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<div class="example" id="defer">
<h2><a href="./">Go by Example</a>: Defer</h2>
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<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="http://play.golang.org/p/9aoHwzHcAo"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;os&quot;</span>
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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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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
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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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<div class="highlight"><pre> <span class="nx">f</span> <span class="o">:=</span> <span class="nx">createFile</span><span class="p">(</span><span class="s">&quot;/tmp/defer.txt&quot;</span><span class="p">)</span>
<span class="k">defer</span> <span class="nx">closeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
<span class="nx">writeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
<span class="p">}</span>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">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="nx">Println</span><span class="p">(</span><span class="s">&quot;creating&quot;</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="nx">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>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">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="nx">Println</span><span class="p">(</span><span class="s">&quot;writing&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintln</span><span class="p">(</span><span class="nx">f</span><span class="p">,</span> <span class="s">&quot;data&quot;</span><span class="p">)</span>
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<div class="highlight"><pre><span class="p">}</span>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">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="nx">Println</span><span class="p">(</span><span class="s">&quot;closing&quot;</span><span class="p">)</span>
<span class="nx">f</span><span class="p">.</span><span class="nx">Close</span><span class="p">()</span>
<span class="p">}</span>
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<p>Running the program confirms that the file is closed
after being written.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run defer.go
<span class="go">creating</span>
<span class="go">writing</span>
<span class="go">closing</span>
</pre></div>
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<p class="next">
Next example: <a href="collection-functions">Collection Functions</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/defer">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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