <code>closeFile</code> 来关闭这个文件。
这会在封闭函数（<code>main</code>）结束时执行，即 <code>writeFile</code> 完成以后。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">f</span> <span class="o">:=</span> <span class="nf">createFile</span><span class="p">(</span><span class="s">"/tmp/defer.txt"</span><span class="p">)</span>
<span class="k">defer</span> <span class="nf">closeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
<span class="nf">writeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">createFile</span><span class="p">(</span><span class="nx">p</span> <span class="kt">string</span><span class="p">)</span> <span class="o">*</span><span class="nx">os</span><span class="p">.</span><span class="nx">File</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"creating"</span><span class="p">)</span>
<span class="nx">f</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nf">Create</span><span class="p">(</span><span class="nx">p</span><span class="p">)</span>
<span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="p">}</span>
<span class="k">return</span> <span class="nx">f</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">writeFile</span><span class="p">(</span><span class="nx">f</span> <span class="o">*</span><span class="nx">os</span><span class="p">.</span><span class="nx">File</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"writing"</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Fprintln</span><span class="p">(</span><span class="nx">f</span><span class="p">,</span> <span class="s">"data"</span><span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>关闭文件时，进行错误检查是非常重要的，
即使在 defer 函数中也是如此。</p>
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">closeFile</span><span class="p">(</span><span class="nx">f</span> <span class="o">*</span><span class="nx">os</span><span class="p">.</span><span class="nx">File</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"closing"</span><span class="p">)</span>
<span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Close</span><span class="p">()</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code">
<pre class="chroma"> <span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Fprintf</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stderr</span><span class="p">,</span> <span class="s">"error: %v\n"</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
<span class="nx">os</span><span class="p">.</span><span class="nf">Exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
<p>执行程序，确认写入数据后，文件已关闭。</p>
</td>
<td class="code">
<pre class="chroma">
<span class="go">go run defer.go
</span><span class="go">creating
</span><span class="go">writing
</span><span class="go">closing</span></pre>
</td>
</tr>
</table>
<p class="next">
下一个例子: <a href="recover.html">Recover</a>
</p>
<p class="footer">
<a href="https://twitter.com/mmcgrana">@mmcgrana</a> 和<a href="https://eli.thegreenplace.net">Eli Bendersky</a>编写 | <a href="https://github.com/gobyexample-cn">gobyexample-cn</a> 翻译 | <a href="https://github.com/gobyexample-cn/gobyexample/issues">反馈</a> | <a href="https://github.com/gobyexample-cn/gobyexample">源码</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a> </p>
</p>
</div>
<script>
var codeLines = [];
codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"os\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' f :\u003D createFile(\"/tmp/defer.txt\")\u000A defer closeFile(f)\u000A writeFile(f)\u000A}\u000A');codeLines.push('func createFile(p string) *os.File {\u000A fmt.Println(\"creating\")\u000A f, err :\u003D os.Create(p)\u000A if err !\u003D nil {\u000A panic(err)\u000A }\u000A return f\u000A}\u000A');codeLines.push('func writeFile(f *os.File) {\u000A fmt.Println(\"writing\")\u000A fmt.Fprintln(f, \"data\")\u000A');codeLines.push('}\u000A');codeLines.push('func closeFile(f *os.File) {\u000A fmt.Println(\"closing\")\u000A err :\u003D f.Close()\u000A');codeLines.push(' if err !\u003D nil {\u000A fmt.Fprintf(os.Stderr, \"error: %v\\n\", err)\u000A os.Exit(1)\u000A }\u000A}\u000A');codeLines.push('');
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