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| 22 | <title>Referencing Fields (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 57 | <body lang="en">
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| 58 | <span id="Referencing-Fields"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Dynamic-Memory-Allocation.html" accesskey="n" rel="next">Dynamic Memory Allocation</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Referencing-Structure-Fields"></span><h3 class="section">15.1 Referencing Structure Fields</h3>
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| 64 | <span id="index-referencing-structure-fields"></span>
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| 65 | <span id="index-structure-fields_002c-referencing"></span>
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| 66 |
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| 67 | <p>To make a structure useful, there has to be a way to examine and store
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| 68 | its fields. The ‘<samp>.</samp>’ (period) operator does that; its use looks
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| 69 | like <code><var>object</var>.<var>field</var></code>.
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| 70 | </p>
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| 71 | <p>Given this structure and variable,
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| 72 | </p>
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| 73 | <div class="example">
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| 74 | <pre class="example">struct intlistlink
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| 75 | {
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| 76 | int datum;
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| 77 | struct intlistlink *next;
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| 78 | };
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| 79 |
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| 80 | struct intlistlink foo;
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>you can write <code>foo.datum</code> and <code>foo.next</code> to refer to the two
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| 84 | fields in the value of <code>foo</code>. These fields are lvalues, so you
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| 85 | can store values into them, and read the values out again.
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| 86 | </p>
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| 87 | <p>Most often, structures are dynamically allocated (see the next
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| 88 | section), and we refer to the objects via pointers.
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| 89 | <code>(*p).<var>field</var></code> is somewhat cumbersome, so there is an
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| 90 | abbreviation: <code>p-><var>field</var></code>. For instance, assume the program
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| 91 | contains this declaration:
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| 92 | </p>
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| 93 | <div class="example">
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| 94 | <pre class="example">struct intlistlink *ptr;
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| 95 | </pre></div>
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| 96 |
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| 97 | <p>You can write <code>ptr->datum</code> and <code>ptr->next</code> to refer
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| 98 | to the two fields in the object that <code>ptr</code> points to.
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| 99 | </p>
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| 100 | <p>If a unary operator precedes an expression using ‘<samp>-></samp>’,
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| 101 | the ‘<samp>-></samp>’ nests inside:
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| 102 | </p>
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| 103 | <div class="example">
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| 104 | <pre class="example"> -ptr->datum <span class="roman">is equivalent to</span> -(ptr->datum)
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| 105 | </pre></div>
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| 106 |
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| 107 | <p>You can intermix ‘<samp>-></samp>’ and ‘<samp>.</samp>’ without parentheses,
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| 108 | as shown here:
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| 109 | </p>
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| 110 | <div class="example">
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| 111 | <pre class="example">struct { double d; struct intlistlink l; } foo;
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| 112 |
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| 113 | <span class="roman">…</span>foo.l.next->next->datum<span class="roman">…</span>
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| 114 | </pre></div>
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| 115 |
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| 116 |
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| 117 |
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| 118 |
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