194 lines
6.9 KiB
Plaintext
194 lines
6.9 KiB
Plaintext
<!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: Pointers</title>
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<div class="example" id="pointers">
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<h2><a href="./">Go by Example</a>: Pointers</h2>
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<table>
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<tr>
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<td class="docs">
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<p>Go supports <em><a href="http://en.wikipedia.org/wiki/Pointer_(computer_programming)">pointers</a></em>,
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allowing you to pass references to values and records
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within your program.</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/KdE4TBbUL2"><img title="Run code" src="play.png" class="run" /></a>
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"fmt"</span>
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</pre></div>
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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>We’ll show how pointers work in contrast to values with
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2 functions: <code>zeroval</code> and <code>zeroptr</code>. <code>zeroval</code> has an
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<code>int</code> parameter, so arguments will be passed to it by
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value. <code>zeroval</code> will get a copy of <code>ival</code> distinct
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from the one in the calling function.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">zeroval</span><span class="p">(</span><span class="nx">ival</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
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<span class="nx">ival</span> <span class="p">=</span> <span class="mi">0</span>
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<span class="p">}</span>
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</pre></div>
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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><code>zeroptr</code> in contrast has an <code>*int</code> parameter, meaning
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that it takes an <code>int</code> pointer. The <code>*iptr</code> code in the
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function body then <em>dereferences</em> the pointer from its
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memory address to the current value at that address.
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Assigning a value to a dereferenced pointer changes the
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value at the referenced address.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">zeroptr</span><span class="p">(</span><span class="nx">iptr</span> <span class="o">*</span><span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
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<span class="o">*</span><span class="nx">iptr</span> <span class="p">=</span> <span class="mi">0</span>
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<span class="p">}</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
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<span class="nx">i</span> <span class="o">:=</span> <span class="mi">1</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"initial:"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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</pre></div>
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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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<div class="highlight"><pre> <span class="nx">zeroval</span><span class="p">(</span><span class="nx">i</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"zeroval:"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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</pre></div>
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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>The <code>&i</code> syntax gives the memory address of <code>i</code>,
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i.e. a pointer to <code>i</code>.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">zeroptr</span><span class="p">(</span><span class="o">&</span><span class="nx">i</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"zeroptr:"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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</pre></div>
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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>Pointers can be printed too.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"pointer:"</span><span class="p">,</span> <span class="o">&</span><span class="nx">i</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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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><code>zeroval</code> doesn’t change the <code>i</code> in <code>main</code>, but
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<code>zeroptr</code> does because it has a reference to
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the memory address for that variable.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre><span class="gp">$</span> go run pointers.go
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<span class="go">initial: 1</span>
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<span class="go">zeroval: 1</span>
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<span class="go">zeroptr: 0</span>
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<span class="go">pointer: 0x42131100</span>
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</pre></div>
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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="structs">Structs</a>.
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</p>
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<p class="footer">
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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/pointers">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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