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| 57 | <body lang="en">
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| 58 | <span id="const-Fields"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Zero-Length.html" accesskey="n" rel="next">Zero Length</a>, Previous: <a href="Bit-Field-Packing.html" accesskey="p" rel="prev">Bit Field Packing</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="const-Fields-1"></span><h3 class="section">15.8 <code>const</code> Fields</h3>
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| 64 | <span id="index-const-fields"></span>
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| 65 | <span id="index-structure-fields_002c-constant"></span>
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| 66 |
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| 67 |
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| 68 | <p>A structure field declared <code>const</code> cannot be assigned to
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| 69 | (see <a href="const.html">const</a>). For instance, let’s define this modified version of
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| 70 | <code>struct intlistlink</code>:
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">struct intlistlink_ro /* <span class="roman">“ro” for read-only.</span> */
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| 74 | {
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| 75 | const int datum;
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| 76 | struct intlistlink *next;
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| 77 | };
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>This structure can be used to prevent part of the code from modifying
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| 81 | the <code>datum</code> field:
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| 82 | </p>
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| 83 | <div class="example">
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| 84 | <pre class="example">/* <span class="roman"><code>p</code> has type <code>struct intlistlink *</code>.</span>
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| 85 | <span class="roman">Convert it to <code>struct intlistlink_ro *</code>.</span> */
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| 86 | struct intlistlink_ro *q
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| 87 | = (struct intlistlink_ro *) p;
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| 88 |
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| 89 | q->datum = 5; /* <span class="roman">Error!</span> */
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| 90 | p->datum = 5; /* <span class="roman">Valid since <code>*p</code> is</span>
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| 91 | <span class="roman">not a <code>struct intlistlink_ro</code>.</span> */
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| 92 | </pre></div>
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| 93 |
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| 94 | <p>A <code>const</code> field can get a value in two ways: by initialization of
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| 95 | the whole structure, and by making a pointer-to-structure point to an object
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| 96 | in which that field already has a value.
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| 97 | </p>
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| 98 | <p>Any <code>const</code> field in a structure type makes assignment impossible
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| 99 | for structures of that type (see <a href="Structure-Assignment.html">Structure Assignment</a>). That is
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| 100 | because structure assignment works by assigning the structure’s
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| 101 | fields, one by one.
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| 102 | </p>
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| 103 |
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| 104 |
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| 105 |
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| 106 | </body>
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