public/defer

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    <div class="example" id="defer">
      <h2><a href="./">Go by Example</a>: Defer</h2>
      
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          <td class="docs">
            <p><em>Defer</em> is used to ensure that a function call is
performed later in a program&rsquo;s execution, usually for
purposes of cleanup. <code>defer</code> is often used where e.g.
<code>ensure</code> and <code>finally</code> would be used in other languages.</p>

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            <a href="https://go.dev/play/p/5SDVfc_jxbg"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">package</span> <span class="nx">main</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="p">(</span>
</span></span><span class="line"><span class="cl">    <span class="s">&#34;fmt&#34;</span>
</span></span><span class="line"><span class="cl">    <span class="s">&#34;os&#34;</span>
</span></span><span class="line"><span class="cl"><span class="p">)</span></span></span></code></pre>
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            <p>Suppose we wanted to create a file, write to it,
and then close when we&rsquo;re done. Here&rsquo;s how we could
do that with <code>defer</code>.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span></span></span></code></pre>
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            <p>Immediately after getting a file object with
<code>createFile</code>, we defer the closing of that file
with <code>closeFile</code>. This will be executed at the end
of the enclosing function (<code>main</code>), after
<code>writeFile</code> has finished.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">f</span> <span class="o">:=</span> <span class="nf">createFile</span><span class="p">(</span><span class="s">&#34;/tmp/defer.txt&#34;</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="k">defer</span> <span class="nf">closeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="nf">writeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><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></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">&#34;creating&#34;</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <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></span><span class="line"><span class="cl">    <span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">        <span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="p">}</span>
</span></span><span class="line"><span class="cl">    <span class="k">return</span> <span class="nx">f</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><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></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">&#34;writing&#34;</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <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">&#34;data&#34;</span><span class="p">)</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <td class="docs">
            <p>It&rsquo;s important to check for errors when closing a
file, even in a deferred function.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl"><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></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">&#34;closing&#34;</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <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></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">        <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">&#34;error: %v\n&#34;</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">        <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></span><span class="line"><span class="cl">    <span class="p">}</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <td class="docs">
            <p>Running the program confirms that the file is closed
after being written.</p>

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          <td class="code">
            
          <pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run defer.go
</span></span><span class="line"><span class="cl"><span class="go">creating
</span></span></span><span class="line"><span class="cl"><span class="go">writing
</span></span></span><span class="line"><span class="cl"><span class="go">closing</span></span></span></code></pre>
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      <p class="next">
        Next example: <a href="recover">Recover</a>.
      </p>
      

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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>
    </p>

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