214 lines
9.1 KiB
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214 lines
9.1 KiB
Plaintext
Generated
<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Defer</title>
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<div class="example" id="defer">
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<h2><a href="./">Go by Example</a>: Defer</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Defer</em> is used to ensure that a function call is
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performed later in a program’s execution, usually for
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purposes of cleanup. <code>defer</code> is often used where e.g.
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<code>ensure</code> and <code>finally</code> would be used in other languages.</p>
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</td>
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<td class="code empty leading">
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/5SDVfc_jxbg"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="s">"os"</span>
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<span class="p">)</span>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Suppose we wanted to create a file, write to it,
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and then close when we’re done. Here’s how we could
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do that with <code>defer</code>.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Immediately after getting a file object with
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<code>createFile</code>, we defer the closing of that file
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with <code>closeFile</code>. This will be executed at the end
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of the enclosing function (<code>main</code>), after
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<code>writeFile</code> has finished.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<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>
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<span class="k">defer</span> <span class="nf">closeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
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<span class="nf">writeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
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<span class="p">}</span>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<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>
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<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>
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<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>
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<span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
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<span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="k">return</span> <span class="nx">f</span>
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<span class="p">}</span>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<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>
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<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>
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<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>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><span class="p">}</span>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>It’s important to check for errors when closing a
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file, even in a deferred function.</p>
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</td>
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<td class="code leading">
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<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>
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<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>
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<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>
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</pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code">
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<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>
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<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>
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<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>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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<p>Running the program confirms that the file is closed
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after being written.</p>
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</td>
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<td class="code">
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<pre class="chroma">
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<span class="gp">$</span> go run defer.go
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<span class="go">creating
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</span><span class="go">writing
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</span><span class="go">closing</span></pre>
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="recover">Recover</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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</p>
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</div>
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<script>
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var codeLines = [];
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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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